"Science, Faculty of"@en . "Zoology, Department of"@en . "DSpace"@en . "UBCV"@en . "McLaren, William David"@en . "2011-10-27T23:29:29Z"@en . "1963"@en . "Master of Science - MSc"@en . "University of British Columbia"@en . "This study was restricted to birds using tree-holes as nest-sites. Of a total of 20 species in the study area, only 13 were sufficiently abundant to merit consideration. These fell into three natural groups on the basis of hole size, with only one euryoecious species (Iridoprocne bicolor) nesting in all three groups. Only the group based on holes made by the Colaptes woodpeckers (Flickers) can presently be construed as showing evidence of nest-site competition. Physical and ecological characteristics of nest-sites are analyzed in terms of intensification or amelioration of nest-site competition. The competing species, all using holes made by Colaptes cafer, are Sturnus vulgaris, Sialia currucoides, Bucephala albeola, Iridoprocne bicolor and Falco sparverius.\r\nThe data suggest that although competition is now present in this group, it may have been absent before the advent of Sturnus in the avifauna. Neither selection for different sites nor competitive exclusion seem to have occurred before the appearance of Sturnus, which now occupies roughly 25% of all available nests, but one or both of these may now be going on."@en . "https://circle.library.ubc.ca/rest/handle/2429/38361?expand=metadata"@en . "A PRELIMINARY STUDY OF NEST-SITE COMPETITION IN A GROUP OP HOLE-NESTING BIRDS by WILLIAM DAVID McLAREN B.Sc, University of British 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 0 THE UNIVERSITY OF BRITISH COLUMBIA January, 1962 In presenting this thesis in p a r t i a l fulfilment of the requirements for an advanced degree at the University of Br i t i sh Columbia, I agree that the Library shal l make i t freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for f inancia l gain shall not be allowed without my written permission. Department of ZOOLOGY The University of Br i t i sh Columbia, Vancouver 8, Canada. Date J A v t / a q y /g, 1963 ABSTRACT This study was res t r i c t e d to birds using tree-holes as nest-sites. 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 consideration. These f e l l into three natural groups on the basis of hole size, with only one euryoecious species (Iridoprocne bicolor) nesting i n a l l three groups. Only the group based on holes made by the Colaptes woodpeckers (Flickers) can presently be construed as showing evidence of nest-si t e competition. Physical and ecological characteristics of nest-sites are analyzed i n terms of i n t e n s i f i c a t i o n or amelioration of nest-site competition. The competing species, a l l using holes made by Colaptes cafer, are Sturnus vulgaris, S i a l i a currucoides, Bucephala albeola, Iridoprocne bicolor and Falco sparverius. The data suggest that although competition i s now present i n this group, i t may have been absent before the advent of Sturnus i n the avifauna. Neither selection f or different sites nor competitive exclusion seem to have occurred before the appearance of Sturnus, which now occupies roughly 25$ of a l l available nests, but one or both of these may now be going on. - i i -ACKNOWLEDGEMENTS Grateful acknowledgement i s due to a l l persons and organizations whose assistance made possible the successful completion of t h i s study. In particular, appreciation i s extended to: Dr. M.D.F. Udvardy for expert guidance throughout the study as supervisor of the project; to Dr. L. von Haartman, for much assistance and encouragement during his residence at the University 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 University of B r i t i s h Columbia, who assumed much of the burden of supervision and administration of the project during Dr. Udvardy*s absence during part of 1958 and 1959; to Drs. W.S. Hoar, K. Graham and J.F. Bendell, a l l of whom also served on my Research Committee; to Miss M.F. Jackson and Dr. M.T. Myres, who provided many useful data and lo c a l information resulting from their e a r l i e r f i e l d work in the study area; to other members of the Department of Zoology who from time to time were most helpful i n rendering advice on numerous questions and problems; to fellow graduate students i n the Department of Zoology for stimulating discussions and general assistance i n many ways; to Dr. A.J. Erskine, then a fellow graduate student, who provided many data from his study areas near 100 Mile House, B.C., and who was at a l l times a valued companion and esteemed colleague; to Lawson G. Sugden and Patrick W, Martin, Game Biologists with the Fish and Game Branch of the B r i t i s h Columbia Department of Recreation and Conservation, for general assistance and many deeply appreciated courtesies; to the owners of the T/H Ranch at Springhouse, B.C., Messrs. G. and C. Tucker, and Mr. W. Herrick, as well as their families, whose unfailing h o s p i t a l i t y made a - i i i -potent i a l l y d i f f i c u l t task not only easier, but decidedly pleasant. Financial support for this study was provided by the National Research Council of Canada through a grant to Dr. Udvardy. My appreciation i s due also to the Department of Zoology for o f f i c e space and f i e l d equipment used during the study, and to the B.C. Nest Records Scheme within that Department for freedom to consult t h e i r f i l e s . - iv -TABLE OF CONTENTS Subject Page INTRODUCTION 1 Objects of the Study 1 Terminology. . . . \u00E2\u0080\u00A2 . . . . . 1 Equipment and Methods 3 Location of Study Areas 4 ECOLOGICAL CONTEXT OF THE PROBLEM 6 Systematic and Ecological Classification of the \"Competitors\". . . 6 General Plant Ecology of the Study Area 9 Non-Avian Hole-Using Biota 15 The \"Life History\" of Nest Sites 16 Recapitulation: Ecological Context 20 ANALYSIS OF NEST-SITE COMPETITION 21 The \"Cycles\" 21 The Large-Hole Cycle 21 The Small-Hole Cycle 23 The Medium-Hole, or \"Flicker\" Cycle 24 Hole Direction 25 Hole Height 27 Entrance Size 30 S i l l Size 32 Hole Depth 34 Cavity Sizes 35 Recapitulation: Analysis of Competitive Situation 38 DISCUSSION 40 Orientation of this Study and Suggested Future Research 40 On the Definition of Competition 41 The Detection and Measurement of Competition 42 Competitive Exclusion. . . . . . . . . . . . . . . . . 44 Nest-Site Competition as an Ecological Factor in Hole-Nesting Birds 47 Habitat Dispersion as an Ecological Factor 48 The Tree Swallow (Iridoprocne bicolor) as a Competitor. . . . . . . 49 The European Starling (Sturnus vulgaris) as a Competitor 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 hole-nesting birds i n the study area 9 III Hole directions of nests measured within the study area. . . . 26 IV Hole heights: summary for F l i c k e r cycle. 29 V S i l l sizes: F l i c k e r cycle summary. . . .33 VI Frequency d i s t r i b u t i o n of hole depths: F l i c k e r cycle. . . . . 36 VII Size-class d i s t r i b u t i o n of nest c a v i t i e s : F l i c k e r cycle. . . 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 lellow-bellied Sapsucker 24 5. The Flicker Cycle: Demands upon hole supply 24 6. Percent frequency distribution of hole directions: Flicker Cycle 27 7. Height-class distribution: Flicker Cycle 27 8. Colaptes: Size-class frequency of entrance parameters (1959 data) 31 9. Sturnus: Size-class frequency of entrance parameters (1959 data). 31 10. Colaptes: Scatter diagram of entrance measurements. . . . 31 11. Sturnus: Scatter diagram of entrance measurements 31 12. Sialia: Scatter diagram of entrance measurements 31 13. Bucephala: Scatter diagram of entrance measurements. . . . 31 14. S i l l sizes: Colaptes and Flicker Cycle 32 15. S i l l sizes: Bucephala and Iridoprocne 32 16. S i l l sizes: Sturnus and Sialia 32 17. Hole depths: Colaptes, Flicker Cycle, and Sturnus. . . . 36 18. Hole depths: Sialia, Bucephala and Iridoprocne. . . . . . 36 19. Cavity Sizes: Flicker Cycle and Colaptes 38 20. Cavity Sizes: Sturnus and Sialia. 38 21. Cavity Sizes: Bucephala and Iridoprocne 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 is found in 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 restrict its use to one well-defined phenomenon or class of phenomena (cf. Birch, 1957; Milne, 1961). Birds utilizing 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 utilized 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 in 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 British 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 in the development of scientific arguments. The following definitions are thus offered for terras used in the present paper: Competition - the demand by more than one individual organism during a given period of time for an environmental resource which is present in insufficient amounts to supply the total demand. - 2 -It is important to note that this definition stresses both spatial and temporal contiguity of the competitors, as well as availability of sites. An expanded discussion of this problem of the definition of competition is included in the discussion section below. \"Hole-nesting bird\"- those species of birds which normally, or occasionally, utilize 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 in 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\u00E2\u0080\u0094nesting species requiring a cavity as a nest-site, but incapable of carrying out the necessary excavation. Ecological niche - the ecological niche is a particular combination of physical factors and biotic relations required by a species for the normal course of its 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 level. This is 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 vertical 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 distance from s i l l to bottom of cavity. In the case of species which place extraneous material i n the cavity, both level of nest and actual depth of cavity were measured wherever possible. Hole dir e c t i o n - direction toward which entrance hole i s oriented. This parameter was determined with a standard forester's compass. Figure 1 i s a diagrammatic representation of the relationship between various nest-site parameters. Equipment and Methods L i t t l e was required i n terms of special equipment for the f i e l d work phases of this study; particular items used are discussed below. Nest sites were located by searching of selected areas on foot, and other sites were located incidentally by sightings from vehicles, reports from interested persons, etc. A l l measurements made i n 1958 were i n feet and inches, and were lat e r converted to metric equivalents. In 1959, a l l measurements were made d i r e c t l y , i n metric units, except tree diameters and tree heights, and a few extremely high nest heights. A l l measurements of tree diameters were made with a standard forester'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 heights were measured with a standard Abney level or were roughly triangulated when characteristics of te r r a i n or vegetation precluded the use of t h i s instrument. Nest-holes were opened when necessary by a procedure described by Erskine (1959 (b)). Various means were u t i l i z e d to reach nest-sites at different heights, including lineman's climbing spurs, a collapsible aluminum u t i l i t y ladder, and improvisation from natural materials at hand. To folllow p. 3 F I G . 2 I N S P E C T I O N M E T H O D - 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 in the technique mentioned above. Long slips of mirror glass were used in conjunction with a pocket flashlight in the manner indicated in Figure 2, and judicious manipulation of both elements in the system provided a lighted view of the interior of a cavity which was sufficiently good in most cases to permit accurate identification of the contents of the cavity. A notable exception occured in the cavities excavated by the sapsucker, Sphyrapicus varius, in which the width of s i l l was. in 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 statistical methods available in 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 this. Location of Study Areas Munro and Cowan (1947) described thirteen terrestrial biotic areas, in British Columbia, using the criteria 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 in 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 is more properly a part of the Interior Douglas Fir bioclimatic zone. This matter is 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, in the vicinity of Clinton, there is a gradual merging with the dry forests of the Ponderosa Pine Zone, and to the north, in the area just south of Quesnel, is 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 in 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 classified according to families, status as secondary or primary hole-nester, and relative abundance. These 17 species occupy an equal number of ecological niches within the ecosystems, and as is shown below for members of one of the three ecological groupings, these niches show l i t t l e overlap except in terms of nest sites. Heretofore, i t has been widely assumed that as a result of this apparent overlap in 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 in the careful evaluation of that assumption. In Table I, order of occurence, common names and scientific names follow the Fifth 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 in the study area are drawn from a wide variety of taxa, and as might be expected, the variety of their ecological niches is equally broad. Clearly, then, the ecological relationships pertaining to nest-site competition within this group are unlikely to be represented by their taxonomic affinities. Therefore i t was essential for the purposes of this study to devise an ecologically-oriented classification in the context of which these relationships could be appreciated. Several factors are relevant to the elaboration of such a classification. Perhaps the most obvious of these is 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 in 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. SPECIES FAMILY STATUS RELATIVE * ABUNDANCE Barrow's Goldeneye (Bucephala islandica) Bufflehead (Bucephala albeola) Sparrowhawk (Kestrel) (Falco sparverius) Screech Owl (Otus asio) Great Horned Owl (Bubo virginianus) Saw\u00E2\u0080\u0094whet Owl (Aegolius acadieus) Flicker (Colaptes cafer) Pileated Woodpecker (Dryocopus pileatus) Yellow-bellied Sapsucker (Sphyrapicus varius) Hairy Woodpecker (Dendrocopos villosus) Downy Woodpecker (Dendrocopos pubescens) Tree Swallow (Iridoprocne bicolor) Black-capped Chickadee (Parus atricapillus) Mountain Chickadee (Parus gambeli) Red-breasted Nuthatch (Sitta canadensis) Mountain Bluebird (Sialia corrueoides) European Starling (Sturnus vulgaris) Anatidae Anatidae Falconidae Strigidae Strigidae Strigidae Picidae Pic idae Picidae Pic idae Picidae Hirundinidae Paridae Paridae Sittidae Turdidae Sturnidae Secondary Secondary Secondary Secondary Secondary Secondary Primary Primary Primary Primary Primary Secondary Primary Primary Primary Secondary Secondary Common Common Uncommon Rare Relatively Common Rare Common Rare Common Rare Rare Common Rare Relatively Common Common Relatively Common Common \u00C2\u00B1 The terms used to refer to relative abundance are extremely general and open to criticism. However, no more suitable expression is available in the absence of extensive census data. and were seen only rarely, i f at alls Screech Owl, Saw-whet Owl, Hairy Woodpecker, and Downy Woodpecker. 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 in 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 divisible into three groups on the basis of size both of the birds themselves and of the cavities in which they nest. Only the Tree Swallow (Iridoprocne bicolor) occurs in a l l three groups, and only one other, the Sparrow Hawk (Falco sparverius) in more than one. In addition to the three types of bird-excavated holes, the Tree Swallow utilizes other natural holes and a wide variety of niches, and is generally highly euryoecious in terms of its nest-site. The overall species\u00E2\u0080\u0094site relationships as they were found to exist on the study area are represented in Figure 3. Table II gives the effective species composition of the three groups: the values given for size represent length measurements as given in inches by Peterson (1961) and are included to emphasize size relationships. Further details on the three groups are provided below in the section on analysis of nest-site competition. To recapitulate, i t has been shown that the taxonomic classification of these hole-nesting bird species is unsuitable for ecological purposes, and they have been classified in terms of the sizes of holes utilized for nest sites. S O M E D E G R E E . E = E N L A R G E D BY C O L A P T E S : B= B R E A K A G E . F IG . 3 : S P E C I E S - S I T E R E L A T I O N S H I P S TABLE II. SPECIES COMPOSITION OF SUB-GROUPS OF HOLE-NESTING BIRDS IN THE STUDY AREA. GROUP SPECIES SIZE * Small-Hole 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* 8-9 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} General Plant Ecology of the Study Area The ecology of any terrestrial animal is intimately related to the ecology of the plant components of the ecosystem in which i t occurs. This is particularly true of many groups of terrestrial birds whose ecological niches involve, among many other factors, gestalt-type behavioural responses to the overall physical aspect of the environment. It is therefore necessary, in considering the ecological context of the present problem, to review briefly the factors which affect the distribution of the various habitat-types. The following section is 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 in the study area* Sizes are given in inches as taken from Peterson (l96l). - 10 -According to Tisdale and McLean (1957), the entire Cariboo region is located well within the limits of the interior Pseudotsuga Zone, They define this zone as being,in British Columbia, in 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 is rather complicated, with topography having pronounced effects upon local conditions, and exposure causing further complications. Splisbury and Tisdale (1944) described vertical zonation of soils, climate and vegetation. On south and west slopes, the boundaries of main vegetation zones show differences in 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 illustrate variation in climate within the region, as weather stations are largely located in the major valleys where settlement is concentrated. They furnish data to illustrate the changes in climate encountered in passing from the dry grassland areas in the Kamloops area to the northern Spruce-Fir zone, at McCulloch. As Tisdale and McLean indicate, many large gaps exist in our knowledge with regard to the climatology of this region, but in a general way, i t can be stated that the Pseudotsuga zone is 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. cit.) indicate that the major portion of the zone is 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 in which the process will be very slow due to lack of Pseudotsuga seed sources. - 11 -Soils in the Cariboo have been l i t t l e studied* Beaton (1953) classified the soil of the upper Douglas f i r zone near Kamloops as \"Brown Podsolic Soil\"* In general, the area is a large lava flow covered by glacial t i l l of varying coarseness* The role of fire in forest succession has been widely studied and the relationship of aspen (Populus tremuloides) to fire is well established (see Baker, 1925; Moss, 1932). Aspen reproduction after fire 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 viability 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 in small, isolated, f a i r l y discrete groups, frequently clonal in nature, or in large semi-open stands in such locations as are well supplied with soil or surface water. Grassy areas of uneven topography have small groves in 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 is 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 fires caused by both Indians and whites* Suppression of fire in recent years in the grazing areas has resulted in the occupation of many acres by Douglas f i r regeneration as a result of the lack of fire destruction of young trees* (L.G. Sugden, pers. comm.). Grazing also is effective in restricting or preventing aspen regeneration. Logging, although carried out in the Douglas F i r zone since the 1860*s, has only recently become important in the Cariboo area. As yet, only Douglas f i r is exploited in quantity, but continually greater use is being made of Pinus contorta and Spruce, Picea engelmanni, as well. Aspen, as a result of its small size, usually rather gnarled growth habit and almost universal susceptibility to attack by Pomes igniarius, a heart\u00E2\u0080\u0094rot, is not likely to become a commercially important species in the foreseeable future. Industrialization on a large scale has not yet appeared in the Cariboo. Small sawmills are scattered throughout the area, and usually are present in 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 fire hazards, and their decomposition also is extremely slow. Insect outbreaks of major proportions have occurred in 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 Fir 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, in the Interior Wet Belt, through the Cariboo and north at least to the vicinity 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 is 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 in 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 is s t i l l rather small, may have considerable significance to the Flicker populations of the aspen groveland areas: the much softer heartwood is attacked for nest\u00E2\u0080\u0094hole excavations much more frequently than are healthier trees; and frequently the birds commence excavation in a scar or broken branch stub in which decay has softened the wood. Sapsuckers, on the other hand, rarely i f ever excavate a hole in previously injured wood on the outside of the tree. Even in these cases, however, they usually do not begin to enlarge the nest-cavity itself until the excavation reaches the softer, decayed heartwood. Lynch (1955) believes that the exhaustion of water available for plant growth is a c r i t i c a l factor in the stabilization of grove boundaries for aspen. This, he feels, is due to the fact that while the main root zone of aspen rarely goes below 2', the roots functioning in shoot production are usually at depths less than 1', and the reduction of soil water past the permanent .wilting percentage results in the failure of shoots to develop in the dry s o i l . He feels also that lack of seed accounts for the failure of aspen to establish in lowland spots where the understory vegetation implies suitable moisture conditions. Lynch again points out the effect of grazing in preventing aspen regeneration. McMinn (1952) studied the effects of soil drought on the distribution of vegetation in the Rocky Mountains of the northern U.S.A., and concluded that in areas where precipitation is mostly in the winter and summer drought is normal, different plant associations are correlated with different extents of soil drought. To summarize the plant ecology of the study area, the following points are relevant. It seems now to be generally agreed that the Cariboo region is characterized by the climatic climax forest type known as the Interior Douglas Fir (or Pseudotsuga) Zone (c.f. Krajina, 1959) which has, however, been extensively altered so that very large areas now support a serai association in which Lodgepole 14 Pine, Pinus contorta. is the dominant and frequently only tree. Some extensive grassland areas also exist; the permanence of these areas is 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 virtually splits 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 is a dominant factor in the ecologies of hole-nesting birds in the area, occurs in 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 in grassland areas around lakes, in low moist-soil areas and in gullies and other natural drainage courses. Although much more work is 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 Fir 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 in 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 in a later section. Further descriptions of habitat\u00E2\u0080\u0094types and plant communities are to be found in Krajina (1959), Tisdale and McLean (1957), Munro and Cowan (1947), and Erskine (i960), and further elaboration is not needed here. - 15 -Finally, then, i t is clear that although the ecology of this complex and much-changed phytocoenosis can be understood in very general terms, there remains much botanical work to be done before the overall ecological context of the present investigation can be fu l l y appreciated, Non-Avian Hole-Using Biota Tree-holes are utilized 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 in a few cases to be nesting in holes of the medium-hole, (see p;'24),, cycle. Numbers of cases, however, were very small, and is speculated that Glaucomys may tend to use niches, cracks, etc, to a considerable extent, and Tamiasciurus is 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 in Flicker\u00E2\u0080\u0094source holes, and these were disregarded in the analysis for competition for the same reasons as were the rare bird species discussed above. A note of caution is advisable in this regard. Local opinion was that Red Squirrel populations were \"low\" during the period of the study, and the possibility must be introduced that during population peaks of this species, a demand upon flicker-cycle holes could develop which might assume significant proportions in 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 is known of the natural history of Flying Squirrels in this area. In fact, many local persons were unaware that this species existed in the region. - 16 -Nothing is known of population levels, amplitude of fluctuations, or any other aspect of the population dynamics of this species, nor is 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 in 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 in this study. No bats were encountered during the two summers, and only one mouse was discovered in 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 locally that this f i l l i n g of holes is done by the Bushy-tailed Woodrat (Neotoma cinerea), usually called the \"Pack Rat\". At no time during the two year period of fi e l d work was any such debris deposited in nest sites under study. The \"Life History\" of Nest Sites Por most bird species, nest-sites are probably a measurable ecological parameter. This is perhaps most obvious for secondary hole-nesting species, but Stein (1958) and Mayfield (i960), to mention but two, have shown the measurable and relatively clearly defined nature of nest-sites for species of other ecological characteristics. Among the hole-nesting birds of this study, perhaps more than in most other cases, the \"life-history\" 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 Pir (Pseudotsuga menziesii) appears to be an important factor - 17 -in the functioning of the Large-Hole Cycle, and may well be a key factor in determining the population density of the Barrow's Goldeneye (Bucephala islandica). Equally clear is the differential role of Aspen (Populus tremuloides) trees in the Small- and Medium-Hole cycles, at least in 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 is clearly of great significance that the areas supporting this combination of Populus and Fomes are also the areas in 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 is suggested that this conditioning of trees, in the sense that heartwood is rendered softer and more readily excavated, is 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 is present, and in a l l nests studied, did not commence to enlarge the cavity until this decayed wood had been reached. The Flicker (Colaptes), on the other hand, rarely began excavation in sound wood, but usually utilized branch stubs, cracks, checks, \"cats-paws\" and other types of scars in which to commence excavation. The predominant feature seemed to be that holes were begun in locations at which decay had occurred in sapwood, thus providing softer wood with which to work* It is clearly significant that Sphyrapicus is a \"generalized\" woodpecker, while Colaptes shows various adaptations for ground-foraging, including a well-\u00C2\u00ABnarked curvature of the b i l l (Burt, 1930), It may be presumed that such a b i l l , primarily adapted for foraging, is 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 is 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 is essential to examine very carefully a l l factors related to the characteristics and availability of holes. Thus, i t is 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 is very clear for a long-term study of a series of fixed plots in order properly to appreciate such factors as rate of production of holes, longevity of individual sites, and the ways in 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 is 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 in soft, well decayed wood, although they do not always do so. Flickers (Colaptes) usually excavate a new hole each year, and were observed in five separate cases to excavate a second hole after expulsion from the original hole by Starlings (Sturnus vulgaris). The Pileated Woodpecker (Dryocopus pileatus) is said to excavate a new hole each year, as well as individual roost holes during winter (Hoyt, 1957). An important consideration is the durability of trees. Many aspen trees * Possibly of several species. - 19 -are in an advanced state of decadence when they become suitable for excavation by Colaptes, and are highly susceptible to breakage, windthrow, etc. This is in marked contrast to Douglas Fir \"snags\", which, even when dead, tend to remain standing for long periods. Thus a difference might be expected between climatic climax Douglas Fir stands and serai Aspen stands, in terms of hole longevity. In addition to breakage and windthrow, as mentioned above, holes tend to become unavailable in 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\u00C2\u00AB) tends to f i l l holes with . , sawdust-like material which results from the tunneling activity of the ants. In a single instance, an old hole in 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 is 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 in 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 is done, would act as a factor conditioning the environment, although i t is possible that actual strife could occur early in the season, when nest sites are being sought. Even this \"conditioning\", however, since i t is 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 is a factor which should be considered in 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 in the study area include 17 species of seven different families, divisible 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 in terras of nest sites. The importance of habitat is 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 Fir 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 briefly, and comments are offered on their role in 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 in general, in terms of factors influencing them. - 21 -ANALYSIS OP NEST-SITE COMPETITION The \"Cycles\" As noted above, the 13 common hole-nesting species in the study area may be placed in three natural ecological groupings in terms of nest-sites. These may, for convenience, be termed the Large-, Medium- and Small-Hole Cycles. It will 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 is discussed separately below. The Large-Hole Cycle This\" is 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 British 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). is comparatively rarely seen, and even less often found nesting. Munro and Cowan (1947) l i s t only two nesting records for British 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 in this study, only one nest was encountered, and even in that case, excavation was not completed. Another species, the Great Horned Owl (Bubo virginianus) is 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 in a hole; this was a natural cavity from a broken branch in 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 in studies involving the Barrow's * Goldeneye, remarkably few data are available regarding its nest-sites. Available \u00E2\u0080\u0094 M.F. Jackson, the University of British Columbia, has data on a number of natural sites of Barrow's Goldeneye. It is virtually 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 is an old Pileated Woodpecker hole. The Goldeneye, however, is one of the more abundant ducks of the study area, and its populations are exceedingly dense in 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\" site. Two factors in the nesting ecology of the Pileated Woodpecker, however, tend to explain this apparent anomaly. Firstly, each pair of Pileated Woodpeckers excavates a new cavity each year, as well as individual roost-holes each winter ( c f , Hoyt, 1957); secondly, in the study area, few trees except Douglas Fir (Pseudotsuga menziesii). are large enough to contain a Pileated Woodpecker nest, and this tree is extremely long-lived. Thus, the supply of available holes is far greater than might be directly indicated by the Dryocopus population at any given time. Additionally, Dryocopus pileatus is a large bird, and might be expected to live several years. J.S.Y. Hoyt (1950) gives an age of 9 years for a captive specimen. 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 in 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 possibility 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 possibility that many sites may be missed, particularly those high in stubs of Douglas Fir, and those at great distances from water. During recent years, the British 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 in 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 in 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 in transit from remote nests to brood lakes, might well be reduced to a significant extent. It is clearly indicated in the foregoing that the Large-Hole Cycle, although clearly defined in general terms, is too poorly known in detail to be considered here. Further studies are needed to elucidate possible competitive relationships. Present knowledge by no means precludes the occurrence of nest\u00E2\u0080\u0094site competition within this group, but data now in hand do not permit any great degree of generalization on this matter. The Small-Hole Cycle This group is composed of 5 species, of which the Tree Swallow (Iridoprocne bicolor), is the only form incapable of excavating its own cavity. Thus i t is dif f i c u l t to envision nest-site competition in this group. This situation is in direct contrast with the equivalent group in Europe, which contains many species of Paridae, Turdidae, Museicapidae, etc., few i f any of which have this ability. The hole-nesting avifauna of Europe is much richer in a l l three groups, although greater attention has been paid to the smaller passerines. No doubt i t is 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 in the study area i s , then, quite different from that found in Europe, and widely studied there. - 24 -The Medium-Hole, or \"Flicker\" Cycle For purposes of brevity, the medium-hole group will 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 in terms of hole sizes and observed species-site affinities. The group now under discussion includes 6 regular component species in the study area, among which only the Flicker (Colaptes) produces holes. The remaining 5 species are, as defined above, secondary hole-nesters. 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 is 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 in terms of the question, \"which factors tend to mitigate, and which to intensify, nest-site competition in this group of birds, a l l of which seem to share a single environmental resource?\" Ideally, each species would be considered in terms of a l l parameters so as to synthesize a \"theoretical ideal nest-site\" for that species, and these statistical entities compared. However, as will be seen in 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 variability in nest-site characteristics from;species to species. The alternative method of analysis, consideration of a l l species together, parameter by parameter, was undertaken, and the analysis is given below. Sample sizes vary from one parameter to another, as i t was not possible in many cases to secure values for a l l Parameters. To f o l l o w p . 24 5 5 < lO .. r- J~ j) cr U J LU Z > J O J J c p at f i 5 r oo oo oo o oo o o o o oo o oo oo ooo o o o o oo o . oo \u00E2\u0080\u00A2 o o o ooo o o o o oo o o ooo o oo o oo o ooo oo ooo o o ooo o oo oo o o ooo o oo \u00E2\u0080\u00A2 '\u00E2\u0080\u00A2' o o o o o o o o o* o o o oo \u00E2\u0080\u00A2 o oo \u00E2\u0080\u00A2 0 4 O \u00E2\u0080\u00A2 o IT) \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 o IT) \u00E2\u0080\u00A2 * CD O ' CD IT) O ff in Q vO in in o< OO o< oo \u00C2\u00AB1 \u00E2\u0080\u00A2 <3 \u00C2\u00AB < \u00C2\u00AB o o |o 0| o_ o o f o o o o o 0( o o o o o o o o o o o 0 o o o o o o o o o 2_2 l l j I I j H K \" L , , M , , N | , 0 , L P \" Q \" F L I C K E R - ^ S T A R L I N G - [ B L U E B I R D - Q \" u M V 1 o o o o o o o o o o o o o o o o b oi o 0 o A o o o o o o o IO 0| a B U F F L E H E A D - , T R E E SW.-R K E S T R E L -n p-q o o o o o o o\u00C2\u00B0o o 23 I J K L M N 0 P TQ\u00C2\u00BB R H E I G H T C L A S S E S o o ol u o o o K> o) o o o o K> \u00C2\u00B0 o |o o| o lo 0| , o o o o o at o 0 o , o (9, .91 n \u00E2\u0080\u00A260 \u00E2\u0080\u009455 j-45 r*35 7-30 I-2S E-20 \u00E2\u0080\u0094 15 \u00E2\u0080\u00A210 t-5 O To ToutouJ P. 2 7 OF HOLE D I R E C T I O N S (two mnvt%) - 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 in 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 in the range over approximately 4.5 metres were a greater proportion of Douglas f i r (Pseudotsuga menziesi) trees. It can be seen from both Table IV and Figure 7 that two \"humps\" occur in 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 is around the level of 1.75 metres, with a drop to f a i r l y low values in 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 is almost certain that these higher height classes are under-represented: l) vastly greater difficulty in locating under fi 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 level. Other measurements were not always obtained for such sites. 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 4 ra) nests appears to parallel the ratio of t a l l to low trees in 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 is a function of the availability of tree-trunk surface with appropriate decay conditions and facing a suitable flight 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 # Flicker Ht. Class Flicker Starling Bluebird Bufflehead Swallow Kestrel Source in Cm. No. No. 1\u00C2\u00B0 No. No. * No. No. 1\u00C2\u00B0 No. 1\u00C2\u00B0 A 0- 25 - - - - - - 1 0.9 1 3.1 - - 2 0.4 B 26- 50 1 1.0 - - - - -. - - - - 1 0.2 C 51- 75 3 3.0 2 1.2 1 2.3 1 0.9 1 3.1 - - 8 3.7 D 76-100 18 18.0 7 4.1 2 4.5 2 1.8 3 9.4 - - 32 6.6 E 101-125 13 13.0 21 12.3 10 22.7 4 3.7 5 15.6 2 8.3 55 11.5 F 126-150 10 10.0 24 14.1 6 13.6 10 9.3 8 25.0 - - 58 12.1 G 151-175 6 6.0 23 13.5 9 20.4 15 14.0 3 9.4 1 4.2 57 11.9 H 176-200 15 15.0 17 10.0 6 13.6 9 8.4 3 9.4 4 16.6 54 11.3 I 201-225 6. 6.0 17 10.0 3 6.8 9 8.4 1 3.1 \u00E2\u0080\u0094 \u00E2\u0080\u0094 36 7.5 J 226-250 3 3.0 12 7.0 2 4.5 8 7.5 3 9.4 3 12.5 31 6.5 K 251-275 3 3.0 7 4.1 1 2.3 8 7.5 2 6.2- 1 4.2 22 4.6 L 276-300 3 3.0 3 1.8 1 2.3 - - \u00E2\u0080\u0094 \u00E2\u0080\u0094 - - 7 1.5 M 301-325 7 7.0 8 4.7 - - 1 0.9 - - 1 4.2 17 3.5 N 326-350 4 4.0 2 1.2 \u00E2\u0080\u0094 - 2 1.8 1 3.1 \u00E2\u0080\u0094 - 9 1.2 0 351-375 2 2.0 5 2.9 - - 8 7.5 - - \u00E2\u0080\u0094 - 15 3.1 P 376-400 \u00E2\u0080\u0094 \u00E2\u0080\u0094 6 3.5 - - 1 0.9 - \u00E2\u0080\u0094 2 8.3 9 1.9 Q 401-425 \u00E2\u0080\u0094 - - \u00E2\u0080\u0094 \u00E2\u0080\u0094 \u00E2\u0080\u0094 \u00E2\u0080\u0094 \u00E2\u0080\u0094 - ' - \u00E2\u0080\u0094 - - -R 426-450 - - 4 2.4 \u00E2\u0080\u0094 - 1 0.9 - \u00E2\u0080\u0094 - - 5 1.1 S 451-475 2 2.0 - \u00E2\u0080\u0094 1 2.3 3 2.8 - \u00E2\u0080\u0094 1 4.2 7 1.5 T 476-500 1 1.0 1 0.5 - - 2 1.8 1 1.31 \u00E2\u0080\u0094 \u00E2\u0080\u0094 5 1.1 U 501-525 3 3.0 2 1.2 \u00E2\u0080\u0094 \u00E2\u0080\u0094 1 0.9 - - 1 4.2 7 1.5 V >525 2 2.0 9 5.2 2 4.5 21 18.6 ' - - 8 33.5 42 8.8 TOTALS 102 nests 170 nests 44 nests 107 nests 32 nests 24 nests 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 its inclusion is 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 in the archives of the Department of Zoology in the University of British Columbia, but for reasons of space economy, they are not presented here. Several factors can be seen, however, which could be of significance in possible nest site competition: l ) virtually 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$, of Sparrowhawk (Falco sparverius) nests occurred below 1.75 metres. Also, no Sparrowhawk nest below about 5 metres was known to be successful in fledging young; 3) other than these points, approximately 80$ of a l l species (Fl.-81$, St.-78$, Bluebird-93$, Buff.-63$, T.S.-90$, Kes-45.8$) were in the 50-300 cm. height range: 77$ of a l l nests in this cycle f e l l within the same range. Thus, except that Sialia 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 in terms of hole height. The departure of hole height distribution in two aspects from normal distribution is referable to the influence of tree growth-form. Entrance Size Clearly, entrance size is quite important to secondary hole-nesting species. A minimum size is imposed by the body size of the species involved, and maximum limits, although largely a function of available sites so far as is now known, may also be set by behavioural responses. The species of the three cycles are separable almost completely in this regard, - 31 -except for two regular exceptions. The Sparrowhawk (Falco sparverius) is 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 in several instances in British 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 in which entrance sizes could not be measured, and 10 for which complete measurements were obtained. The Tree Swallow (Iridoprocne bicolor), nested not only in a l l three cycles, but in many other situations as well. Figure 4 shows the relationship of Tree Swallow nests to some sites occupied by Flicker and Sapsucker in 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 in histogram form, and in Figures 10 to 14, horizontal diameters are plotted against vertical 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 is 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 in the range from 5.0 to 6.0 cm. This is not unexpected, as the Bufflehead is somewhat larger, particularly in girth, than the other species of this group (see Table II for length measurements). g H O R I Z O N T A L DIAM.(cm.) Q V E R T I C A L D I A M . (cm.) 1 5 \u00E2\u0080\u0094 5 -4 3 2. I t-9 O o o \u00E2\u0080\u0094[ 1\u00E2\u0080\u0094i 1\u00E2\u0080\u0094i 1\u00E2\u0080\u0094i 1\u00E2\u0080\u0094I 1\u00E2\u0080\u0094I 1\u00E2\u0080\u0094i ' i \u00E2\u0080\u0094 r 4.7 -5.( 5 . 2 - 5 . 6 5.1- 6 l 62 -6* . 6 6.7-7.1 7 \u00C2\u00A3 - 7.6 7.7-8.1 a 2 - 8 . 6 F I G . 8*. F L I C K E R : H O L E S I Z E S , 1 9 5 9 2 0 - , 1 5 -l o -> u z LLl a LLl CC U_ 5 -4 Z I 25 21 ^ H O R I Z O N T A L D I A M . (cm.) ( ^ V E R T I C A L DIAM.(c/v\.) 3 OVll vo.l 4 . 0 - 4 . . 4 . 1 - J . I 5 . 2 - 5 . 6 SI- 6.1 6 . 1 - 6 . 6 4 . 1 - 1 1 7 . 1 - 7 6 7 . 7 - 8 . 8-l~ S.m F I G 9 : S T A R L I N G ! H O L E S I Z E S , 1959 O o o I\u00E2\u0080\u00941 l I I HOLIS ovtt 5 5 vcwr. S I A M . t. m ;H \u00E2\u0080\u00A2 : : \u00E2\u0080\u00A2\u00C2\u00BB\u00E2\u0080\u00A2 \u00E2\u0080\u00A2\u00E2\u0080\u00A2 r .4 3 -\u00E2\u0080\u0094I 1 1 1 1 1 1 1 1 1 1 1 1 4.0 4.S 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 '0.0 H O R I Z O N T A L !b JAM ere* (CM) F I G . I O : E N T R A N C E S I Z E S : F L I C K E R s v c < u 3 5 n a.o \u00E2\u0080\u00A2 8 5 8 0 ' 7 . 5 7 . 0 \u00E2\u0080\u00A2 6.5 6o-5.5-5-0-AS \u00E2\u0080\u00A2 4.o \u00E2\u0080\u00A2 3.5-t 3 Hekts O V M 9.5 V l t t \u00C2\u00A9I A M . \u00E2\u0080\u00A2 a \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2 * \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2 mm* \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2\u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 r 4.o 4.5 S.0 5.5 6.0 6.S 7.0 7 5 8.0 8 5 9 0 3.5 l o o H O R I Z O N T A L ^ l A M t r v f t (tju) o o o F I G . I I : E N T R A N C E S I Z E S ! S T A R L I N G f 5 M\u00C2\u00AB\u00C2\u00ABTS o v i t IO.O CM- vtnT. DIAM-(INCL. 2 o v u lo . a e/M. H\u00C2\u00ABRt3. S I\u00C2\u00BBM.) \"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 \u00C2\u00A35 9.0 9.5 10.0 H O R I Z O N T A L DIAMCTII (cm-) FIG. 1 2 : E N T R A N C E S I Z E S : B L U E B I R D m M m 1 \u00C2\u00AB < u i c o -3 . 5 -3 . 0 -6 . 5 - \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 8 . 0 - \u00E2\u0080\u00A2 - \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 7-5 - \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2\u00E2\u0080\u00A2* 7 . 0 - \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 6 * . S - \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2\u00E2\u0080\u00A2 mm m mmm - \u00E2\u0080\u00A2\u00E2\u0080\u00A2\u00E2\u0080\u00A2 m mm mmm m \u00E2\u0080\u00A2 5 - 5 - \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 5-0 -4 - 5 -4 * 0 -i 1 1 1 1 1 4 . 0 4.5 5.0 5.5 6\0 6.5 \u00E2\u0080\u00A2 i 1 12 MISTS O V U IO-O C M VCKT. \u00C2\u00A9i A M . (INCL. 4> O V H IO.O e * . Hoi is. T 1 1 1\u00E2\u0080\u0094I 1 1 HORIZONTAL DIAMCTCR (CM.) i-3 O O o o V>4 FIG 13*. E N T R A N C E SIZES: B U F F L E H E A D - 32 -Vertical diameter exhibited somewhat greater variability than did horizontal diameter, particularly in terms of the upper limit. This tendency may well be imposed by the nature of the sites selected, as a considerable proportion (approximately 10$) of Flicker holes could s t i l l be seen, even when complete, to have been started in a vertical crack. S i l l Size Data on s i l l size are summarized in Table V and Figures 14 to 16. This parameter showed one of the most sharply defined ranges in values, with 94$ of a l l measurements in a sample of 436 falling within the range between 1.2 and 7.1 cm. 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) in Flicker-source sites are shown in Figure 16 and clearly follow the same pattern of s i l l size distribution. Figure 15 , however, is somewhat less clear: the data shown in 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 in Sturnus vulgaris to select somewhat thicker s i l l s , while the Sialia 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 Sialia. then, seems striking, but at present i t cannot be explained. A possible explanation would be competitive exclusion by Sturnus of Sialia from holes with thicker s i l l s : this is militated against, but not precluded, by the fact that 49.1$ of Sturnus nests also occur in the size range occupied by 81.8$ of Sialia nests. It would be most instructive to examine data on a series of Sialia nests To follow p. 32 S I L L S I Z E S I A I V I D I V 1 V 1 V 1 \u00C2\u00A3 F L . HOLES 447 NESTS a ' b ' c ' d 1 T FLIC ICES*-OCCUPIED 35 MKTS Cr- H ) i 1 1 1 1 1 r\u00E2\u0080\u00941 I \~ l.l 2.1 31 41 5.1 l.\ 7.1 8.1 31 10. I SIZE C L A S S E S OF S I L L S , C M . \u00E2\u0080\u00A2 3 5 % \u00E2\u0080\u00A2 3 0 \u00C2\u00B0 / 0 \u00E2\u0080\u00A2 2 5 % .20 Io .15 7* -/\u00C2\u00A3> % -5 X O \u00E2\u0080\u00A235% -3o % -25% -20% -/5 % E - 5 \u00C2\u00A3 - o F i g . 14 To follow p. 32 1 r T \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 r T \u00E2\u0080\u0094 i \u00E2\u0080\u0094 r F F L E H E AJ) 3 7 NUTS \u00E2\u0080\u0094 , , , 1 1 1 O M 2.1 VI 4.1 S.I 6.1 1.1 8.1 9.1 lot H\u00E2\u0080\u0094h 1-20 = - 5 % TREE 5>NALLow(ri . . -yoo\u00C2\u00ABct ) 33 N\u00C2\u00AB9TJ ~J 1 1 1 1 1 1\u00E2\u0080\u0094 l . l I .I 3.1 4.1 5.t 6.1 7.1 8 1 9.1 l o . l j f - 3 5 % r~ 3 \u00C2\u00B0 = - 2 0 = - l O -o FIG 15: SIZE CLASSES O F S I L L S , C M . To follow p. 32 SILL Sizes 0 I.I 2 \ 3.1 4.1 5,1 l.l 7.1 6.1 9.1 lo.l 1 i i i i i i i i i i / 6 7 NfSTS 1\u00E2\u0080\u0094i\u00E2\u0080\u0094r M T. E>Lo tS lRJ> 4 - 4 W E S T S * i 1 1 r 1. 1 - J A ' B ' C ' D ' E ' F ' S ' H ' I ' 3-0 I I 21 3-\ 4.1 5.1 6.1 7J 0.1 ft/ i o . / S I Z E C L A S S E S O F S I L L S *. C M . 35 % -3o '-25 -=-5 E-35 \u00C2\u00AB z-30 5-20 \u00E2\u0080\u0094 IO - 5 - O F i g . 16 TABLE V. SILL SIZES: FLICKER CYCLE SUMMARY (BOTH YEARS). Code Size Class Limits Flicker Starling Bufflehead Bluebird Tree Swallow Flicker No. $ No. $> No. % No. fo No. % No. $ A 0-1.1 cm. 1 1.1 4 2.4 3 3.1 3 6.8 5 15.2 16 3.7 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 2 4.6 5 15.2 77 17.6 F 5.2-6.1 cm. 3 3.3 22 13.1 4 4.1 2 4.6 2 6.2 33 7.6 Gr 6.2-7.1 cm. 7 7.4 10 5.9 4 4.1 1 2.3 2 6.2 24 5.5 H 7.2-8.1 cm. 2 2.2 5 2.9 1 1.0 - - 1 3.1 9 2.1 I 8.2-9.1 cm. \u00E2\u0080\u00A2- - 1 0.6 - - - - - - 1 .02 J 9.2-10.1 cm. - - 2 1.2 - - - - - - 2 .04 K 10.1 - - - - - - - - 1 3.1 1 .02 TOTALS 95 168 97 44 33 (Fl. Source 436 Only) - 34 \u00C2\u00AB from an area not occupied by Sturnus. Also, the curve for Sialia might change somewhat with enlargement of the sample: the nature and extent of such a change, i f in 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 in terms of this parameter, although partial competitive exclusion may be hinted at in this one species pair. Finally, i t is clear from Tabled and Figures 14 to 16 that, in general, there is very l i t t l e separation of species in 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 in a total of 446 Flicker-cycle nests occupied by Flicker, Starling, Mountain Bluebird, Bufflehead and Tree Swallow. Depths of 11 Kestrel (Falco sparverius) sites were also measured, but this small sample is not included in 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 in 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. in 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. in 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. in depth. - 35 -84,1 $ of Bluebird nests were less than 20 cm. The placing by secondary hole-nesters of nest material in the cavity is clearly a significant ecological factor in 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 Sialia or Iridoprocne. However, use of a site by Bluebird (Sialia) or Tree Swallow (Iridoprocne) renders a cavity virtually unsuitable for Starling or Bufflehead. 84.1$ of Bluebird and 69.4$ of Tree Swallow nests were less than 20 cm. in depth, and only 17.0$ of Starling and 3.8$ of Bufflehead nests were in this range. This definitely constitutes a significant factor in terms of competition, and would tend to reduce competitive pressure on Sialia and Iridoprocne. It could, at the same time, intensify competition between Bucephala and Sturnus; i t is important that Bucephala begins laying early in the spring almost before the arrival of Starlings in the area, incubates very closely, and has ceased to use the nest-site before the nesting of Sturnus is completed. Again, however, i t is quite clear that the requirements of a l l species in the Flicker cycle are very similar. Few cases of actual inter- or intra-specific strife 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 is described below, in suggested further lines of research. Cavity Sizes The final parameter related to nest-holes is that defined above as \"cavity\". It was thought that this could be an important feature, as i t is 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 in Table VII, TABLE VI. FREQUENCY DISTRIBUTION OF HOLE DEPTHS: FLICKER CYCLE. SPECIES DEPTH-CLASSES 0-5 5-10 10--15 15--20 20-25 25-30 30--35 35-40 40-45 45 No. No. $ No. No. No. No. No. % No. 1\u00C2\u00B0 No. No. * Flicker N a 95 - - - - - - - 5 5.3 34 35.6 28 29.5 17 17.9 6 6.3 5 5.3 Starling N = 65 - - - 3 1.8 25 15.2 44 26.8 42 25.4 30 18.2 13 7.9 1 0.6 7 3,3 Bluebird N = 44 10 22.7 11 25.0 7 15.9 9 20.5 3 6.8 1 2.3 1 2.3 1 2.3 0 - 1 2.3 Bufflehead N = 104 - - - - - 4 3.85 9 8.6 22 21.2 33 31,8 17 16.4 6 5.8 13 12.5 Tree Swallow N = 38 5 14.2 8 21.0 7 18.4 6 15.8 4 10.3 5 13,2 2 5.6 0 - 0 - 1 2.6 Flicker Cycle 15 3.4 19 4.5 17 3.8 44 9.6 65 14.6 104 23.4 94 21.0 48 10.8 13 3.0 27 6.0 % values refer to species. To follow p. 36 H O L E D E P T H S I N C M . O 5 I O 15 \u00C2\u00A3 0 25 ZO 35 AO 4 5 + T F L I C K E R M \u00C2\u00BB 9 5 T l\u00E2\u0080\u0094r T T T T T . I r : 3 2 28 ZA ZO 16 IO 6 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 r \u00C2\u00A3 FhlCKftft CYCLI N \u00E2\u0080\u00A2 4 4 \u00C2\u00AB Z$ ZO (6 M o 6 z% S T A C L I N O N \u00E2\u0080\u00A2 165 \u00E2\u0080\u00A230% \u00E2\u0080\u00A22 6 \u00E2\u0080\u00A22.0 : 1 6 \u00E2\u0080\u00A210 ; 6 : 2 % n \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 i \u00E2\u0080\u0094 r O 5 l o 15 2 0 25 3 0 35 4 o \u00E2\u0080\u00A2 *\u00C2\u00BB FIG. 17 H O L E D E P T H S FLICKER C Y C L E (I) To follow p. 36 2 u N \u00C2\u00BB - I \u00E2\u0080\u00A2X V < z -j M w z 0 3 I-2 H O L E DEATHS IN C M . O 5 10 15 2 0 ZS 3 0 3 5 <40 4 5 4 -T T T 1 r T \" \u00E2\u0080\u0094 r T T T T Bi-ufRif t I> N= -44 1\u00E2\u0080\u0094r BuVfUt HEAD 14 = I 0 4 1 1\u00E2\u0080\u0094r T 1 1 1 1 r M\u00C2\u00AB 3 8 \u00E2\u0080\u0094!\u00E2\u0080\u00941\u00E2\u0080\u00941\u00E2\u0080\u0094j\u00E2\u0080\u00941\u00E2\u0080\u00941\u00E2\u0080\u0094p\u00E2\u0080\u0094r O 5 IO 15 2 0 25 3 0 35 4 0 4 5 4-|36% S O '20 \u00E2\u0080\u00A2 to \u00E2\u0080\u00A2 6 = 3 6 % - 3 0 2 4 2 0 14 I o 6 36% I\u00E2\u0080\u0094 30 14 IO '\u00E2\u0080\u00A2 I * \u00E2\u0080\u00A2 1 o \u00E2\u0080\u00A2 6 : 1% F I G . I B H O L E D E P T H S F L I C K E R C Y C L E (u) TABLE VII. SIZE-CLASS DISTRIBUTIONS OP NEST CAVITIES: FLICKER CYCLE. SPECIES AND SIZE CLASSES: CM. SAMPLE SIZE 10 10--12 12-14 14-16 16--18 18-20 20--22 22--24 24--26 26 No. * No. % No. No. 1\u00C2\u00B0 No. % No. No. % No. * No. % No. % Flicker Cycle N = 446 27 6.0 83 18.6 96 21.5 68 15.2 56 12.7 36 8.2 42 9.8 15 3.4 14 3.2 9 2.0 Flicker (occ.) N = 95 6 6.3 17 17.9 19 20.0 17 17.9 8 8.4 11 11.6 9 9.5 - - 5 5.5 3 3.2 Starling N = 166 8 4.8 37 22.4 33 20.0 25 15.1 22 13.3 10 6.0 16 9.9 7 4.2 4 2.4 4 2.4 Bluebird N m 44 4 9.2 13 29,5 9 20,5 7 15.9 3 6.8 4 9.0 2 4.5 - - 2 4.5 -Bufflehead N = 107 4 3.7 12 11.2 25 23.4 _15 14.0 20 18.6 8 7.5 12 11.2 7 6,5 2 1.8 2 1.8 Tree Swallow 5 14.7 4 11.8 10 29.4 4 11.8 3 8.8 3 8.8 3 8.8 1 2.9 1 2.9 _ \u00E2\u0080\u0094 N = 34 - 38 -and shown graphically in Figures 19, 20, and 21. Several points are abundantly clear. 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 is occupied by roughly the same proportion of the sample for each species. In short, there seems to be no evidence in terms of this parameter to indicate any sort of amelioration of competition. Another obvious parameter is that of hole volume, which undoubtedly is 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 in volume will parallel those in hole depth, and since the latter parameter is analyzed above, a special analysis of hole volumes is redundant. Recapitulation: Analysis of Competitive Situation It has now been shown that the 13 common hole-nesting species of the study area are divisible into three natural groups, one of which shows no reason to assume the existence of nest site competition among its component species. Of the remaining two, the large-hole group is too poorly known to permit the forming of definite conclusions, although some qualitative suggestions have been offered. The final group, that composing of species utilizing nest-holes made by the Flicker (Colaptes), has been examined in considerable detail in terms of some measurable characteristics of nest-sites. It has become clear that, in general, the requirements of the various species, in terms of the parameters considered, show an impressive degree of overlap. A few cases exist in which some evidence is seen for amelioration of competition, but these are minor in comparison with the extent to which broad overlap occurs. Thus i t can be presumed on a somewhat more quantitative basis than is usually possible that competition as defined above may be occurring within the medium-hole group of hole-nesting birds in the study area. To follow p. 38 O Io V V * rg 3 i u z O vt III V in V> C V vO u \u00C2\u00ABM Id N LO V o h > \u00E2\u0080\u00A2 < 0 O V c n o m 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 or 14 u 3 \u00C2\u00AB0 m a U V u at \u00C2\u00AB X \u00C2\u00ABl J It I 1 \0 _ w v w .\u00C2\u00AB! M _ o _<0 .0 V V M -3 (VI .0 M -00 _o V 11111111[ 1111111111111'l 111 o m o in J T LU _i o >-u or LU ox y. LL in LU M cn > < To follow p. 38 X u 2 -*1 III (A In a j U ml H in >\u00E2\u0080\u00A2 \u00C2\u00BB-> < U o CM J=l. LU _ l U > u Q: LU u cn LU M cn > > < CZ. A V I T Y S i x m C m \u00C2\u00BB E S I N d h*. 107 i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094r Tk\u00C2\u00AB\u00C2\u00AB 5vw ALLOW 34 o -i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094i\u00E2\u0080\u0094r~ "Thesis/Dissertation"@en . "10.14288/1.0105473"@en . "eng"@en . "Zoology"@en . "Vancouver : University of British Columbia Library"@en . "University of British Columbia"@en . "For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use."@en . "Graduate"@en . "A preliminary study of nest-site competition in a group of hole-nesting birds"@en . "Text"@en . "http://hdl.handle.net/2429/38361"@en .