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A preliminary study of nest-site competition in a group of hole-nesting birds McLaren, William David 1963

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A PRELIMINARY STUDY OF NEST-SITE COMPETITION IN A GROUP OP HOLE-NESTING BIRDS  by  WILLIAM DAVID McLAREN B.Sc, University of B r i t i s h Columbia, 1958  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Zoology  We accept this thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA  0  January, 1962  In presenting t h i s thesis i n p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t available for reference and study.  freely  I further agree that permission  for extensive copying of t h i s thesis for scholarly purposes may be granted by the Head of my Department or by his  representatives.  It i s understood that copying or publication of this thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission.  Department of  ZOOLOGY  The University of B r i t i s h Columbia, Vancouver 8, Canada. Date  JAvt/aqy  /g,  1963  ABSTRACT  T h i s study was r e s t r i c t e d to b i r d s using tree-holes as n e s t - s i t e s . Of a t o t a l of 20 species i n the study area, only 13 were s u f f i c i e n t l y abundant to merit c o n s i d e r a t i o n .  These f e l l i n t o three n a t u r a l groups on the b a s i s of  hole s i z e , with only one euryoecious species (Iridoprocne b i c o l o r ) n e s t i n g i n a l l three groups.  Only the group based on holes made by the Colaptes  woodpeckers ( F l i c k e r s ) can p r e s e n t l y be construed as showing evidence of n e s t s i t e competition.  P h y s i c a l and e c o l o g i c a l c h a r a c t e r i s t i c s of n e s t - s i t e s are  analyzed i n terms of i n t e n s i f i c a t i o n or a m e l i o r a t i o n of n e s t - s i t e  competition.  The competing s p e c i e s , a l l using holes made by Colaptes c a f e r , are Sturnus v u l g a r i s , S i a l i a currucoides, Bucephala a l b e o l a , Iridoprocne b i c o l o r and F a l c o sparverius. The data suggest t h a t although competition i s now present i n t h i s group, i t may have been absent before the advent of Sturnus  i n the a v i f a u n a .  Neither  s e l e c t i o n f o r d i f f e r e n t s i t e s nor competitive e x c l u s i o n seem to have occurred before the appearance of Sturnus, which now occupies roughly 25$ of a l l a v a i l a b l e n e s t s , but one or both of these may now be going on.  - ii  -  ACKNOWLEDGEMENTS  G r a t e f u l acknowledgement i s due  to a l l persons and organizations whose  a s s i s t a n c e made p o s s i b l e the s u c c e s s f u l completion of t h i s study.  In p a r t i c u l a r ,  a p p r e c i a t i o n i s extended t o : Dr. M.D.F. Udvardy f o r expert guidance throughout the study as supervisor of the p r o j e c t ; to Dr. L. von Haartman, f o r much assistance and encouragement during h i s residence a t the U n i v e r s i t y of B r i t i s h Columbia i n 1958-59; to Dr. I . McT.  Cowan, Head of the Department of Zoology of the U n i v e r s i t y of  B r i t i s h Columbia, who  assumed much of the burden of s u p e r v i s i o n and  of the p r o j e c t during Dr. Udvardy*s absence during p a r t of 1958 to Drs. W.S.  and  administration 1959;  Hoar, K. Graham and J.F. B e n d e l l , a l l of whom a l s o served on  my  Research Committee; to Miss M.F.  Jackson and Dr. M.T.  Myres, who  provided many u s e f u l data and  local  information r e s u l t i n g from t h e i r e a r l i e r f i e l d work i n the study area; to other members of the Department of Zoology who  from time to time were most  h e l p f u l i n rendering advice on numerous questions and problems; to f e l l o w graduate students  i n the Department of Zoology f o r s t i m u l a t i n g d i s c u s s i o n s  and general a s s i s t a n c e i n many ways; to Dr. A . J . E r s k i n e , then a f e l l o w graduate student, who h i s study areas near 100 M i l e House, B.C.,  and who  was  provided many data from  at a l l times a valued  companion and esteemed colleague; to Lawson G. Sugden and P a t r i c k W, M a r t i n , Game B i o l o g i s t s with the F i s h and Game Branch of the B r i t i s h Columbia Department of Recreation and Conservation,  f o r general  a s s i s t a n c e and many deeply appreciated c o u r t e s i e s ; to the owners of the T/H and Mr. W.  Ranch at Springhouse, B.C.,  Messrs. G. and C. Tucker,  H e r r i c k , as w e l l as t h e i r f a m i l i e s , whose u n f a i l i n g h o s p i t a l i t y made a  - i i i -  p o t e n t i a l l y d i f f i c u l t t a s k not only e a s i e r , but decidedly pleasant. F i n a n c i a l support f o r t h i s study was  provided by the N a t i o n a l Research  C o u n c i l of Canada through a grant to Dr. Udvardy. My  a p p r e c i a t i o n i s due a l s o to the Department of Zoology f o r o f f i c e space  and f i e l d equipment used during the study, and to the B.C. w i t h i n t h a t Department f o r freedom to c o n s u l t t h e i r f i l e s .  Nest Records Scheme  - iv -  TABLE OF CONTENTS  Subject INTRODUCTION Objects of the Study Terminology. . . . • . Equipment and Methods Location of Study Areas  Page  . . . .  1 1 1 3 4  ECOLOGICAL CONTEXT OF THE PROBLEM Systematic and Ecological Classification of the "Competitors". . . General Plant Ecology of the Study Area Non-Avian Hole-Using Biota The "Life History" of Nest Sites Recapitulation: Ecological Context  6 6 9 15 16 20  ANALYSIS OF NEST-SITE COMPETITION The "Cycles" The Large-Hole Cycle The Small-Hole Cycle The Medium-Hole, or "Flicker" Cycle Hole Direction Hole Height Entrance Size S i l l Size Hole Depth Cavity Sizes Recapitulation: Analysis of Competitive Situation  21 21 21 23 24 25 27 30 32 34 35 38  DISCUSSION Orientation of this Study and Suggested Future Research On the Definition of Competition The Detection and Measurement of Competition Competitive Exclusion. . . . . . . . . . . . . . . . . Nest-Site Competition as an Ecological Factor i n Hole-Nesting Birds Habitat Dispersion as an Ecological Factor The Tree Swallow (Iridoprocne bicolor) as a Competitor. . . . . . . The European Starling (Sturnus vulgaris) as a Competitor  40 40 41 42 44 47 48 49 50  SUMMARY AND CONCLUSIONS  52  REFERENCES CITED  54  APPENDIX.-  57  - V -  LIST OF TABLES  Table  Page  I  Hole-nesting species i n the study area  7  II  Species composition of sub-groups of h o l e - n e s t i n g b i r d s  i n the  study area  9  III  Hole d i r e c t i o n s  IV  Hole h e i g h t s :  V  Sill  VI  Frequency d i s t r i b u t i o n of hole depths:  VII  Size-class  sizes:  of nests measured w i t h i n the study a r e a . . . . summary f o r F l i c k e r c y c l e .  F l i c k e r cycle  26 29  summary. . .  .33 Flicker cycle.  d i s t r i b u t i o n of nest c a v i t i e s :  . . . .  Flicker cycle.  . .  36 37  LIST OP FIGURES Figure  to follow Page  1.  Diagram of nest-site measurements. . . .  3  2.  Diagram of nest inspection technique  3  3.  Natural species-site relationships  8  4.  Hole-size relationships of Tree Swallow, Flicker and lellowbellied Sapsucker  24  5.  The Flicker Cycle:  Demands upon hole supply  6.  Percent frequency distribution of hole directions:  24 Flicker  Cycle  27  7.  Height-class distribution:  8.  Colaptes: Size-class frequency of entrance parameters (1959 data) Sturnus: Size-class frequency of entrance parameters  9.  Flicker Cycle  (1959 data).  27  31 31  10.  Colaptes:  Scatter diagram of entrance measurements. . . .  11.  Sturnus:  12.  Sialia:  13.  Bucephala:  14.  S i l l sizes:  Colaptes and  15.  S i l l sizes:  Bucephala and Iridoprocne  32  16.  S i l l sizes:  Sturnus and S i a l i a  32  17.  Hole depths:  Scatter diagram of entrance measurements Scatter diagram of entrance measurements Scatter diagram of entrance measurements. . . .  18. Hole depths: 19.  Cavity Sizes:  20. Cavity Sizes: 21.  Colaptes,  Flicker Cycle  31 31 31 31 32  Flicker Cycle, and Sturnus. . . .  36  S i a l i a , Bucephala and Iridoprocne. . . . . .  36  Flicker Cycle and Colaptes Sturnus and S i a l i a .  Cavity Sizes: Bucephala and Iridoprocne  38 38 38  INTRODUCTION  Objects of the Study The type of ecological relationship commonly designated as competition has long been a subject of both interest and controversy.  Ample documentation  i s found i n the ecological literature of the fact that this term has been widely and variously used to refer to a broad spectrum of ecological relationships, . although attempts are now being made to r e s t r i c t i t s use to one well-defined phenomenon or class of phenomena (cf. Birch, 1957; Milne, 1961). Birds u t i l i z i n g tree-holes as nest-sites are frequently referred to as typical cases of inter- or intra-specific competition for the use of a limited number of available nest-sites, although opportunities are rarely u t i l i z e d for careful evaluation of this situation under natural conditions. The present work represents the f i r s t stage of a series of studies designed to explore the nature and extent of such competition i n local avifaunas, and consists basically of an attempt to define, on as quantitative a basis as possible, the extent and nature of nest-site competition among the tree-hole-nesting birds of the Cariboo region of the interior of B r i t i s h Columbia. Terminology Frequently, considerable misunderstanding  and controversy arise as a  result of poor or incomplete exposition of the exact meanings attributed to crucial terms and concepts used i n the development of s c i e n t i f i c arguments. The following definitions are thus offered for terras used i n the present paper: Competition - the demand by more than one individual organism during a given period of time for an environmental resource which i s present i n insufficient amounts to supply the total demand.  - 2-  It i s important to note that this definition stresses both spatial and temporal contiguity of the competitors, as well as a v a i l a b i l i t y of s i t e s . An expanded discussion of this problem of the definition of competition i s included i n the discussion section below. "Hole-nesting bird"- those species of birds which normally, or occasionally, u t i l i z e tree-cavities as nest sites, whether able or not to construct such cavities.  Although many other species, such as various Swallows (Hirundinidae),  Kingfishers (Alcedinidae), Petrels (Hydrobatidae), and Shearwaters (Procellariidae), to name but a few examples, also nest i n holes, this study was restricted to tree hole breeders for the obvious reason of ecological continuity. Primary hole-nester - a hole-nesting species capable of excavating the nest cavity. Secondary hole-nester - a hole—nesting species requiring a cavity as a nest-site, but incapable of carrying out the necessary excavation. Ecological niche - the ecological niche i s a particular combination of physical factors and biotic relations required by a species for the normal course of i t s l i f e (Kendeigh, 1961). Diameter at breast height (dbh) - the diameter of any tree as measured at a standard height of four and one-half feet above ground l e v e l .  This i s a  standard forester's measure, and was made with a standard diameter tape (see below). Hole height - height above ground of the bottom of the entrance of any given nest. Hole size - this parameter was always recorded with the horizontal value before the v e r t i c a l value, as, 2,5 X 2.7 cm. S i l l size - thickness of the base of the entrance hole of any cavity. Cavity - horizontal distance from inside edge of s i l l to opposite wall of the cavity.  - 3-  Depth - v e r t i c a l d i s t a n c e from s i l l t o bottom of c a v i t y .  In the case of  species which place extraneous m a t e r i a l i n the c a v i t y , both l e v e l of nest and a c t u a l depth of c a v i t y were measured wherever p o s s i b l e . Hole d i r e c t i o n - d i r e c t i o n toward which entrance hole i s o r i e n t e d . parameter was determined  This  with a standard f o r e s t e r ' s compass.  Figure 1 i s a diagrammatic r e p r e s e n t a t i o n of the r e l a t i o n s h i p between v a r i o u s n e s t - s i t e parameters. Equipment and Methods L i t t l e was r e q u i r e d i n terms of s p e c i a l equipment f o r the f i e l d work phases of t h i s study; p a r t i c u l a r items used are discussed below. Nest s i t e s were l o c a t e d by searching of s e l e c t e d areas on f o o t , and other s i t e s were l o c a t e d i n c i d e n t a l l y by s i g h t i n g s from v e h i c l e s , r e p o r t s from i n t e r e s t e d persons, e t c . A l l measurements made i n 1958 converted to metric e q u i v a l e n t s .  were i n f e e t and inches, and were l a t e r In 1959,  a l l measurements were made d i r e c t l y ,  i n metric u n i t s , except t r e e diameters and t r e e h e i g h t s , and a few extremely h i g h nest h e i g h t s .  A l l measurements of t r e e diameters were made with a standard  f o r e s t e r ' s diameter tape which was graduated  i n diameter-equivalents of  circumference, expressed i n inches and tenths of inches.  Tree heights and  extreme nest h e i g h t s were measured with a standard Abney l e v e l or were roughly t r i a n g u l a t e d when c h a r a c t e r i s t i c s o f t e r r a i n or v e g e t a t i o n precluded the use of t h i s  instrument.  Nest-holes were opened when necessary by a procedure  d e s c r i b e d by E r s k i n e  (1959 ( b ) ) . Various means were u t i l i z e d t o reach n e s t - s i t e s a t d i f f e r e n t h e i g h t s , i n c l u d i n g lineman's climbing spurs, a c o l l a p s i b l e aluminum u t i l i t y ladder, and improvisation from n a t u r a l m a t e r i a l s a t hand.  To folllow p. 3  FIG.  2  INSPECTION  METHOD  - 4-  In collaboration with L.G. Sugden, then Regional Game Biologist, B.C. Fish and Game Branch, Williams Lake, B.C., and Dr. A.J. Erskine, a special technique was devised to permit examination of the interiors of nest holes without opening as i n the technique mentioned above. Long slips of mirror glass were used i n conjunction with a pocket flashlight i n the manner indicated i n Figure 2, and judicious manipulation of both elements i n the system provided a lighted view of the interior of a cavity which was s u f f i c i e n t l y good i n most cases to permit accurate identification of the contents of the cavity.  A  notable exception occured i n the cavities excavated by the sapsucker, Sphyrapicus varius, i n which the width of s i l l was. i n most cases so great as to preclude the use of this technique. In the analysis of data, a l l techniques used were drawn from the standard repertoire of graphic and s t a t i s t i c a l methods available i n most standard books (e.g., Snedecor, 1956; Bailey, 1959; etc.).  Calculations and computations were  made to an accuracy of three significant figures; under f i e l d conditions i t was rarely possible to achieve measurements of greater refinement than t h i s . Location of Study Areas Munro and Cowan (1947) described thirteen t e r r e s t r i a l biotic areas, i n B r i t i s h Columbia, using the c r i t e r i a of presence of distinctive plant species, presence of distinctive animal species, and absence of plant and animal species of other zones.  The study area used i n the present investigation was that named  by Munro and Cowan as the Cariboo Farklands Biotic Region.  Recent botanical  studies tend to indicate that this area i s more properly a part of the Interior Douglas F i r bioclimatic zone.  This matter i s discussed at greater length below.  Geographic limits of the Cariboo area are as follows: to the east and west, the area meets relatively definite limits on the slopes of the coast mountains, and along the North Thompson River; to the south, i n the v i c i n i t y of  Clinton, there i s a gradual merging with the dry forests of the Ponderosa Pine Zone, and to the north, i n the area just south of Quesnel, i s a broad region of intergrading with the Boreal Forest, Clearly, the limits of this area are not well defined, particularly along the north and south boundaries.  More  exact definition of these boundaries awaits more detailed ecological study of these ecotone areas.  - 6 -  ECOLOGICAL CONTEXT OF THE PROBLEM  Systematic and Ecological Classification of the  "Competitors"  Included i n the avifauna of the study area are 17 species which satisfy the above definition of "hole-nesting" species.  In Table I these are summarized  and c l a s s i f i e d according to families, status as secondary or primary hole-nester, and relative abundance. niches within the  These 17 species occupy an equal number of ecological  ecosystems, and as i s shown below for members of one of  the three ecological groupings, these niches show l i t t l e overlap except i n terms of nest s i t e s . Heretofore, i t has been widely assumed that as a result of this apparent overlap i n one aspect of their respective niches, hole-nesting birds must "compete" for hest-sites.  As noted above, this study was designed  as a preliminary step i n the careful evaluation of that assumption. In Table I, order of occurence, common names and s c i e n t i f i c names follow the F i f t h Edition of the A.O.U. Checklist of North American Birds (1957).  As  can readily be seen, the bird species using tree-holes as nest sites i n the study area are drawn from a wide variety of taxa, and as might be expected, the variety of their ecological niches i s equally broad.  Clearly, then, the ecological  relationships pertaining to nest-site competition within this group are unlikely to be represented by their taxonomic a f f i n i t i e s .  Therefore i t was essential  for the purposes of this study to devise an ecologically-oriented c l a s s i f i c a t i o n in the context of which these relationships could be appreciated. Several factors are relevant to the elaboration of such a c l a s s i f i c a t i o n . Perhaps the most obvious of these i s relative population densities of the species involved: clearly, a very rare species cannot exert an appreciable effect upon large numbers of competing individuals or pairs.  The following species, although  represented i n faunal l i s t s for the area were not found nesting during the study  TABLE I. HOLE-NESTING SPECIES IN THE STUDY AREA.  FAMILY  SPECIES  STATUS  RELATIVE * ABUNDANCE  Barrow's Goldeneye (Bucephala islandica)  Anatidae  Secondary  Common  Bufflehead (Bucephala albeola)  Anatidae  Secondary  Common  Sparrowhawk (Kestrel) (Falco sparverius)  Falconidae  Secondary  Uncommon  Screech Owl (Otus asio)  Strigidae  Secondary  Rare  Great Horned Owl (Bubo virginianus)  Strigidae  Secondary  Relatively Common  Saw—whet Owl (Aegolius acadieus)  Strigidae  Secondary  Flicker (Colaptes cafer)  Picidae  Primary  Pileated Woodpecker (Dryocopus pileatus)  Pic idae  Primary  Yellow-bellied Sapsucker (Sphyrapicus varius)  Picidae  Primary  Hairy Woodpecker (Dendrocopos villosus)  Pic idae  Primary  Downy Woodpecker (Dendrocopos pubescens)  Picidae  Primary  Tree Swallow (Iridoprocne bicolor)  Hirundinidae  Secondary  Black-capped Chickadee (Parus a t r i c a p i l l u s )  Paridae  Primary  Mountain Chickadee (Parus gambeli)  Paridae  Primary  Relatively Common  Red-breasted Nuthatch (Sitta canadensis)  Sittidae  Primary  Common  Mountain Bluebird ( S i a l i a corrueoides)  Turdidae  Secondary  Relatively Common  European Starling (Sturnus vulgaris)  Sturnidae  Secondary  Common  ±  Rare Common Rare Common Rare Rare Common Rare  The terms used to refer to relative abundance are extremely general and open to c r i t i c i s m .  However, no more suitable expression i s available i n  the absence of extensive census data.  and were seen only rarely, i f at a l l s Woodpecker, and Downy Woodpecker.  Screech Owl, Saw-whet Owl, Hairy  In addition to these species, the Hooded  Merganser (Lophodytes cucullatus) was represented by one brood at P h i l i l l o Lake, and the Black-backed Three-toed Woodpecker (Picoides arcticus) by one nest, also at Phililoo Lake.  These species were ignored i n the analysis of data  since for a l l practical purposes they were absent from the area (c.f. Andrewartha and Birch, 1954). The remaining species are immediately d i v i s i b l e into three groups on the basis of size both of the birds themselves and of the cavities i n which they nest.  Only the Tree Swallow (Iridoprocne bicolor) occurs i n a l l three groups,  and only one other, the Sparrow Hawk (Falco sparverius) i n more than one.  In  addition to the three types of bird-excavated holes, the Tree Swallow u t i l i z e s other natural holes and a wide variety of niches, and i s generally highly euryoecious i n terms of i t s nest-site.  The overall species—site relationships  as they were found to exist on the study area are represented i n Figure 3. Table II gives the effective species composition of the three groups: the values given for size represent length measurements as given i n inches by Peterson (1961) and are included to emphasize size relationships. Further details on the three groups are provided below i n the section on analysis of nest-site competition. To recapitulate, i t has been shown that the taxonomic c l a s s i f i c a t i o n of these hole-nesting bird species i s unsuitable for ecological purposes, and they have been c l a s s i f i e d i n terms of the sizes of holes  u t i l i z e d for nest sites.  SOME E= ENLARGED  FIG.  3 :  BY C O L A P T E S  SPECIES-SITE  :  B=  DEGREE. BREAKAGE.  RELATIONSHIPS  TABLE I I . SPECIES COMPOSITION OF SUB-GROUPS OF HOLE-NESTING BIRDS IN THE STUDY AREA.  GROUP  SPECIES  SIZE *  Tree Swallow (iridoprocne bicolor) Mountain Chickadee (Parus gambeli) Black-Capped Chickadee (Parus atricapillus) Red-breasted Nuthatch (Sitta canadensis) Yellow-bellied Sapsucker (Sphvranicus varius)  5-6*  Medium-Hole  Red-shafted Flicker (Colaptes cafer) Bufflehead (Bucephala albeola) European Starling (Sturnus vulgaris) Mountain Bluebird (Sialia currucoides) Sparrow Hawk (Falco sparverius) Tree Swallow (iridoprocne bicolor)  12^-14 13-15i 7i-8i 6i-7| 9-12 5-6*  Large-Hole  Pileated Woodpecker (Dryocopus pileatus) Barrow's Goldeneye (Bucephala islandica) Sparrow Hawk (Falco sparnerius) Great Horned Owl (Bubo virginianus) Tree Swallow (Iridoprocne bicolor)  16-I9i 16^-20 9-12 18-25 5-6}  Small-Hole  8-9  General Plant Ecology of the Study Area The ecology of any t e r r e s t r i a l animal i s intimately related to the ecology of the plant components of the ecosystem i n which i t occurs.  This i s particularly  true of many groups of t e r r e s t r i a l birds whose ecological niches involve, among many other factors, gestalt-type behavioural responses to the overall physical aspect of the environment.  I t i s therefore necessary, i n considering the  ecological context of the present problem, to review b r i e f l y the factors which affect the distribution of the various habitat-types.  The following section  i s thus presented as an attempt to summarize the results of some relevant studies and to apply these results to a brief consideration of the habitats i n the study area* Sizes are given i n inches as taken from Peterson (l96l).  - 10 -  According to Tisdale and McLean (1957), the entire Cariboo region i s located well within the limits of the interior Pseudotsuga Zone,  They define  this zone as being,in B r i t i s h Columbia, i n the southern interior between the Rocky Mountains and the Coast Range on the East and West, ending at about Latitude 53 degrees to the north, and composing the northern extremity of the Rocky Mountain Douglas f i r forest,, The climate of the area i s rather complicated, with topography having pronounced effects upon local conditions, and exposure causing further complications.  Splisbury and Tisdale (1944) described vertical zonation of  s o i l s , climate and vegetation.  On south and west slopes, the boundaries of  main vegetation zones show differences i n elevation of as much as 800 feet from those of north and east exposures (Tisdale and McLean, 1957).  The same authors  also point out that regular meteorological data f a i l to i l l u s t r a t e variation i n climate within the region, as weather stations are largely located i n the major valleys where settlement i s concentrated.  They furnish data to i l l u s t r a t e the  changes i n climate encountered i n passing from the dry grassland areas i n the Kamloops area to the northern Spruce-Fir zone, at McCulloch.  As Tisdale and  McLean indicate, many large gaps exist i n our knowledge with regard to the climatology of this region, but i n a general way,  i t can be stated that the  Pseudotsuga zone i s cooler and moister than the grasslands-Yellow Pine (Pinus ponderosa) areas, and drier and warmer (in summer at least) than the Picea-Abies zone.  No data are available for zonal ecotones.  Tisdale and McLean (op. c i t . ) indicate that the major portion of the zone i s now, as a result of the effects of f i r e , dominated by serai tree species, particularly Pinus contorta and Populus tremuloides.  They further state that  although present conditions favour the establishment of the climax tree species over the region, there are large areas i n which the process w i l l be very slow due to lack of Pseudotsuga seed sources.  - 11 -  Soils i n the Cariboo have been l i t t l e studied*  Beaton (1953) c l a s s i f i e d  the s o i l of the upper Douglas f i r zone near Kamloops as "Brown Podsolic S o i l " * In general, the area i s a large lava flow covered by g l a c i a l t i l l of varying coarseness* The role of f i r e i n forest succession has been widely studied and the relationship of aspen (Populus tremuloides) to f i r e i s well established (see Baker, 1925; Moss, 1932). Aspen reproduction after f i r e seems mainly to occur by means of adventitious shoots arising from the roots. Among reasons put forward for.the scarcity of cases of aspen reproduction by seed are the shortness of the period of seed v i a b i l i t y and slow i n i t i a l root growth (Moss, 1932), and a frequent lack of seed trees after f i r e (Tisdale and McLean* 1957).  In the region of  Kamloops, however, Tisdale and McLean observed "considerable numbers" of aspen seedlings in burned areas of the Douglas f i r zone during the summer of 1952* In the grassland openings of the Cariboo Parklands, aspen groves can be found on level ground either i n small, isolated, f a i r l y discrete groups, frequently clonal i n nature, or i n large semi-open stands i n such locations as are well supplied with s o i l or surface water.  Grassy areas of uneven topography have  small groves i n gullies and other drainage routes, while low-lying wetlands are frequently covered with thick growths of willow thickets, etc. Past use by man has had a profound effect upon the entire Douglas f i r zone, and the Cariboo Parklands portion i s not excepted*  Both Macoun (1876) and Dawson  (1894) remarked on the replacement by poplar (or aspen) and Pinus contorta of the original vegetation, and Dawson refers to the abundance of f i r e s caused by both Indians and whites*  Suppression of f i r e i n recent years i n the grazing  areas has resulted i n the occupation of many acres by Douglas f i r regeneration as a result of the lack of f i r e destruction of young trees* (L.G. Sugden, pers. comm.). Grazing also i s effective i n restricting or preventing aspen regeneration. Logging, although carried out i n the Douglas F i r zone since the 1860*s, has only  recently become important i n the Cariboo area.  As yet, only Douglas f i r i s  exploited i n quantity, but continually greater use i s being made of Pinus contorta and Spruce, Picea engelmanni, as well.  Aspen, as a result of i t s small size,  usually rather gnarled growth habit and almost universal susceptibility to attack by Pomes igniarius, a heart—rot, i s not l i k e l y to become a commercially species i n the foreseeable future. appeared i n the Cariboo.  important  Industrialization on a large scale has not yet  Small sawmills are scattered throughout the area, and  usually are present i n a given location for only a relatively short period, leaving behind sawdust piles and large heaps of slab, scrap and reject material. These refuse deposits constitute serious f i r e hazards, and their decomposition also i s extremely slow. Insect outbreaks of major proportions have occurred i n recent decades. The most destructive of these have been the infestations of Lodgepole Pine (Pinus contorta) by the bark beetles, Dendroctonus and Ips.  Parts of the Douglas F i r  zone other than the Cariboo were hardest h i t . The Aspen Leaf Miner, Phyllocnistis populiella. a lepidopteran, has caused widespread heavy attacks of aspen from Golden, i n the Interior Wet Belt, through the Cariboo and north at least to the v i c i n i t y of Quesnel (c.f, Tisdale and McLean).  Infestation by this insect seems  to cause no particular damage, and some trees at least were seen during the present study to produce new leaves in midsummer when infestation was particularly high. Lynch (1955) discusses the role of the wood rotting fungus, Pomes igniarius, in the aspen groveland of Glacier County, Montana. He states that this fungus causes widespread damage and that i t i s exceptional for aspen not to have discolored heartwood.  Schmitz and Jackson (1927) also found this to hold for Minnesota.  Moss (1932) and Bird (1930) found P. igniarius to be abundant i n Alberta and Manitoba respectively, and Bird states that i t causes the "premature" death of many trees. Moss found sporophores rare on 35-year-old aspens, but not uncommon  13 -  on 50 year-old trees.  The decay of heartwood caused by this fungus, which  starts while the tree i s s t i l l rather small, may have considerable significance to the Flicker populations of the aspen groveland areas:  the much softer heartwood  i s attacked for nest—hole excavations much more frequently than are healthier trees; and frequently the birds commence excavation i n a scar or broken branch stub i n which decay has softened the wood. Sapsuckers, on the other hand, rarely i f ever excavate a hole i n previously injured wood on the outside of the tree. Even i n these cases, however, they usually do not begin to enlarge the nest-cavity i t s e l f u n t i l the excavation reaches the softer, decayed heartwood. Lynch (1955) believes that the exhaustion of water available for plant growth i s a c r i t i c a l factor i n the stabilization of grove boundaries for aspen.  This,  he feels, i s due to the fact that while the main root zone of aspen rarely goes below 2', the roots functioning i n shoot production are usually at depths less than 1', and the reduction of s o i l water past the permanent .wilting percentage results i n the failure of shoots to develop i n the dry s o i l .  He feels also that  lack of seed accounts for the f a i l u r e of aspen to establish i n lowland spots where the understory vegetation implies suitable moisture conditions.  Lynch  again points out the effect of grazing i n preventing aspen regeneration. McMinn (1952) studied the effects of s o i l drought on the distribution of vegetation i n the Rocky Mountains of the northern U.S.A., and concluded that i n areas where precipitation i s mostly i n the winter and summer drought i s normal, different plant associations are correlated with different extents of s o i l drought. To summarize the plant ecology of the study area, the following points are relevant.  I t seems now to be generally agreed that the Cariboo region i s  characterized by the climatic climax forest type known as the Interior Douglas F i r (or Pseudotsuga) Zone (c.f. Krajina, 1959) which has, however, been extensively altered so that very large areas now support a serai association i n which Lodgepole  14  Pine, Pinus contorta. is the dominant and frequently only tree.  Some extensive  grassland areas also exist; the permanence of these areas i s doubtful, however, except in the southerly ecotone with the Ponderosa Pine-Grassland Zone, and along the great trench of the Fraser River.  The latter area v i r t u a l l y s p l i t s the  Cariboo-Chilcotin plateau into two separate areas, and requires special study from an ecological and biogeographical viewpoint.  Trembling Aspen (Populus  tremuloides), the presence of which i s a dominant factor i n the ecologies of hole-nesting birds i n the area, occurs i n stands of several different types. "Fringe" stands frequently occur along edges between grasslands and coniferous stands, large pure stands are found occasionally, and scattered "groves" are found i n grassland areas around lakes, i n low moist-soil areas and i n gullies and other natural drainage courses.  Although much more work i s needed, the hypothesis  seems favoured that not only the Lodgepole Pine, but also Grassland and Aspen habitats are serai stages resulting from the effects of f i r e .  Various types of  edaphic wetland communities occur also, but these are very l i t t l e used by the bird species under study. Aspen and Douglas F i r trees contained more than 90$ of a l l nests studied, which indicates clearly the importance of these two species.  The distribution  and abundance of both appears to have been profoundly affected by f i r e , and that of Aspen also by soil-water considerations.  It might be suggested also that  extensive range-use studies would be of great value i n assessing the effect of grazing by domestic livestock upon tree regeneration, particularly of Aspen. The overall effects of these considerations of plant ecology upon distribution and abundance of hole-nesting birds are discussed i n a later section. Further descriptions of habitat—types and plant communities are to be found in Krajina (1959), Tisdale and McLean (1957), Munro and Cowan (1947), and Erskine (i960), and further elaboration i s not needed here.  - 15 -  F i n a l l y , then, i t i s clear that although the ecology of this complex and much-changed phytocoenosis  can be understood i n very general terms, there remains  much botanical work to be done before the overall ecological context of the present investigation can be f u l l y appreciated, Non-Avian Hole-Using Biota Tree-holes are u t i l i z e d for various purposes by many members of the fauna other than birds. The most numerous of those encountered during this study were two species of sciurid rodents, the Flying Squirrel (Glaucomys sabrinus) and the Red Squirrel (Tamiasciurus hudsonicus), both of which were found i n a few cases to be nesting i n holes of the medium-hole, (see p;'24),, cycle.  Numbers of cases,  however, were very small, and i s speculated that Glaucomys may tend to use niches, cracks, etc, to a considerable extent, and Tamiasciurus  i s also known to build  large nests of twigs, leaves, etc., as well as to use woodpecker holes*  Only six  nests of both Squirrel species were i n Flicker—source holes, and these were disregarded i n the analysis for competition for the same reasons as were the rare bird species discussed above. A note of caution i s advisable i n this regard.  Local opinion was that Red  Squirrel populations were "low" during the period of the study, and the p o s s i b i l i t y must be introduced that during population peaks of this species, a demand upon flicker-cycle holes could develop which might assume significant proportions i n the overall competitive situation.  Such a situation could conceivably result  also in an increased rate of non-specific predation by squirrels on both eggs and young of the hole-nesting birds as well as those of other nesting habits.  These  factors were apparently not operative to a significant degree during the present study, but they remain nevertheless as potentially significant ecological parameters. Very l i t t l e i s known of the natural history of Flying Squirrels i n this area. In fact, many local persons were unaware that this species existed i n the region.  - 16 -  Nothing i s known of population levels, amplitude of fluctuations, or any other aspect of the population dynamics of this species, nor i s i t known what proportion of nests f a l l within the flicker-cycle.  In short, essentially nothing  is known of this species except that i t occurs i n the area, and that occasional individuals were flushed from flicker-source holes during the investigation. Other workers arid casual informants have reported verbally the occasional finding of Bats (family Vespertilionidae), Deer Mice (Peromyscus maniculatus: family Cricetidae), Bushy-tailed Woodrats (Neotoma cinerea: family Cricetidae) and assorted insects, largely Hymenoptera. Again, only scattered evidence of such use was encountered i n this study.  No bats were encountered during the two  summers, and only one mouse was discovered i n an empty, incomplete  nest-hole.  Several holes were found to be completely f i l l e d with assorted debris, and i t is widely held l o c a l l y that this f i l l i n g of holes i s done by the Bushy-tailed Woodrat (Neotoma cinerea), usually called the "Pack Rat".  At no time during the  two year period of f i e l d work was any such debris deposited i n nest sites under study. The "Life History" of Nest Sites Por most bird species, nest-sites are probably a measurable ecological parameter.  This i s perhaps most obvious for secondary hole-nesting species, but  Stein (1958) and Mayfield (i960), to mention but two, have shown the measurable and r e l a t i v e l y clearly defined nature of nest-sites for species of other ecological characteristics. Among the hole-nesting birds of this study, perhaps more than i n most other cases, the " l i f e - h i s t o r y " of the sites themselves can clearly be seen to have an important relationship to the population ecologies of the species involved. It has already been noted (c.f. above) that the durability of stumps and "snags" of Douglas P i r (Pseudotsuga menziesii) appears to be an important factor  - 17 -  in the functioning of the Large-Hole Cycle, and may well be a key factor i n determining the population density of the Barrow's Goldeneye (Bucephala islandica). Equally clear i s the d i f f e r e n t i a l role of Aspen (Populus tremuloides) trees in the Small- and Medium-Hole cycles, at least i n terms of the Yellow-bellied Sapsucker (Sphyrapicus varius) in the former cycle*  As noted above, essentially  a l l trees of Aspen of a diameter at breast height of about five inches or greater are infected by the heart-rot fungus, Fomes igniarius.  This may  be  regarded as a conditioning factor, and i t i s clearly of great significance that the areas supporting this combination of Populus and Fomes are also the areas i n which hole-nesting birds are known to be vastly more abundant than elsewhere.  It  is concurrently true that other features of the phytocoenosis which includes Populus tremuloides are no doubt also essential to the hole-nesting species under study, but i t i s suggested that this conditioning of trees, i n the sense that heartwood i s rendered softer and more readily excavated, i s of considerable significance.  The Sapsucker (Sphyrapicus) normally commences nest-site excavation  in sound sapwood, but always at a sufficiently great diameter (in 40 nests, never less than 7.0",  except one of 6.5")  that decayed heartwood i s present,  and in a l l nests studied, did not commence to enlarge the cavity u n t i l this decayed wood had been reached.  The Flicker (Colaptes), on the other hand, rarely  began excavation i n sound wood, but usually u t i l i z e d branch stubs, cracks, checks, "cats-paws" and other types of scars i n which to commence excavation.  The  predominant feature seemed to be that holes were begun i n locations at which decay had occurred i n sapwood, thus providing softer wood with which to work*  It  is clearly significant that Sphyrapicus i s a "generalized" woodpecker, while Colaptes shows various adaptations for ground-foraging, curvature of the b i l l  including a well-«narked  (Burt, 1930), It may be presumed that such a b i l l , primarily  adapted for foraging, i s less suitable for excavating sound wood.  - 18 -  As noted below, there seems to be l i t t l e evidence for the existence of competition among members of the small-hole cycle, and l i t t l e i s known of the large-hole group, except that the durability of Douglas f i r stumps could be an important factor.  With respect to the medium, or Flicker cycle, however,  i t i s essential to examine very carefully a l l factors related to the characteristics and a v a i l a b i l i t y of holes.  Thus, i t i s necessary to review the " l i f e histories"  of Flicker nests after they are excavated.  It was not possible during the time  alloted to this study to gather many relevant data, and the need i s very clear for  a long-term study of a series of fixed plots i n order properly to appreciate  such factors as rate of production of holes, longevity of individual sites, and the ways i n which and rate at which sites become unavailable, as well as to detect any differences which may exist between habitat types.  However, personal  observations over two years, together with some data from other sources, do permit a qualitative description of the situation, as rather a sideline to the central purpose of this study. The rate of production of holes i s naturally of considerable interest. The Sapsucker (Sphyrapicus) excavates a new hole each year, and many trees may be found to contain a series of nests.  Among the other small-hole species, the  Red-breasted Nuthatch (Sitta canadensis), the Black-capped Chickadee (Parus atricapillus)and the Mountain Chickadee (Parus gambeli) are a l l known to be capable of excavating holes i n soft, well decayed wood, although they do not always do so.  Flickers (Colaptes) usually excavate a new hole each year, and  were observed i n five separate cases to excavate a second hole after expulsion from the original hole by Starlings (Sturnus vulgaris).  The Pileated Woodpecker  (Dryocopus pileatus) i s said to excavate a new hole each year, as well as individual roost holes during winter (Hoyt, 1957). An important consideration i s the durability of trees. Many aspen trees *  Possibly of several species.  - 19 -  are i n an advanced state of decadence when they become suitable for excavation by Colaptes, and are highly susceptible to breakage, windthrow, etc.  This i s  in marked contrast to Douglas F i r "snags", which, even when dead, tend to remain standing for long periods.  Thus a difference might be expected between climatic  climax Douglas F i r stands and serai Aspen stands, i n terms of hole longevity. In addition to breakage and windthrow, as mentioned above, holes tend to become unavailable i n a number of ways. As previously noted, f i l l i n g of cavities by Bluebird (Sialia) and Tree Swallow (iridoprocne) has the effective result of rendering holes unavailable for other species.  In decadent and dead trees,  tunneling by Carpenter Ants (Camponotus sp«) tends to f i l l holes with  .  ,  sawdust-like material which results from the tunneling a c t i v i t y of the ants. In a single instance, an old hole i n a dead aspen tree was observed over a six hour period to be f i l l e d at a rate of nearly 1 cm. per 2 hours.  This was  almost certainly an exceptional case, but i t does serve to emphasize the role of this phenomenon as a "hole mortality" factor.  Various persons have also  informed^me that f i l l i n g of holes i s carried out by Wood Rats (Neotoma), Deer Mice (Peromyscus), Flying Squirrels (Glaucomys) and Red Squirrels (Tamiasciurus), (c.f.  above).  Nests of the latter two species encountered i n this study were  composed largely of lichens and mosses, and nearly f i l l e d the holes which they occupied.  This again, once the f i l l i n g of the holes i s done, would act  as a  factor conditioning the environment, although i t i s possible that actual s t r i f e could occur early i n the season, when nest sites are being sought. Even this "conditioning", however, since i t i s done by hole-using animals, may be regarded as an aspect of competition. In summary, the " l i f e history" of the holes themselves i s a factor which should be considered i n reviewing the ecological context of the present study.  - 20 -  Aspects to be considered include rate of production, longevity, and rate at which holes become unavailable. Factors affecting these are also important, and they have been reviewed qualitatively at least, pending a detailed study for the purpose of establishing more specific values. Recapitulation:  Ecological Context  Hole-nesting birds i n the study area include 17 species of seven different families, d i v i s i b l e into three natural groups.  The group based upon holes of  medium size includes 6 common species, none of whose ecological niches are particularly similar except i n terras of nest s i t e s . The importance of habitat i s recognized with a brief review of the plant ecology of the study area, with particular attention to factors thought to influence the distribution of various plant communities.  Of these, the climatic  climax Douglas F i r Zone supports considerable hole-nester populations, as do the Aspen groves of the serai Aspen-Grassland "parkland" regions.  The serai Lodgepole  Pine association supports few birds of this nesting habitus. Non-avian hole-using biota are reviewed b r i e f l y , and comments are offered on their role i n the ecosystem, with respect to nest-site competition. Factors affecting the " l i f e history" of nest sites themselves are discussed, and the need is pointed out for a definitive study of this aspect of the problem. The matters of rate of production, longevity and rate of loss of holes are reviewed i n general, i n terms of factors influencing them.  - 21 -  ANALYSIS OP NEST-SITE COMPETITION The "Cycles" As noted above, the 13 common hole-nesting species i n the study area may be placed i n three natural ecological groupings i n terms of nest-sites. for convenience, be termed the Large-, Medium- and Small-Hole Cycles.  These  may,  It w i l l be  clear from the following passages that of these three groups, only the Medium-Hole Cycle may presently be construed as showing evidence that competition could be occurring for nest-sites.  Each group i s discussed separately below.  The Large-Hole Cycle This" i s the least well-known of the three groups, despite extensive efforts in terms of f i e l d work by several workers representing the University of B r i t i s h Columbia, as^well as f i e l d representatives of both Provincial and Federal wildlife agencies.  The producer species, the Pileated Woodpecker (Dryocopus pileatus). i s  comparatively rarely seen, and even less often found nesting.  Munro and Cowan  (1947) l i s t only two nesting records for B r i t i s h Columbia, and the B.C. Nest Records Scheme contains only 5 further entries for the Province.  During the two  seasons of f i e l d work i n this study, only one nest was encountered, and even i n that case, excavation was not completed.  Another species, the Great Horned Owl  (Bubo virginianus) i s quite commonly encounteredL in. the study area, but few nests have been found.  The f i l e s of the B.C. Nest Records Scheme to date contain 8  records of the latter species, only one of which was i n a hole; this was a natural cavity from a broken branch i n a Cottonwood (Populus trichocarpa) tree near Fraser Lake, B.C.  Of the remaining 7 records, 4 were platform-type stick nests, and three  were records of fledged young. In view of the degree of effort expended i n studies involving the Barrow's  * Goldeneye, remarkably few data are available regarding i t s nest-sites. Available — M.F. Jackson, the University of B r i t i s h Columbia, has data on a number of natural sites of Barrow's Goldeneye. It i s v i r t u a l l y impossible to estimate the time spent searching for these nests, as many records resulted from casual encounters, but the efforts of many people over many years are involved (oral comm.).  - 22 -  data do suggest very strongly, however, that the "typical" nest-site i s an old Pileated Woodpecker hole.  The Goldeneye, however, i s one of the more abundant  ducks of the study area, and i t s populations are exceedingly dense i n comparison with populations of the Pileated Woodpecker.  This fact, on f i r s t  examination,  might well be thought to militate against the above-mentioned "typical" s i t e . Two factors i n the nesting ecology of the Pileated Woodpecker, however, tend to explain this apparent anomaly.  F i r s t l y , each pair of Pileated Woodpeckers  excavates a new cavity each year, as well as individual roost-holes each winter ( c f , Hoyt, 1957); secondly, i n the study area, few trees except Douglas F i r (Pseudotsuga menziesii). are large enough to contain a Pileated Woodpecker nest, and this tree i s extremely long-lived. Thus, the supply of available holes i s far greater than might be directly indicated by the Dryocopus population at any given time.  Additionally, Dryocopus pileatus i s a large bird, and might be expected to  live several years. specimen.  J.S.Y. Hoyt (1950) gives an age of 9 years for a captive  No age records are given in Bent (1939).  Jackson (pers. comm.) also  feels that since 1952, populations of both Pileated Woodpeckers and Barrow's Goldeneye have declined i n the study area.  As pointed out earlier, logging,  which was just gaining momentum by 1952, has been concentrated on Douglas F i r , and a strong p o s s i b i l i t y exists that removal of large trees of this species may be a primary reason for these population reductions. Jackson also feels that, although her data cover a wide range of site characteristics, there is a considerable p o s s i b i l i t y that many sites may be missed, particularly those high i n stubs of Douglas F i r , and those at great distances from water. During recent years, the B r i t i s h Columbia Fish & Game Branch has been conducting a programme providing nest boxes for Barrow's Goldeneye over much of the study area.  - 23 -  Up to 20$ of these boxes have been used i n the f i r s t year after placement (verbal communication, L.G. Sugden).  Such heavy use may be interpreted as  indirect evidence that natural nest-sites must be i n short supply:  however, i t  could equally well be argued that boxes are used merely by virtue of being close to the lakes on which Goldeneye broods are raised, and are thus more "desirable" than distant natural sites.  This could be advantageous to Goldeneye populations  in that brood mortality i n transit from remote nests to brood lakes, might well be reduced to a significant extent. It i s clearly indicated i n the foregoing that the Large-Hole Cycle, although clearly defined i n general terms, i s too poorly known i n detail to be considered here.  Further studies are needed to elucidate possible competitive relationships.  Present knowledge by no means precludes the occurrence of nest—site competition within this group, but data now i n hand do not permit any great degree of generalization on this matter. The Small-Hole Cycle This group i s composed of 5 species, of which the Tree Swallow (Iridoprocne bicolor), i s the only form incapable of excavating i t s own cavity. Thus i t i s d i f f i c u l t to envision nest-site competition i n this group.  This situation i s i n  direct contrast with the equivalent group i n Europe, which contains many species of Paridae, Turdidae, Museicapidae, etc., few i f any of which have this a b i l i t y . The hole-nesting avifauna of Europe i s much richer i n a l l three groups, although greater attention has been paid to the smaller passerines.  No doubt i t i s from  this situation that the current widely-held views have arisen on nest-site competition among hole-nesting birds (c.f. Udvardy, 1951).  This situation i n the  study area i s , then, quite different from that found i n Europe, and widely studied there.  - 24 -  The Medium-Hole, or "Flicker" Cycle For purposes of brevity, the medium-hole group w i l l be referred to hereafter as the "Flicker" cycle, to emphasize that this species is the hole-producer for the group. Figures 3 and 4 show rather clearly the separation of the three cycles i n terms of hole sizes and observed species-site a f f i n i t i e s .  The group now under  discussion includes 6 regular component species i n the study area, among which only the Flicker (Colaptes) produces holes. defined above, secondary hole-nesters.  The remaining 5 species are, as  Figure 3 includes a diagrammatic  representation of the species-site relationships within the Flicker cycle, and of this cycle to the other two groups.  Figure 5 i s an attempt to describe the  ecological niches of the component species of the Flicker cycle, and to show how these niches combine with other environmental factors to produce an overall "ecological demand" upon the supply of tree-holes excavated by the Woodpecker, Colaptes.  Mensurational data were collected concerning ecological and physical  characteristics of these holes and were analyzed i n terms of the question, "which factors tend to mitigate, and which to intensify, nest-site competition i n this group of birds, a l l of which seem to share a single environmental  resource?"  Ideally, each species would be considered i n terms of a l l parameters so as to synthesize a "theoretical ideal nest-site" for that species, and these s t a t i s t i c a l entities compared.  However, as w i l l be seen i n the following treatment, actual  f i e l d conditions rendered such idealized treatment unrealistic. Additional complications are introduced by the combination of differing sample sizes and widely divergent degrees of v a r i a b i l i t y species.  i n nest-site characteristics from;species to  The alternative method of analysis, consideration of a l l species together,  parameter by parameter, was undertaken, and the analysis i s given below.  Sample  sizes vary from one parameter to another, as i t was not possible i n many cases to secure values for a l l Parameters.  To f o l l o w p .  24  5  O  5 <  •o  IT)  lO .. r- J~  • •  j) cr  J  O J J  UJ LU  Z  >  c  o  at  f  i  5 r  oo  o  o  •  o  oo  •  o  oo  oo ooo o o oo o  o oo  oo  o . oo  o ooo o  oo  o  o ooo o  oo oo o  o  oo  o oo  o  o ooo o  o o  ooo  oo ooo o  o  • '•'  oo  o o  o  o  o  p • 0 4  o  o ooo o o o* o  o o o  oo  oo  in  oo  Q  ff  vO  in in .Q  oo  «1  < •<  OS  m cb  o  O  CD  FIG HOLE-SIZE  1 """ o vO  in  'WD  O ' CD  O  oo  o<  O  *  CD  oo  o<  o b  IT)  IT)  OO  ID  •  NI  41  RELATIONSHIPS  1 in in  V 1 •=  Q in  ti313WVId  m  • <3  «  in in  «<o 0«< •o o  O  <<3 1 q  in  IVDIldBA  m ro ro  To f o l l o w pH. 24  BUFFLEHEAD  (ANSERIFORMES)  only numerous duck this size -precocious -aquatic -adult tends brood but young feed independently Passeriformes MOUNTAIN BLUEBIRD (Turdidae) only medium-large passerine -altricial -open-feeder; hovers, feeds on smaller insects  KESTREL, , (FALCONIFORMES) only Falcon -altricial -hunts by hovering; takes small vertebrates, larger insects  STARLING (Sturnidae) Alien -altricial -open-feeder; largely probes i n ground -large open areas  FLICKER (PICIFORMES) only Woodpecker this size -altricial -ground-feeder, largely on ants -mixed semi-open cover -producer & facultatively 2«ary  Resource: Flicker Holes  Si  HOLE "MORTALITI"  TREE SWALLOW (Hirundinidae) only small passerine -altricial -aerial feeding by chasing insects  -tree f a l l i n g or cut -tree breakage -flooding of low holes - f i l l i n g : - b y Camponotus -by birds -by mammals -etc.  NON-AVIAN BIOTA -Red Squirrel -Flying Squirrel -Bats -Packrat -Wasps, Bees Other Arthropods  Figure 5.  The Flicker Cycle:  Demands Upon Hole Supply.  - 25 -  Hole., Direction One of the f i r s t considerations to arise i s usually the suggestion that certain species might favor a particular directional orientation of the nest entrance, or that primary hole-nesters would show a preponderance of holes facing so as to u t i l i z e warming by the sun.  In the study area, this would encompass the  bearings between 090° (East) and 180° (West).  In Table III are presented the  actual data recorded. Pynnonen (1939) found that the majority of woodpecker nests which he studied in Finland showed a northward orientation, but attributed this to the fact that prevailing winds i n the area were from that direction, and that the bark of trees was thereby roughened, permitting the birds to cling more easily while the nest was being constructed. In the present study, nests of Flicker cycle species i n solitary trees showed no particular orientation (see Table III) and trees i n groves were characterized by two attributes of holes: l ) space; 2)  a l l holes faced toward an open  holes tended to be at or near grove-edges, except i n very sparse  groupings of trees. This i s readily explained by the characteristics of the f l i g h t of the hole producer, the Flicker (Colaptes). This species f l i e s heavily in bounding swoops, and carries the wings nearly closed during a great proportion of the time spent i n f l i g h t .  Such a mode of f l i g h t imposes considerable  restrictions on lateral manoe.uverability, with the result that a "clear run" i s required by a Flicker f l y i n g toward or from a nest entrance. Burns (1900) gives values of one to two metres f o r depth of swoops and three to five metres for length i n the Yellow-shafted Flicker (Coloptes auratus): these values may be taken as quite acceptable for the Flickers of the study area. the entrance to the nest must leave a f l i g h t path of at least these  Thus  dimensions.  A l l secondary species of this cycle i n the study area are more manoeuverable than  TABLE III. HOLE DIRECTIONS OF NESTS MEASURED WITHIN STUDY AREA.  Directions N  Species  NE  No. Flicker  5  No. 5.3  11  E %  No.  11.4  10  SE  # 10.4  S  No.  #  No.  8  8.3  27  *•  28.1  SW No.  *  17  17.6  W No. 12  NW  %  No.  %  12.4  6  6.2  Species :Total 96  Starling  13  7.7  21  12.6  11  6.5  29  17.2  30  17.8  30  17.8  17  10.1  17  10.1  168  Bufflehead  14  13.5  11  10.6  14  13.5  11  10.6  17  16.5  15  14.5  11  10.6  10  9.5  103  Bluebird  5  11.1  4  9.1  2  4.5  3  6.8  5 11.1  13  29.5  5  11.1  7  15.9  44  Tree Swallow  2  5.9  6  17.7  2  5.9  5  14.7  5 14.7  7  20.5  1  3.0  6  17.7  34  39  3.8  53  14.5  39  8.8  56  12.6  82  18.4  46  11.8  46  11.8  445  F l . Source (Excl. Kestrel)  No, = nests measured % = io of nests f o r that species. F l . source = holes made by Flicker.  84  18.4  l  to o l  - 27 -  the Flicker and thus have less exacting flightway requirements.  However, i n  terms of compass direction, the choice available i s that found i n old Flicker sites.  Table III shows hole directions measured for each species.  Figure 6, a graphic representation of the data i n Table III, shows that a tendency exists for hole directions to face the arc between 90° and 180°.  However,  as can be seen from the data i n both Table III and Figure 6, there i s l i t t l e deviation within the medium-hole group from the overall pattern of holes produced by the Flicker. When hole directions of a l l species were tested by chi-square for goodness-of-fit against the overall pattern of Flicker holes, only the Bluebird showed a significant deviation, (X^ = 23.81 at probl. level of 5%, 7 degree of freedom), and then only i n one direction (SW).  Since there i s no depression i n that direction i n any  other species, nor, i n any other direction, any marked depression i n Bluebirds accompanied by a "bulge" i n another species, this deviation i n Bluebirds seems more l i k e l y to be an a r t i f a c t of sample size than an indication of competitive' exclusion. In any event, i t i s clear that differences i n hole directions are hardly of such magnitude as to be of sweeping biological significance.  The importance of these  data rests i n the fact that hole directions are essentially similar i n a l l species. Hole Height The distribution of hole heights i s subject to a greater degree than other physical characteristics of the cavity to alteration i n response to the species and growth forms of the trees i n which cavities are excavated.  In Table IV and  Figure 7, the overall height distribution i s given i n 25 cm. height classes for  FLICKER CYCLE*. HOLE HEIGHT DIST'N. BY SPECIES •A " B " C  " D " E  "F  "G  " H ll j II j H  K  " L  ,  ,  M  ,  N  ,  |  ,  0  ,  L  P  "Q"  "u  M  V  1  FLICKER-^ o o o o o o 0( o o o o o o fc> o o o o o o |o 0| o o o o_ o o o o o ooo o 0 o o o 2_2  IT  cr UJ ' a  —55  STARLING- [ BLUEBIRD- Q  j-45  BUFFLEHEAD-, TREE  SW.-  R  KESTREL-  r*35  o o o o o o o o o o o o  < _l. u H  o o o o b oi o 0 o o A o o o o o o  </) • p-  U) UJ Z '  p-q o o o o o o o°o o  IO 0|  or LU  a  ID  5 Z  n  o o o K> o) o o o o K> ° o |o o| o lo 0| ,ooo o o at o 0 o ,o (9, .91  n  23  D  E  I  J  HEIGHT  K  L  M  CLASSES  N  0  P TQ»  R  7-30 I-2S  E-20 — 15 •10 t-5  o o ol  f  f ^ ' C  •60  u  O  To  OF H O L E D I R E C T I O N S  (two  ToutouJ  mnvt%)  P. 2 7  - 28 -  479 nests constructed by Flickers (Colaptes) and measured while being used by species of the Flicker cycle.  In both Table IV and Figure 7, the only Tree Swallow  (Iridoprocne bicolor) nests considered were those i n holes originally made by Flickers.  In the height range between 0 to 4 metres, most nests were in Aspen  (Populus tremuloides) trunks and stubs, and i n the range over approximately 4.5 metres were a greater proportion of Douglas f i r (Pseudotsuga menziesi) trees. I t can be seen from both Table IV and Figure 7 that two "humps" occur i n the height distribution of holes excavated by Flickers, the second being much more diffuse, and covering a wide range; the f i r s t i s around the level of 1.75 metres, with a drop to f a i r l y low values i n the 4 to 5 metre range, followed by about 10$ of a l l nests at scattered heights over 5 metres.  For two reasons, i t i s almost certain  that these higher height classes are under-represented: l ) vastly greater d i f f i c u l t y i n locating under f i e l d conditions, 2) many high holes,- even though located, could not be reached safely.  In the latter cases, however, i t was usually possible  to establish occupancy and determine height by use of an Abney l e v e l .  Other  measurements were not always obtained for such s i t e s . The form of the height distribution curve can thus be seen to be related to growth-form and thus species of trees used, rather than to an innate height preference of Colaptes.  Although no quantitative data are available, i t can be  stated on a basis of common sense and experience that the ratio of high (over 5 m) to low (under 4ra)nests appears to parallel the ratio of t a l l to low trees i n the study area.  Again, detailed census and survey data are badly needed, despite the  very great efforts necessary to secure them.  In short, the form of the height d i s t r i -  bution of holes made by Colaptes i s a function of the a v a i l a b i l i t y of tree-trunk surface with appropriate decay conditions and facing a suitable f l i g h t corridor. It was realized nearly at the conclusion of the study that this would have been shown clearly by the recording of the height of the bottom of the tree canopy over  TABLE IV. HOLE HEIGHTS: SUMMARY FOR FLICKER CYCLE. Tree Swallow #  Ht. Class in Cm. A B C D E F G H I J K L M N 0 P Q R S T U V  0- 25 26- 50 51- 75 76-100 101-125 126-150 151-175 176-200 201-225 226-250 251-275 276-300 301-325 326-350 351-375 376-400 401-425 426-450 451-475 476-500 501-525 >525 TOTALS  Flicker No.  -1  Starling No.  1.0 3.0 18.0 13.0 10.0 6.0 15.0 6.0 3.0 3.0 3.0 7.0 4.0 2.0 —  -  1° -  2.0 1.0 3.0 2.0  2 1.2 4.1 7 21 12.3 24 14.1 23 13.5 17 10.0 17 10.0 12 7.0 4.1 7 1.8 3 8 4.7 2 1.2 5 2.9 6 3.5 — 4 2.4 — 1 0.5 1.2 2 5.2 9  102 nests  170 nests  3 18 13 10 6 15 6. 3 3 3 7 4 2 — —  2 1 3 2  -  -  Bluebird No.  -  -  1 2.3 2 4.5 10 22.7 6 13.6 9 20.4 6 13.6 6.8 3 2 4.5 1 2.3 1 2.3  -  -  Bufflehead No.  *  1  0.9  -.  1 2 4 10 15 9 9 8 8  -1  -  0.9 1.8 3.7 9.3 14.0 8.4 8.4 7.5 7.5  -  — — 1  2.3  — 2  — 4.5  0.9 1.8 2 8 7.5 1 0.9 — — 1 0.9 2.8 3 2 1.8 1 0.9 21 18.6  44 nests  107 nests  —  --  -  --  —  -  -  No. 1  -  Kestrel No.  3.1  -  -  1 3.1 9.4 3 5 15.6 8 25.0 9.4 3 9.4 3 1 3.1 9.4 3 2 6.2— —  --  -  1  -'  -1  '-  -  3.1  -  —  -  — — 1.31  -  -  32 nests  1° -  -  2  8.3  1 4 — 3 1  4.2 16.6 — 12.5 4.2  1 — — 2 —  4.2  -  -  -1  — 1 8  -  -  -  8.3  --  4.2 — 4.2 33.5  24 nests  Flicker Source No.  1°  0.4 2 1 0.2 8 3.7 6.6 32 55 11.5 58 12.1 57 11.9 54 11.3 36 7.5 31 6.5 4.6 22 1.5 7 17 3.5 1.2 9 3.1 15 9 1.9  -5 7 5 7 42  -  1.1 1.5 1.1 1.5 8.8  479 nests  Only Flicker-source Tree Swallow nests were used here. fo values are for total nests of each species and are calculated to three significant figures.  - 30 -  each nest*  This measurement, however, was not taken for most nests, although  i t s inclusion i s recommended for future workers.  Tree heights were recorded  for a l l nests, but these seem to bear no relationship to hole height distribution. Along with a l l other data, tree height measurements are deposited i n the archives of the Department of Zoology i n the University of B r i t i s h Columbia, but for reasons of space economy, they are not presented here. Several factors can be seen, however, which could be of significance i n possible nest site competition: l )  v i r t u a l l y no Bluebird (Sialia) or Tree  Swallow (Iridoprocne) nests were found above the 3 to 4 metre level; 2)  only  3 of 24, or approximately 13$, of Sparrowhawk (Falco sparverius) nests occurred below 1.75 metres.  Also, no Sparrowhawk nest below about 5 metres was known to  be successful i n fledging young; 3)  other than these points, approximately  80$  of a l l species (Fl.-81$, St.-78$, Bluebird-93$, Buff.-63$, T.S.-90$, Kes-45.8$) were i n the 50-300 cm. height range:  77$ of a l l nests i n this cycle f e l l within  the same range. Thus, except that S i a l i a and, to a much lesser extent, Iridoprocne. may tend to be restricted to lower nests, there appears not to be any significant separation of species i n terms of hole height.  The departure of hole height distribution i n  two aspects from normal distribution i s referable to the influence of tree growthform. Entrance Size Clearly, entrance size i s quite important to secondary hole-nesting species. A minimum size i s imposed by the body size of the species involved, and maximum l i m i t s , although largely a function of available sites so far as i s now known, may also be set by behavioural responses. The species of the three cycles are separable almost completely i n this regard,  - 31 -  except for two regular exceptions.  The Sparrowhawk (Falco sparverius) i s known  occasionally to use holes made by the Pileated Woodpecker, although no cases of this were found during the present study.  The use by Falco of boxes designed for  Wood Duck (Aix sponsa) and Barrow's Goldeneye (Bucephala islandica) has been reported i n several instances i n B r i t i s h Columbia (pers. comm: J . Mack, Enderby, B.C.;  Game Biologists L.G. Sugden and P.W. Martin, B.C. Fish and Game Branch).  Data obtained during this study included 4 cases of the use of Game Branch duck boxes, 10 nests i n which entrance sizes could not be measured, and 10 for which complete measurements were obtained. The Tree Swallow (Iridoprocne bicolor), nested not only i n a l l three cycles, but i n many other situations as well. Figure 4 shows the relationship of Tree Swallow nests to some sites occupied by Flicker and Sapsucker i n terms of entrance sizes. Purely as a space consideration, not a l l entrance-size data are plotted here. In Figures 8 and 9, absolute frequencies of 1959 data for Flicker and Starling are given i n histogram form, and i n Figures 10 to 14, horizontal diameters are plotted against v e r t i c a l diameters for Flicker (Figure 10), Starling (Figure 11), Mountain Bluebird (Figure 12), and Bufflehead (Figure 13).  Figure 4,should be  referred to again for partial Tree Swallow data. The major point to be made here i s , again, that very l i t t l e separation i s evident from species to species, except that few Bufflehead nests were smaller than,6.0 X 6.0 cm., and none were smaller than 6.0 X 5.0 cm, (only one at this value).  No Bufflehead nest showed a horizontal component of less than 6,0 cm,,  while many nests of the other species f e l l i n the range from 5.0 to 6.0 cm. This i s not unexpected, as the Bufflehead i s somewhat larger, particularly i n girth, than the other species of this group (see Table II for length measurements).  g  HORIZONTAL  Q  VERTICAL  DIAM.(cm.)  DIAM.  (cm.)  15—  t-9 O  o o  5 4  3 2.  I —[ 1—i 1—i 1—i 1—I 1—I 1—i ' i — r 4.7 -5.( 5 . 2 - 5 . 6 5 . 1 - 6 l 6 2 - 6 * . 6 6.7-7.1 7 £ - 7.6 7 . 7 - 8 . 1 a2-8.6 FIG.  8*.  FLICKER  :  H O L E  SIZES  ,  1959  ^ H O R I Z O N T A L  DIAM.  (cm.)  2 0 - ,  21  25  ( ^ V E R T I C A L  DIAM.(c/v\.)  15-  > u  lo-  z  LLl  a  LLl CC U_  O  5-  4 3 OVll vo.l  Z I 4 . 0 - 4 . . 4 . 1 - J . I  5.2-5.6  SI-  6.1  6.1-6.6  4.1-11  7.1-76  7.7-8.  8-l~  o o  S.m  I— 1  FIG  9:  STARLING!  H O L E  SIZES  ,  1959  l  I  I  ovtt  HOLIS  55  m  t. ;H  •  :  :  1  1  1  •»• ••  r  .4  3 -  —I 4.0  1  4.S 5.0 5.5  1  1  6.0 6.5 7.0  HORIZONTAL  FIG.  IO:  ENTRANCE  1  1  1  7.5 8.0  8.5  1  1  9.0 9.5  !b JAM e r e * (CM)  SIZES:  FLICKER  1 '0.0  vcwr.  SIAM.  3 5  t  n  a.o •  3 Hekts O V M  9.5 V l t t ©I A M .  85  80' s  •a• •  7.5 7.0 •  v  < c u  •  6.5 6o5.55-0-  • •••  • •mm* • •*• ••• •  • •  •  • •• •  o  AS •  o  4.o •  o  3.5-  —i—i—i—i—i—i—i—i—i—i—i—i—r 4.o  4.5 S.0 5.5  6.0 6.S 7.0  HORIZONTAL FIG.  II:  ENTRANCE  75  8.0  85  ^lAMtrvft SIZES !  9  0  3.5  (tju)  STARLING  loo  f  M««TS  5  ovit  IO.O CM- vtnT. DIAM(INCL. 2 o v u  l o . a e/M.  H«Rt3. S I » M . )  "T 1 1 1 1 1 1 1 1 1 1 1 1 A.O 4.5 5.0 5.5 6.0 65 7.0 7.5 ftO £5 9.0 9.5 10.0 HORIZONTAL FIG. 12:  ENTRANCE  DIAMCTII (cm-)  SIZES:  BLUEBIRD  ico  •  -  1  i  3.5 3.0 -  • • • -•• • •• • •• • ••* • ••m • mm  6.58.0-  7-5 m  M m  1 «  7.06*.S-  5-5-  u  5-0  IO-O C M VCKT. ©i A M . (INCL. 4> O V H IO.O e * .  Hoi is.  mmm m  -  <  12 MISTS O V U  ••• • • ••  m mm mmm  i-3 O  -  4-5 -  O  4*0 -  o i  1  1  1  4 . 0 4.5 5.0 5.5  1  1 T  13*.  1  1—I  1  1 o  6\0 6.5  HORIZONTAL  FIG  1  ENTRANCE  DIAMCTCR  SIZES:  (CM.)  BUFFLEHEAD  V>4  - 32 -  Vertical diameter exhibited somewhat greater v a r i a b i l i t y than did horizontal diameter, particularly i n terms of the upper l i m i t .  This tendency may well be  imposed by the nature of the sites selected, as a considerable proportion (approximately 10$) of Flicker holes could s t i l l be seen, even when complete, to have been started i n a v e r t i c a l crack. S i l l Size Data on s i l l size are summarized i n Table V and Figures 14 to 16.  This  parameter showed one of the most sharply defined ranges i n values, with 94$ of a l l measurements i n a sample of 436 f a l l i n g within the range between 1.2 and cm.  7.1  Figure 14 represents per cent frequency distribution of data on occupied  Flicker nests, and on a l l sites established as having been constructed by Flickers. Data for Bufflehead (Bucephala albeola) and Tree Swallow (Iridoprocne bicolor) i n Flicker-source sites are shown i n Figure 16 and clearly follow the same pattern of s i l l size distribution.  Figure 15 , however, i s somewhat less clear: the data  shown i n this figure are from nests occupied by Starling (Sturnus vulgaris) and Mountain Bluebird (Sialia currucoides).  The sample for Sturnus, representing 168  nests, should be representative for the species, and seems on f i r s t inspection to represent a tendency i n Sturnus vulgaris to select somewhat thicker s i l l s , while the S i a l i a data seem to show a skew toward thinner s i l l s .  The size range from 1  to 4 cm, includes 49.1$ of a l l Starling nests, and 81.8$ of Bluebird nests.  The  interval 4 to 6 cm. includes a further 37.6$ of Starling and 9.2$ of Bluebird nests. The difference between the s i l l size-class distributions for Sturnus and S i a l i a . then, seems striking, but at present i t cannot be explained.  A possible  explanation would be competitive exclusion by Sturnus of S i a l i a from holes with thicker s i l l s : 49.1$ nests.  this i s militated against, but not precluded, by the fact that  of Sturnus nests also occur i n the size range occupied by 81.8$ of Sialia It would be most instructive to examine data on a series of S i a l i a nests  To f o l l o w p. 32 SILL  I £  A  SIZES  I V I I F L . HOLES 447 NESTS D  V 1 V 1 V 1 • 35% • 30°/  0  •25%  .20 Io .15  7*  -/£> % -5 X a ' b ' c ' d  FLIC ICES*OCCUPIED  T  1  Cr-  H  O  )  •35%  35 MKTS  -3o % -25% -20% -/5 %  Ei  1  1  1  1  l.l  2.1  31  41  5.1  SIZE  CLASSES  F i g . 14  OF  1 l.\  r—1 7.1  SILLS,  8.1  I  \~  31  10. I  C M .  5£  -o  To f o l l o w p. 32  1  T — i — r  T—i—i—r  r  F F L E H E AJ) 3 7 NUTS  1-20  =-5%  O  —, M  , 2.1  , VI  1 4.1  1 S.I  1 6.1  1.1  8.1  H—h  9.1  lot  jf-35%  TREE  5>NALLow(ri..-yoo«ct)  r~ ° 3  33  N«9TJ  =-20  =-lO  ~J l.l  FIG  15:  -o  1 5.t  1 6.1  1— 7.1 8 1  9.1  SIZE C L A S S E S  OF  SILLS,  CM.  1 I.I  1 3.1  1 4.1  lo.l  To f o l l o w p. 32  SILL 0  I.I  Sizes  2 \  4.1 5,1  3.1  l.l  7.1 6.1  9.1  lo.l  1 i i i i i i i i i i / 67  35 %  NfSTS  -3o  '-25  -=-5  MT. 4-4  1  i  1—i—r  1  r  E-35  E>LotSlRJ> WESTS  *  z-30  5-20  — IO - 5 0  A  ' II  SIZE  B  '  21  C  '  D  3-\  CLASSES  '  E  '  F  4.1 5.1 OF  F i g . 16  '  S  '  H  6.1 7J  SILLS  1. '  0.1  *. C M .  I  1 3-J ft/ i o . / '  - O  «  TABLE V.  Code  Size Class Limits  SILL SIZES: FLICKER CYCLE SUMMARY (BOTH YEARS).  Flicker  Starling  Bufflehead  Bluebird  Tree Swallow  Flicker  No.  $  No.  $>  No.  %  No.  fo  No.  %  No.  $  2.4  3  3.1  3  6.8  5  15.2  16  3.7  A  0-1.1 cm.  1  1.1  4  B  1.2-2.1 cm.  20  22.0  22  13.1  33  34.0  9  20.5  6  18.2  90  20.5  C  2.2-3.1 cm.  25  27.0  26  15.5  16  16.5  14  31.8  5  15.2  86  19.7  D  3.2-4.1 cm.  19  20.0  34  20.5  25  25.8  13  29.5  6  18.2  97  22.2  E  4.2-5.1 cm.  18  19.0  41  24.5  11  11.3  17.6  F  5.2-6.1 cm.  3  3.3  22  13.1  4  Gr  6.2-7.1 cm.  7  7.4  10  5.9  H  7.2-8.1 cm.  2  2.2  5  I  8.2-9.1 cm.  J  9.2-10.1 cm.  -  K  10.1 TOTALS  •95  -  2  4.6  5  15.2  77  4.1  2  4.6  2  6.2  33  7.6  4  4.1  1  2.3  2  6.2  24  5.5  2.9  1  1.0  1  3.1  9  2.1  1  0.6  2  1.2  -  -  -  -  -  168  97  44  -  -  1  -  3.1  33 ( F l . Source Only)  1  .02  2  .04  1  .02  436  - 34 «  from an area not occupied by Sturnus.  Also, the curve for S i a l i a might change  somewhat with enlargement of the sample: the nature and extent of such a change, i f i n fact i t would occur at a l l , cannot of course be predicted.  In any event,  there can hardly be said to be a separation of species i n terms of this parameter, although partial competitive exclusion may be hinted at i n this one species pair. F i n a l l y , i t i s clear from Tabled and Figures 14 to 16 that, i n general, there i s very l i t t l e separation of species i n terms of s i l l sizes (except that Starlings (Sturnus vulgaris) seem to show a slight tendency to use wider s i l l s at a rate possibly greater than that at which they are produced). Hole Depth Hole depth measurements were made i n a total of 446 Flicker-cycle nests occupied by F l i c k e r , Starling, Mountain Bluebird, Bufflehead and Tree Swallow. Depths of 11 Kestrel (Falco sparverius) sites were also measured, but this small sample i s not included i n the treatment here. A l l depth measurements were made without disturbing nests or nest materials, in order to establish the actual space used by the bird. Data obtained on hole depths are summarized i n Table VI,  perusal of which  quickly reveals several salient factors: 1)  Holes occupied by Colaptes show that 94.7% of holes were between 20 cm. and 45 cm. i n depth, and 83% were between 25 cm. and 40 cm.  2)  92.9% of Tree Swallow nests were f i l l e d with nest material to the range between 0 and 30 cm.; and 69.4% of the total were between 0 and 20 cm.  3)  No Starling nests were found to be less than 10 cm. i n depth, and only 28 or 17.0% were less than 20 cm. The range 15 to 35 cm. entraced 75.6% of the 165 Starling nests measured.  4)  No Bufflehead nests were measured with a depth of less than 15 cm., and only 4, or 3.8%, were less than 20 cm. i n depth.  - 35 -  84,1 $ of Bluebird nests were less than 20 cm. The placing by secondary hole-nesters of nest material in the cavity i s clearly a significant ecological factor i n this "competitive" situation.  Perusal  of Figures 17 and 18, which present the same data as Table VI, shows, as confirmed by direct observation, that the Starling, Sturnus, adds only enough nest material to cover the bottom of the cavity.  In most cases, this would not alter a cavity  enough to render i t unsuitable (in terms of depth) for Bufflehead, or for adoption and further f i l l i n g by S i a l i a or Iridoprocne.  However, use of a site by Bluebird  (Sialia) or Tree Swallow (Iridoprocne) renders a cavity v i r t u a l l y unsuitable for Starling or Bufflehead.  84.1$ of Bluebird and 69.4$ of Tree Swallow nests were  less than 20 cm. i n depth, and only 17.0$ of Starling and 3.8$ of Bufflehead nests were i n this range.  This definitely constitutes a significant factor in  terms of competition, and would tend to reduce competitive pressure on S i a l i a and Iridoprocne.  It could, at the same time, intensify competition between  Bucephala and Sturnus; i t i s important that Bucephala begins laying early i n the spring almost before the a r r i v a l of Starlings i n the area, incubates very closely, and has ceased to use the nest-site before the nesting of Sturnus i s completed. Again, however, i t i s quite clear that the requirements of a l l species i n the Flicker cycle are very similar.  Few cases of actual inter- or intra-specific  s t r i f e were actually observed during this study, as i t s mensurational nature required examination of many nests and did not permit detailed observation of single sites.  The need for such studies i s described below, i n suggested further  lines of research. Cavity Sizes The f i n a l parameter related to nest-holes i s that defined above as "cavity". It was thought that this could be an important feature, as i t i s an expression of one aspect of the space within the nest cavity.  Measurements were obtained for a  total of 446 nests within the Flicker cycle, and these are summarized i n Table VII,  TABLE VI.  FREQUENCY DISTRIBUTION OF HOLE DEPTHS: FLICKER CYCLE.  SPECIES  DEPTH-CLASSES 0-5 No.  Flicker N a 95 Starling N = 65  No.  No.  No.  -  -  -  -  - -  -  3  1.8  25  7  15.9  9  -  -  -  4  8 21.0  7  18.4  6  17  3.8  44  Bufflehead N = 104  -  -  Tree Swallow N = 38  5  14.2  15  3.4  % values refer to species.  11 25.0  19  4.5  20-25 No.  -  22.7  Flicker Cycle  $  15--20  - -  10  Bluebird N = 44  10--15  5-10  25-30 No.  30--35 No.  %  35-40 No.  1°  40-45 No.  45 No.  *  5  5.3  34  35.6  28  29.5  17  17.9  6  6.3  5  5.3  15.2  44  26.8  42  25.4  30  18.2  13  7.9  1  0.6  7  3,3  20.5  3  6.8  1  2.3  1  2.3  1  2.3  0  -  1  2.3  9  8.6  22  21.2  33  31,8  17  16.4  6  5.8  13  12.5  15.8  4  10.3  5  13,2  2  5.6  0  -  0  -  1  2.6  9.6  65  14.6  104  23.4  94  21.0  48  10.8  13  3.0  27  6.0  3.85  To f o l l o w p. 36  HOLE O  DEPTHS  5 T  IO  15  T  l—r  IN £0  25 T  CM. ZO  35  AO  T  T  T  45+ T :  FLICKER M»  95  .  FhlCKftft N •  IO 6  I  r  i—i—i—r £  32 28 ZA ZO 16  CYCLI Z$  44«  ZO (6 M o 6  z% STACLINO N•  •30% •2 6  165  •2.0 : •10 1  n—i—i—i—i—i—i—r O  HOLE  5  lo  15  DEPTHS  20  25  FIG.  17  30  FLICKER  35  4o  CYCLE  •*»  (I)  6  ;  6  :  2 %  To f o l l o w p. 36  HOLE  DEATHS  O  5  10  T  T  T  IN  15  20  1  r  CM.  ZS  30  T"—r  35  <40  T  T  454T  T  |36% SO  Bi-ufRiftI> N= -44  '20 • to • 6  2  1—r u N  »  -I  •X  V  = 36% -  BuVfUt HEAD 14 = I 0 4  30 24 20  <  z  14 I o 6  -j  M  w  z 0  1—r  1  T  1  1  1  1  r  36%  3  30  — I  I2  M«  38  14 IO '• I  *  • 1 •  O  —!—1—1—j—1—1—p—r 5  IO  FIG. HOLE  15  20  25  30  35  40  : 4 5 4-  IB  DEPTHS  FLICKER  C Y C L E  (u)  o  6  1%  TABLE VII.  SIZE-CLASS DISTRIBUTIONS OP NEST CAVITIES: FLICKER CYCLE. SIZE CLASSES:  SPECIES AND SAMPLE SIZE  10--12  10  12-14  14-16  18-20  No.  %  No.  %  8.2  42  9.8  15  3.4  14  3.2  9  2.0  11  11.6  9  9.5  -  -  5  5.5  3  3.2  13.3  10  6.0  16  9.9  7  4.2  4  2.4  4  2.4  3  6.8  4  9.0  2  4.5  -  -  2  4.5  -  14.0  20  18.6  8  7.5  12  11.2  7  6,5  2  1.8  2  11.8  3  8.8  3  8.8  3  8.8  1  2.9  1  2.9  _  56  12.7  36  17  17.9  8  8.4  20.0  25  15.1  22  9  20,5  7  15.9  11.2  25  23.4  _15  11.8  10  29.4  4  No.  27  6.0  83  18.6  96  21.5  68  Flicker (occ.) N = 95  6  6.3  17  17.9  19  20.0  Starling N = 166  8  4.8  37  22.4  33  Bluebird N m 44  4  9.2  13  29,5  Bufflehead N = 107  4  3.7  12  Tree Swallow N = 34  5  14.7  4  No.  26  *  15.2  %  24--26  No.  No.  No.  22--24  %  %  *  20--22 No.  No.  No. Flicker Cycle N = 446  16--18  CM.  1°  1.8  —  - 38 -  and shown graphically in Figures 19, 20, and 21. clear.  Several points are abundantly  Only about 12$ of these 446 nests had cavities of less than 10 cm. inside  diameter, and only 18, or about 4.5$ were over 26 cm. inside diameter.  Reference  to Figures 19, 20, and 21 shows immediately, as does study of Table VII, that each size class i s occupied by roughly the same proportion of the sample for each species.  In short, there seems to be no evidence i n terms of this parameter to  indicate any sort of amelioration of competition. Another obvious parameter i s that of hole volume, which undoubtedly i s of importance with relation to body size.  However, i t can easily be seen that since  the cavity diameters selected by the various species are so nearly similar, variations i n volume w i l l parallel those i n hole depth, and since the latter parameter i s analyzed above, a special analysis of hole volumes i s redundant. Recapitulation:  Analysis of Competitive Situation  It has now been shown that the 13 common hole-nesting species of the study area are d i v i s i b l e into three natural groups, one of which shows no reason to assume the existence of nest site competition among i t s component species.  Of the  remaining two, the large-hole group i s too poorly known to permit the forming of definite conclusions, although some qualitative suggestions have been offered. The f i n a l group, that composing of species u t i l i z i n g nest-holes made by the Flicker (Colaptes), has been examined i n considerable detail i n terms of some measurable characteristics of nest-sites.  It has become clear that, i n general,  the requirements of the various species, i n terms of the parameters considered, show an impressive degree of overlap.  A few cases exist i n which some evidence i s  seen for amelioration of competition, but these are minor i n comparison with the extent to which broad overlap occurs.  Thus i t can be presumed on a somewhat more  quantitative basis than i s usually possible that competition as defined above may be occurring within the medium-hole group of hole-nesting birds i n the study area.  To f o l l o w p. 38  cn  o  m  O  Io  11111111111111111111111111  v w  V V*  rg  .«! M  _o  3  _<0  LU  i  14  _i o >u  z  u 3  LU  «0 or  u vt  III  in V>  or  O  .0  V  V  V  V  «M  M  C u vO  -3  Id  N  LO  (VI  V o h  >  <  0  •  m a U V  u at « X  «l J It O  V  _  ox  y.  LL  in LU M  cn  .0 M  -00  I  \0 1  w  _o  11111111[ 1111111111111'l 111 o m o in  JT  V  > <  To f o l l o w p. 38  J=l. LU _l  U >  u  Q: LU  u X  u  2  *1 III  (A In  a j U ml inH >• »-  > <  U  -  o CM  cn LU M  cn >  > <  CZ. A V I T Y <XO IZ I * 16  Sixm  18 2 0 2 2  C m » E S  24 16<  T—i—i—i—i—i—i—i—r  IN  d h*.  <\o IZ 14 16  i—i—i—i—r  Tk««  &UFFLKHKA2>  25-  IS 2Q 22 24 2 l <  25  5vw ALLOW  20-  20  15-  15  IO-  IO  u  M 3  a *  at  U-  5-  5  34  107  o  i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—r~  <lO 121 14 16 18 20 22 Z4 26<  <  FIG.  21  CAVITY  SIZES  (O 12. 14 16 18 20 22 24  FLICKER  Z6<  -  . O 1-3 O  O M CYCLE  (nfl  O  VJ4 00  - 39 -  In the following discussion, the implications and ramifications of this are explored, and a special section has been prepared to suggest future research to examine i n greater detail the mechanisms of this competition,  as well as i t s  implications i n distribution and abundance of the species involved.  It should  be reiterated at this point that the main role of the work reported here has been to "show the way" i n the sense of establishing the extent of occurrence of nest-site competition  i n the study area.  - 40 -  DISCUSSION  Orientation of.This Study and Suggested Future Research As can be seen i n the above presentation, the major role of this study i n the overall consideration of nest-site competition among hole-nesting birds i n the study area has been to show that the phenomenon may be assumed to occur, and to delimit i t s extent i n terms of the species involved. that:  (a)  It i s now clear  not a l l hole-nesting species may be regarded as being  this competition; (b)  involved in  on the basis of a quantitative study of nest sites, the  group designated as the "Flicker cycle" may be accepted as one i n which the existance of nest—site competition may reasonably be assumed. Furthermore, since there i s broad overlap of a l l species i n the Flicker cycle i n terms of nest—site parameters, i t i s clear that, provided other factors do not limit, populations below the level permitted by the supply of holes, the species of this group are l i k e l y to remain i n competition. Several avenues of exploration, both experimental and observational, now suggest themselves for further study, particularly, of Flicker cycle species. Among those of an observational nature are detailed studies of t e r r i t o r i a l i t y and breeding biology and detailed analysis of inter- and intraspecific behaviour. If possible, a general study similar to the present one i n an area not yet colonized by the Starling (Sturnus vulgaris) would be most instructive, not only in terms of competition per se. but also i n terms of the general biology of alien species, and their impact upon the ecosystems which they "invade". experimental studies are also required:  Numerous  perhaps the most v i t a l and most  interesting of these would be to test the hypothesis that the overall population of Flicker cycle species is limited by the number of holes available.  This could  be accomplished by saturating an area with "standard" nest boxes designed to  - 41 -  approximate the mean measurements of a l l Flicker-cycle holes, and placed i n a randomized arrangement of combinations of height and direction, provided only that each hole have an appropriate flightway before i t .  Such an experiment  should, of course, include an untreated control area of similar ecological characteristics, and would permit the gathering, i n addition to population data, of useful information on competitive mechanisms and comparative success.  competitive  Comparative studies on fecundity and a b i l i t y to feed young would also  be of great value.  A further refinement of nest-box studies might involve the  providing of boxes of systematically varied geometries, i n order to assess the ultimate limits of acceptability of site characteristics for the various species. Detailed l i f e history studies of individual species are needed to provide data for the f u l l e r description of the ecological niches of the competing species, in order to f a c i l i t a t e better appreciation of the various relationships which may e x i s t  0  Many other possible studies could be outlined but i t i s suggested that those indicated above would be logical steps to follow the present background study. On the Definition of Competition The concept of biological competition has been familiar to biologists for many years.  This one term has been used i n a variety of ways, has been given many  different meanings, and i n fact has frequently been introduced without definition. This has, of course, given rise to much confusion, and recently, attempts have been made to c l a r i f y this situation.  Birch (1957) described the broad range of  meanings given to the term, ranging from the etymological equivalent of "environment" through a narrow and restricted meaning such as the one used for this study.  As i s ever the case with terminology, the individual's preference  in choice of meaning i s largely a matter of taste, but i f only for the sake of economy i n writing and i n the interest of enhanced understanding among ecologists, the plea must be entered for the adoption of the limited and hence clearer  - 42 -  definition.  Again, as Birch has also indicated, there exists no need to replace  with "competition" the concepts of "environment",  "interference phenomena", etc.  On the other hand, the perfectly clear r e a l i t y of the phenomenon described by the restricted definition of competition does require a suitable term. Milne (1961) reviewed the etymology of the term, as well as the ways in which i t has been used, and proposed a definition of hiw own for use in ecology, genetics and evolution, to cover both inter- and intra-specific relations. The definition given on Page 1  i s essentially similar to those proposed  by Birch and Milne, and agrees with them, as well as with Udvardy (1951), in the adoption of a limited meaning.  The concept has much potential usefulness,  but in order for this to be realized, i t must be used consistently.  There i s ,  furthermore, as Milne (1961) has pointed out, a need for discerning interpretation. The Detection and Measurement of Competition Much of the d i f f i c u l t y i n the study of any form of biological competition arises from confusion i n communication as a result of variations i n definition (see pages 1 and 2).  However, even when this i s overcome, there remain the  problems of detecting and measuring competition.  These matters are by no means  standardized or even well understood, and this study may be regarded as an attempt to f a c i l i t a t e the detection of competition for nest-sites i n hole-nesting birds. In t h i s , some success has been achieved, but the measurement or assessment of the competition depends upon the carrying out of a series of experimental studies such as those outlined above.  It is suggested that highly interesting and informative  data w i l l result from this combination of experimental and observational study. The method used here to detect nest—site competition is based upon the fact that, although the philosophical concept of "competition" is highly subjective at best, and made more confusing by the variety of uses to which i t has been put, there are a number of essentially permanent parameters associated with nest-sites studied,  -  43-  and the sites chosen by each species could be described i n a relatively quantitative fashion. If the requirements of each species i n terms of nest-sites were quite different, then either (a)  they were originally quite different or (b)  exclusion has occurred i n terms of nest-sites.  competitive  Within the Flicker-based group,  as shown by the data presented earlier, this did not prove to be the case.  Rather,  there appeared to be almost universal overlap in nest-site characteristics within this group,all of which are dependant upon one species for nest-sites. If differences had been detected, there would have remained the problem of whether these differences were due to inherent tendencies of the birds, or whether they were the result of competitive exclusion.  In either event, an experimental  programme similar to that outlined above would s t i l l have been required.  In the  actual situation, as revealed by the analysis given above, i t is clear that in fact nest-site requirements are essentially similar i n a l l species of the Flicker cycle, and there i s no reason to assume that competitive exclusion has occurred or is very l i k e l y to occur, i n terms of nest-sites, or particular parameters of them.  There does remain the logical possibility that some species might already  have been entirely excluded from the area as a result of nest-site competition. This aspect i s discussed separately below. The actual situation described above i s open to another interpretation. The view is widely held by ecologists that i f any competition is present between two or more species, one of two outcomes must result:  either the species involved  w i l l evolve different requirements; or competitive exclusion w i l l occur.  As w i l l  be shown below, i t is v i r t u a l l y impossible to decide whether the range of a species is primarily determined by competitive resistance at i t s boundary, or by some other limiting factor.  In the present study, the European Starling had only recently  "invaded" the area, and was probably s t i l l increasing i t s population.  There  appeared to be few i f any surplus holes during the study period but roughly 160 of  - 44 -  450 nests studied (or about 20 to 25$) were of this species. The present study showed nearly a l l holes examined to be occupied, and the requirements of a l l species were v i r t u a l l y identical.  Thus i t i s reasonable to assume that competition  might now be possible, but what of the situation before the addition of the Starling to the avifauna?  The other Flicker cycle species have presumably been  in the area since i t s reforestation after the last glaciation, and have no doubt had time to evolve different nest-site requirements.  That this appears  not to have occurred suggests that, at least u n t i l the advent of the European Starling in the study area, competition for nest-sites i n the Flicker cycle may not have occurred, or at least was insignificant as an ecological factor.  Thus  we find the intriguing p o s s i b i l i t y that i n the avifauna of the Cariboo region we have an opportunity to study the effects upon a group of species, related i n only one aspect of their ecological niches, to the relatively abrupt addition to their ecologies of a new and potentially very potent factor.  It i s unfortunate  that data are not available from similar areas not yet supporting Starling populations, i n order to better assess the role of the Starling.  However, i t  seems very clear that the impact of any species, which i n just over ten years manages to occupy 25$ of a l l holes examined, must be quite considerable.  This i s  particularly so when, as i s the case here, i t s nest site requirements are essentially identical to those of the indigenous;species. Competitive Exclusion It could be suggested that certain species which are rare or absent from the study area, but present nearby, might have assumed such status as a result of competitive exclusion, and the suggestion should therefore be considered. Simply stated, the "competitive exclusion principle", or "exclusion principle" states that complete competitors cannot co-exist. Hardin (i960), i n a discussion of the nature of this principle, contends that i t s "truth" rests in theory and  - 45 -  i s not subject to proof or disproof by facts, as ordinarily understood.  He  presents an ingenious argument i n defence of this, and points out that isolated laws such as this (and, for example, Newton's f i r s t law) are not tested per se, but are tested as parts of a whole conceptual model.  From such models, predictions  may be made, tested against empirical data, and the model changed i f i t s predictions cannot be v e r i f i e d .  He further points out that there i s no procedural rule to  t e l l which element of the model should be changed.  His f i n a l point i n this  section i s that the exclusion principle i s one element i n a system of ecological thought.  Not only are we unable to test the principle i t s e l f , but we do not yet  know what the whole ecological system i s .  Cole (i960) offered further remarks  on this matter, pointing out that u n c r i t i c a l acceptance of the exclusion principle could lead to neglect of important ecological evidence.  Patten (1961)  advocated the relegation of this principle from the state of importance given i t by some workers to that of a small segment of a broad class of interspecific phenomena. This, then, i s at least a partial delineation of the theoretical context i n which to consider the p o s s i b i l i t y that certain species may be absent from the avifauna of the study area as a result of competitive exclusion. Several distinct situations which should be reviewed here are presented below.  In each such case, i t must be borne i n mind that no f i n a l decision i s  possible u n t i l more information i s available on the ecological niches of the species i n question.  Study of a paper by Bond (1957) has suggested that although  two or more species may be i n competition for one environmental resource, differences in other aspects of their ecological niches could tend to reduce this competition, and i f these differences were great enough, they could even be reflected i n different geographical ranges.  However, the point can also be made that the  presence i n a given area of only one of a pair of species which compete elsewhere  - 46 -  could well be a result of competitive exclusion. This question , when i t arises, can only be settled with detailed knowledge of the niches of both species. The Hooded Merganser (Lophodytes cucullatus) i s represented i n Munro and Cowan (1947) by scattered summer records i n the study area. breeding are known to the author at this time: brood i n the v i c i n i t y of Springhouse, B.C., brood at P h i l i l l o o Lake i n 1958  Only two cases of  Jackson (pers. comm.) noted a  i n 1952, and A.J. Erskine recorded a  (B.C. Nest Records Scheme).  Whether this species  is excluded from the Cariboo area by nest-site competition is open to dispute. In view of the fact, however, that many more records are available (c.f. B.C. Nest Records Scheme, as well as other sources) from regions to the south of the study area with less severe climates, i t appears probable that the study area merely represents marginal habitat. This species i s r e l a t i v e l y close to the Bufflehead both taxonomically and i n terms of size (Johnsgard, 1961), but no conclusions are possible without more data on i t s ecological niche i n B.C. The Wood Duck (Aix sponsa) represents a case similar to that of the Hooded Merganser, except that no records of nesting are known to me from the study area. The two species of Chickadee (Parus atricapillus and P. gambeli) are both able to excavate holes, and insofar as i s known, seem to exist sympatrically i n a stable relationship.  No data were obtained on differences i n the niches of  these two species, but in the light of the exclusion principle, as discussed above, i f i t i s accepted i n i t s usual form, there must be l i t t l e or no competition i n this closely related pair of species. The possible effects of increasing Starling (Sturnus vulgaris) populations upon those of the Mountain Bluebird (Sialia currucoides) have been of considerable public concern.  It now seems clear, i n the light of data analyzed in this study,  that these two species might be  i n competition for nest-sites i n the study area.  The effects upon Bluebird populations, however, cannot be determined without the  - 47 -  aid of census data from times before the appearance of the Starling, are not available.  These  Some further circumstantial evidence might be forthcoming  from a survey of nest-sites i n an area occupied by Bluebirds, but not yet reached by Starlings.  Some such areas are suggested by Myres (1958).  Competitive exclusion of Woodpeckers v i a nest-site competition i s d i f f i c u l t to envision, nor does i t seem possible, at least in summer, that other factors such as food, predation, etc., could v i r t u a l l y exclude the Black-backed  Three-toed  Woodpecker (Picaides arcticus) and the Hairy and Downy Woodpeckers (Dendrocopos villosus) and D. pubescens).  For the former, Jobin (1952) published the f i r s t  record as a winter v i s i t a n t , and the only hest record for the study area was made by A.J. Erskine i n 1958 (B.C. Nest Records Scheme).  The two Dendrocopos  species i n the study area are very rare, with only occasional nestings reported, and no nests were found i n the two years of this study.  The two species are,  however, present sympatrically i n much the same way as the Parus species above. Without further study of the niches of a l l three woodpecker species, i t appears impossible to determine the role of competitive exclusion i n determining their abundance i n the study area.  It i s suggested, however, that the reason for their  apparent scarcity might more profitably be sought i n considerations related to aspects of the niche other than nest-site competition. Nest-Site Competition as an Ecological Factor i n Hole-Nesting Birds In a situation such as that exemplified by the Flicker cycle, with closely p a r a l l e l requirements and few primary species, i t i s easy to envision the supply of nest-sites as a factor controlling the overall population of hole-nesting birds, and to imagine that competition might be occurring among them for these sites. It certainly follows, then, that relative competitive a b i l i t y may have an important bearing upon the abundance of a given species relative to the number of available holes.  Within the study area, few data are yet available on this point, but  scattered records are available such as cases of joint clutches of different species  - 48 -  (e.g.  Erskine, 1959a) i n several combinations, cases of usurpation, etc.  (As mentioned above, the requirements of the present study were such as to preclude the p o s s i b i l i t y of obtaining many observations of this sort), and there i s a clear need f o r detailed studies on fixed plots to gather more such data. Competitive success need not be entirely a function of size or agressiveness. Lohrl (1956) points out that Starlings i n northern Europe, although they regularly usurp woodpeckers nests, are less successful i n competing with Nuthatches which p a r t i a l l y block nest entrances with hard mud.  This behaviour i s noted also by  Seilmann (1958) and has been described to me verbally (von Haartman; Udvardy). This behaviour i s represented i n the Red-breasted Nuthatch (Sitta canadensis) only by the placing spottily around the hole of "pitch" from coniferous trees. In the present study, then, i t can be seen that at least i n the Flicker cycle, nest-site competition appears to limit the overall hole-nester population (except that of the f l i c k e r ) , and i t may be speculated that within the group, comparative a b i l i t y to compete may determine the proportion of the total population taken up by each component species. As noted above, however, i t i s possible to suggest that this competition may not have been present before the advent of the European Starling i n the study area. The results of the experimental studies suggested above may be expected to reveal many interesting data on this subject.  The overall population response  to the provision of many new holes should be highly instructive per se, and should also reveal much concerning comparative a b i l i t i e s of the different species to compete one with the other for nest s i t e s . Habitat Dispersion as an Ecological Factor Of the six species of the Flicker cycle, a l l but the Bufflehead may be described as "edge" species, and even i t , as i s evident from Erskine (i960), as well as from the fact that i t i s dependent upon the Flicker f o r nest-sites, may be so regarded.  I t i s well known that such species are able to achieve much higher  - 49 -  population densities per unit area i n a mixed habitat than i n a continuous phytocoenosis.  Thus, i n a very real sense, the population density of any  secondary species i s dependant not only upon i t s comparative competitive a b i l i t y as noted above, but also upon two other sets of factors:  (a)  those factors  .satisfying, i n terms of the phytocoenosis, various aspects of i t s own ecological niche; (b)  factors permitting the presence and survival of the Flicker (Colaptes).  The latter two groups are, of course, closely related and i n most cases there are some aspects in common. Exactly what many of these factors are i s not known at present, but i n any event, whatever the proximal causation of habitat selection, each species clearly must satisfy a set of habitat requirements,  and  the amount of "good"-habitat available for each species can be supposed to be related to the interspersion of various phytocoenoses.  The f i n a l evaluation of  the role of habitat dispersion i n the ecology of the hole-nesting avifauna of the study area awaits the results of detailed l i f e history studies of each of the component species, to reveal more clearly the details of their ecological niches. The Tree Swallow (Iridoprocne bicolor) as a Competitor Two points are of importance i n the consideration of this species as a nest competitor.  F i r s t l y , i n terms of the Flicker cycle, i s the fact that, so  far as i s known at present, i t i s , i n a sense, a facultative member of that group. Without more detailed l i f e history data, i t must be assumed that individual Tree Swallow pairs choose nest sites at random, and are no more or less l i k e l y to use a Flicker hole than one of any other size.  This i s by no means certain, but  i t must be accepted as a working hypothesis, although the p o s s i b i l i t y should be borne i n mind that only a proportion of the total population of this species may be potential users of Flicker holes.  Secondly, the habit of this species of f i l l i n g  nest holes would be regarded by many workers as an aspect of competition.  Whether  or not i t is accepted as such may be a matter of individual choice, but the ecological effect of rendering the hole unsuitable for certain species is s t i l l the same.  -50  -  F i n a l l y , i t should be noted that the- overall abundance of the Tree Swallow (Iridoprocne) i n the study area i s definitely not reflected by the sample of nests i n the Flicker cycle.  These, however, were the only nests of the species  which had relevance to the competitive situation.  Thus the Tree Swallow i s a  species which forms part of, but i s not confined to, the competing complex. The European Starling (Sturnus vulgaris) as a Competitor In i t s original range, this species i s well known as a nest-site competitor (c.f. Lohrl, 1956 et al) and might be expected to show similar characteristics here.  In the analysis of data above, i t i s clear that the parameters of the nest  site are similar to those of the other species of the Flicker cycle, and since there seemed  to be very few unoccupied holes, i t i s strongly suggested that  this species i s now a competitor for nest-sites in the study area.  Myres (1958)  summarized essentially a l l data on the spread and increase of this species i n B.C. from 1947, when i t was f i r s t noted, to 1957, by which time the population wasnumbered i n tens of thousands.  Even i f , as suggested above, nest-site  competition was absent or negligible before the appearance of this species, the probability that i t now exists is greatly enhanced, i f only by the sheer numbers of Starlings now  present.  The Results of this Study i n Relation to General Theory Much of the previous work on nest-site competition among hole-nesting birds has been done i n Europe, with an avifauna considerably richer i n hole-nesters of a l l sizes, and i n addition has largely been concerned with nest-box studies. The present study serves to point out various differences i n the extent of competition for nest—holes, which were found i n the study area, and was essentially an exploration of a natural situation.  On the basis of the finding reported here,  i t w i l l now be possible to design experiments to study the various mechanisms of this competition, such as agression, egg-foisting (mixed clutches) (c.f, P, Busse  - 51 -  and J» Crotzman, "Nesting competition and mixed clutches among some birds inhabiting the nest boxes", Acta Orn. 7 ( l ) :  1-32, 1962), etc.  Various casual  observations made during the f i e l d work are included above, and these suggest that detailed study w i l l probably show that at least i n broad terms, these mechanisms are essentially similar.  It has already been pointed out that the  data given here may be interpreted to mean that the hole-nesters of the area may not have been in competition before the a r r i v a l of the European Starling.  This  proposal might be further illuminated by study of areas not yet reached by the Starling.  If i t i s correct, we find ourselves i n the excellent position of being  able to study at f i r s t hand the effects of the introduction of biological competition in a situation where i t was previously absent.  This w i l l , as  the quantitative data show essentially no differences i n nest requirements,  permit  the study on a long-term basis of the effects of this newly introduced competition. In short, then, the present study has revealed the existence of an excellent opportunity to conduct a series of studies, not only on the mechanisms of competition i n this case, but, perhaps even more significantly, on the response of populations to this new situation.  Apparently no selection of different  nest-sites has previously occurred i n Flicker cycle species; now, with a new aspect of nest—site competition introduced, an excellent opportunity has arisen to study, from a f a i r l y well-defined time of commencement, a situation i n which selection for different nest-sites may well be expected to occur.  - 52 -  SUMMARY AND CONCLUSIONS  1.  This work was undertaken as the f i r s t of a series of studies designed to  explore the extent and nature of nest-site competition among tree-hole-nesting birds i n the Cariboo region of B r i t i s h Columbia. 2.  The method used was direct measurement of several parameters associated with  nest-sites as they were found i n nature, and comparison of sites used by each species i n terms of overlap i n site 3.  requirements.  The 20 species recorded from the area f e l l into three distinct natural  groups on the basis of hole size, with the exception of one euryoecious species. Of the three groups, one was highly unlikely to show nest-site competition, one i s too poorly known to permit generalization, and one clearly shows a likelihood of competition. 4.  In the group using medium-sized holes, site characteristics of a l l 6 species  are essentially similar, and only the Flicker (Colaptes cafer) produces holes. 5. Nest-site competition may be deemed to be present among the following species: Flicker (Colaptes cafer), European Starling (Sturnus vulgaris), Mountain Bluebird (Sialia currucoides), Bufflehead (Bucephala albeola). Sparrowhawk (Falco sparverius). and Tree Swallow (Iridoprocne bicolor). 6.  Interpretation of quantitative data on nest-sites i n the medium-hole group  suggests that competition was absent or inconsiderable before the addition of the European Starling (Sturnus) to the avifauna of the area.  Data gathered during  this study, while populations of this species were s t i l l increasing, show that there i s good reason to assume that competition i s now occurring i n that group. It i s suggested that i f this i s the case, one, or a combination of both, of two courses of events may occur: in numbers; or ( i i )  (i) one or more species may be eliminated or reduced  selection for differing nest sites may occur.  In either event,  - 53 -  there exists an excellent opportunity to study these events as they occur. 7.  The competitive exclusion principle i s b r i e f l y reviewed, and several possible  cases i n the study area are considered.  - 54 -  REFERENCES CITED  American Ornithologist's Union (1957), "Check-list of North American Birds, F i f t h Ed". Andrewartha, H.G. and L.C, Birch (1954), "The Distribution and Abundance of Animals". University of Chicago Press, 782 pp. Bailey, N.T. (1959), " S t a t i s t i c a l Methods i n Biology". Press, 200 pp.  English Universities  Baker, F.S. (1925), "Aspen i n the Central Rocky Mountain Region". Agr. B u l l . 1291: 45 pp.  U.S. Dept.  Beaton, J.D. (1953), "Influence of Burning on Soil i n Timber Range Area of Lac le Jeune, B.C.". M.S. Thesis, University of B r i t i s h Columbia. Bent, A.C. (1939), "Life Histories of North American Woodpeckers". Mus. B u l l . 174, 334 pp. Birch, L.C. (1957), "The Meanings of Competition". 91: 5-18.  U.S. Natl.  The American Naturalist  Bird, R.D. (1930), "Biotic Communities 6'f the Aspen Parkland of Central Canada". Ecology 11: 356-442. Bond, R.R. (1957), "Ecological Distribution of Breeding Birds i n the Upland Forests of Southern Wisconsin". Ecological Monographs 27: 351-384. Burns, F.L. (1900), "A Monograph of the Flicker (Colaptes auratus) .' Wilson B u l l . 12: 1-82. 1  Burt, W.H. (1930), "Adaptive Modifications i n the Woodpeckers". Univ. C a l i f . Publ. Zool. 32: 455-524. Cole, Lamont C.  (i960), "Competitive Exclusion".  Science 132 (3423):  Dawson, G.M, (1894), "Report on the Area of the Kamloops Map Sheet". Geol. Sur. Canad. 7: 78-163.  348-349. Rep.  Erskine, A.J. (1959a), "A Joint Clutch of Barrow's Goldeneye and Bufflehead Eggs". Can. Field-Nat. 73: 131. Erskine, A.J.  (1959b), "A Method for Opening Nesting Holes".  Bird-Banding 30:  181.  Erskine, A.J. (i960), "A Discussion of the Distributional Ecology of the Bufflehead (Bucephala albeola; Anatidae; Aves) Based Upon Breeding Biology Studies i n B r i t i s h Columbia". M.A. Thesis, University of B r i t i s h Columbia. Hardin, Garrett (i960), "The Competitive Exclusion Principle". 1292-1298.  Science 131 (3409):  - 55 -  Hoyt, J.S.Y. (1950), "Nine Years Experience i n Keeping the Pileated Woodpecker in Captivity". Aviculture 20$ 41-46. Hoyt, S.F. (1957), "The Ecology of the Pileated Woodpecker". Ecology 38 (2): 246-256. Jobin, L. (1952), "Some Bird Records From the Cariboo D i s t r i c t , B r i t i s h Columbia". Condor 54 (3): 171-172. Johnsgard, Paul A. (1961), "The Sexual Behaviour and Systematic Position of the Hooded Merganser". Wilson B u l l . 73 (3): 226-236. Kendeigh, S.C. (1961), "Animal Ecology".  Prentice-Hall, 468 pp.  Krajina, V.J. (1959), "Bioclimatic Zones i n B r i t i s h Columbia". of B r i t i s h Columbia Bot. Ser. 1: 47 pp.  University  L'ohrl, Hans (1956), "The Starling as a Competitor for Nest Holes". Die Vogelwelt 77 (2): 47-50. (Original not seen: from review in Bird-Banding 28 (2)). Lynch, Brother D. (1955), "Ecology of the Aspen Groveland i n Glacier County, Montana". Ecol. Monogr. 25: 321-344. Macoun, J . (1876), "Report on the Botanical Features of the Country from Vancouver Island to Carleton". Rept. Geol. Sur, Canad. 110-232. Mayfield, H. (i960), "The K i r t l a n d ^ Warbler". H i l l , Mich., 242 pp.  Cranbrook Inst. S c i . Bloomfield  McMinn, R.G. (1952), "The Role of Soil Drought i n the Distribution of Vegetation in the Northern Rocky Mountains". Ecology 33 ( l ) : 1-15. Milne, A. ( l 9 6 l ) . "Definition of Competition Among Animals". B i o l . XVj 40-61.  Symp. Soc. Exp.  Moss, E,H. (1932), "The Vegetation of Alberta. IV. The Poplar Association and Related Vegetation of Central Alberta". Jour. Ecol. 20: 380-415. Moss, E.H. (1938), "Longevity of Seed and Establishment of Seedlings i n Species of Populus". Bot. Gaz. 99: 529-542. Munro, J.A. and I. McT. Cowan. (1947), "A Review of the Bird Fauna of B r i t i s h Columbia". Spec. Pub. B.C. Prov. Mus. No. 2: 285 pp. Myres, M.T. (1958), "The European Starling i n B r i t i s h Columbia: Occ. Pap. Prov. Mus. B.C. No. 11: 60 pp. Patten, Bernard C. 1601.  (1961), "Competitive Exclusion".  1947-1957".  Science 134 (3490):  Peterson, R.T. (1961), "Field Guide to Western Birds". Co., Boston.  1599-  366 pp., Houghton M i f f l i n  Pynnbnen, A. (1939), "Beitrage zur Kenntnis der Biologie Finnischer Spechte". Ann. Soc. Zool.-Bot, Fenn. Vanamo, 7 (2): 1-166.  - 56 -  Schraitz, H. and L.W.R. Jackson (1927), "Heart-rot of Aspen With Special Reference to Forest Management i n Minnesota". Minn. Agr. Expt. Sta. Tech. B u l l . 50: 43 pp. Seilmann, Heinz  (1958), "Das Jahr Mit Den Spechten".  Snedecor, George W.  (1956), "Statistical Methods".  U l l s t e i n A.G., Berlin. Iowa State C o l l . Press, 534 pp.  Spilsbury, R.H. and E.W. Tisdale (1944), "Soil-plant Relationships and Vertical Zonation i n the Southern Interior of B r i t i s h Columbia". S c i . Agr. 24: 395-436. Stein, Robert C. (1958), "The Behavioural, Ecological and Morphological Characteristics of Two Populations of the Alder Flycatcher, Empidonax t r a i l I i i (Audubon)". N.I. State Museum and Science Service B u l l . 37H  63 pp.  Tisdale, E.W. and A. McLean. (1957), "The Douglas-Fir Zone of Southern Interior B r i t i s h Columbia". Ecol. Monogr. 27: 247-266. Udvardy, M.D.F. (1951), "The Significance of Interspecific Competition i n Bird L i f e " . 0 i k o s 3 ( l ) : 98-123.  Appendix I  :  Map o f B r i t i s h Columbia t o show Approximate L o c a t i o n o f Cariboo P l a t e a u . Major study L o c a t i o n s I n d i c a t e d fay Symbol X .  

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