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Influence of brood-size on reproductive success of two species of cormorant, Phalacrocorax auritus &… Robertson, Ian 1971

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THE INFLUENCE OF BROOD-SIZE ON REPRODUCTIVE SUCCESS IN TWO SPECIES OF CORMORANT, Phalacrocorax auritus & P_. pelagicus, AND ITS RELATION TO THE PROBLEM OF CLUTCH-SIZE by IAN ROBERTSON B.A., Un ivers i ty of B r i t i s h Columbia, 1966 B . S c , Un iver s i ty of B r i t i s h Columbia, 1968 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department o r Zoology We accept th i s thes is as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA August, 1971 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h Co lumb ia , I a g ree tha t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s tudy . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d tha t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department The U n i v e r s i t y o f B r i t i s h Co lumbia Vancouver 8, Canada i i ABSTRACT Brood-rearing capab i l i t i e s and the i r re la t ion to the problem of c lutch-s i ze were investigated in two cormorant species, Phalacrocorax  auritus and P_. pelagicus, at Mandarte Is land, B r i t i s h Columbia. Extra chicks were added to normal sized broods (1 to 4 chicks) so that the range of brood-sizes studied was 1 to 8 in Double-crests (auritus) and 1 to 7 in Pelagics (pelagicus). Fledging success in Double-crests i s generally 90 per cent or higher in broods of up to 6 chicks. In Pelagics f ledging success drops sharply below 90 per cent in broods greater than 4. In no Pelagic nest did more than 4 chicks f ledge, whereas up to 7 chicks fledged in Double-crest nests. Growth rates in both species are lower in supernormal broods. However, th is decline amounts to only 12 per cent in Double-crest broods. It i s 35 per cent in Pelagic broods. There i s a strong re lat ionsh ip between mortal i ty and the low growth rates of certa in Pelagic chicks in supernormal broods. Slower growth in large broods is ref lected in a f ledging period which i s longer by several days in both species. The return to Mandarte Island of year l ing Double-crests ( in 1970) indicates a post-f ledging mortal i ty of approximately 50 per cent in normal broods and 47 per cent in supernormal broods, although the l a t t e r sample includes only one brood of 7. This s l i g h t survivorship difference i s not enough to of fset the reproductive advantage of Marge broods. Observation of feeding rates in both species indicates an increase in feeding t r ip s with increasing brood-size which is less than i i i proport ional. As this does not appear to be enough to explain the a b i l i t y of parent Double-crests to feed supernormal broods i t was hypothesized that these parents return with more food. Several other aspects of feed-ing frequency were examined and found consistent with th i s hypothesis. Study of the success of indiv idual feedings indicated no tendency for a higher success level in normal broods. The s t ra in on parents ra i s ing large broods in both species was shown by the decreasing nest - s i te attend-ance as broods get larger and as the chicks grow older. The high brood-rearing capab i l i t i e s of the Double-crests were d i s -cussed and two problems on which there was l i t t l e or no data were ra ised. It was concluded that Double-crest parents do not endanger the i r future survivorship in order to raise supernormal broods, although data is poor on this question. A possible breakdown in the feeding of broods greater than 5 may occur under certa in conditions. The d i f f e r i n g brood-rearing capab i l i t i e s of the two species were tentat ive ly explained in terms of the i r population status, i . e . , growing versus s tab le , with the Double-crests the growing population. This idea corresponds with one raised by Lack (1965) who has suggested a corre lat ion between higher than normal brood-rearing capab i l i t i e s and a growing population. As such, the findings are consistent with Lack's c lutch-s i ze hypothesis. The occur-rence of most of the nest l ing mortal i ty in the f i r s t two weeks of the nest l ing period suggest factors other than food may l i m i t brood-rearing success. iv TABLE OF.CONTENTS Page LIST OF TABLES v LIST OF FIGURES vi Section INTRODUCTION 1 Egg laying and Incubation . 4 MATERIALS AND METHODS 6 RESULTS 8 Fledging success and Chick Production . . . . 8 Growth Rates . 1 3 Length of Fledging Period 20 Post-f ledging Survival . . . 22 Feeding Rates , . 26 Nest-s i te Attendance . 3 5 DISCUSSION 39 LITERATURE CITED 45 V LIST OF TABLES Table Page 1. The Frequency of C lutch-s izes, the Hatching Success, and the Frequency of the Brood-sizes at Hatching in the Pelagic and Double-crested Cormorants 5 2. Number of Broods studied during the 1969 and 1970 F ie ld Seasons 8 3. Chick Morta l i ty in Double-crested and Pelagic Cormorants related to Age and Brood-size 12 4. Growth rates of Individual Chicks, measured in grams per day increase, ranked from Fastest to Slowest Chick 18 5. Morta l i ty of Pelagic Cormorant chicks correlated with Growth Rate 19 6. Weights of chicks from Normal and Supernormal Broods late in the i r F i r s t Week 35 vi LIST OF FIGURES Figure Page 1. The influence of brood-size on fledging success and chick production in the Double-crested and Pelagic Cormorants 10 2. Growth curves from normal broods of Double-crested Cormorant (upper l ine) and the Pelagic Cormorant (lower l ine) 14 3. The influence of brood-size on growth rate in Double-crested and Pelagic Cormorants . 16 4. The influence of brood-size on age at fledging in Double-crested Cormorants 21 5. The influence of brood-size on age at f ledging in Pelagic> Cormorants 21 6. F i r s t year survivorship (upper l i ne ) and progeny a l i ve at one year (lower l ine) in Double-crests as a function of brood-size at f ledging 25 7. Daily feeding t r ip s as a percentage of fourth week feeding t r i p s in Double-crested and Pelagic Cormorants 28 8. Daily brood growth as a function of brood-size in Double-crested and Pelagic Cormorants 28 9. Daily number of feeding t r ip s as a function of brood-s ize in Double-crested and Pelagic-Cormorants 30 10. Daily number of feeding t r ip s per chick as a function of brood-size in Double-crested and Pelagic Cormorants 30 11... .The. influence ..of. brood-size on the number of indiv idual feedings per feeding t r i p in Double-crested and Pelagic,Cormorants 33 12. The influence of brood-size on the number of feedings per day per chick in Double-crested and Pelagic Cormorants 33 v i i Figure Page 13. Frequency of unsuccessful feedings as a function of age in normal and supernormal broods of the Double-crested Cormorant 34 14. Nest-s i te attendance in the Double-crested Cormorant as a function of age and s ize of brood . . . . 3 7 15. Nest-s i te attendance in the Pelagic Cormorant as a function of brood-size 38 ACKNOWLEDGMENTS I am deeply indebted to my research advisor, Dr. R. H. Drent, whose advice and encouragement were a con-stant insp i rat ion during the course of th is study. Dr. J . F. Bendell and Dr. J ; M. Taylor kindly read the manu-s c r i p t and offered many helpful suggestions. For permission to carry out the study at Mandarte Island I am thankful to the East Saanich band of Indians. On the is land my work load was eased considerably by the help of a number of colleagues and research ass i stants. These include A. F. Koelink, J . 0. Anvik, N. A i tch i son, J . 6. Ward, B. A. Henderson, and C. Inkster. To the Inksters and Mrs. R. Mathews of Sidney I wish to extend my appreciation for the i r ho sp i t a l i t y . This study was supported by a Canadian W i l d l i f e Service Research contract and by National Research Council Grants to.Dr. Drent. THE INFLUENCE OF BROOD-SIZE ON REPRODUCTIVE SUCCESS IN TWO SPECIES OF CORMORANT, Phalacrocorax auritus AND P. pelagicus, AND ITS RELATION TO THE PROBLEM OF CLUTCH-SIZE INTRODUCTION This study investigates the role played by brood-size in the reproductive success of birds which feed the i r young (nidicolous b i rd s ) . The purpose of th i s inquiry is to gain understanding of the re la t ion between c lu tch- s i ze and brood-rearing success at various brood-sizes, including experimentally enlarged broods. According to Lack's theory, the broodrsize contr ibut ing the most progeny to the breeding population should correspond to the most common c lutch-s i ze (Lack, 1954). Presum-ably, th is i s the resu l t of se lect ion for the c lutch-s ize which gives r i se to the most reproducing of f spr ing. The theory predicts that broods larger than normal, e i ther a r t i f i c i a l l y enlarged or natura l , w i l l not produce more reproducing progeny than that produced by the normal brood-s i z e ^ ) . This means that even i f the large broods produce more fledged offspr ing than normal, post-fledging mortal i ty w i l l be higher in the former thus el iminat ing the or ig ina l advantage of the large broods. This i s precise ly what Lack (1948) found in his study of the S t a r l i n g , Sturnus vu lgar i s . In the Great T i t , Parus major, the large broods pro-duce' even fewer progeny than"normal'broods (Lack, Gibb, and Owen, 1957). Only in the Glaucous-winged G u l l , Larus glaucescens, is there, evidence that supernormal broods do not suf fer a higher post-fledging mortal i ty than normal (Vermeer, 1963). Owing to the d i f f i c u l t i e s in obtaining r e l i ab l e estimates of post-f ledging survival , studies on the c lutch-s i ze problem have been largely re s t r i c ted to the incubation and nest l ing periods. Most of these studies 1 2 have shown that the brood-size producing the most fledged offspr ing cor-responds to the most common c lu t ch - s i ze , even when a sample of broods i s experimentally enlarged (Rice and Kenyon, 1962; Nelson, 1966a; Huntington, in Lack, 1966; Har r i s , 1966; and Summers, 1970). The large broods suffer abnormally high nest l ing losses thus confirming the pre-dict ions of Lack's theory. In studies Tike these where the compensatory mortal i ty takes place at the nest l ing stage estimates of post-f ledging survival are less c r i t i c a l in assessing the potential reproductive value of large broods. In cases where supernormal broods produce more fledged offspr ing than normal (Vermeer, 1963; Nelson, 1964; and Harris and Plump, 1965), i t i s important to have an estimate of post-f ledging morta l i ty . Unfortunately, i t i s only avai lable for the Glaucous-winged Gull (Vermeer, 1963). Without such information i t i s impossible to preclude the p o s s i b i l i t y that fledged offspr ing from supernormal broods may suffer a higher mortal i ty in the i r f i r s t year, pa r t i cu l a r l y i f they fledge l i gh te r than normal. To avoid th i s def ic iency, considerable e f f o r t has been taken in th is study to obtain r e l i ab l e estimates of post-f ledging surv ival in the Double-crested Cormorant, Phalacrocorax aur i tus , as i t i s capable of rearing more offspr ing than normal. The examples of brood-rearing capab i l i t i e s greater than predicted have been attr ibuted by Lack (1965) to special conditions s i g n i f i c an t l y d i f fe rent from those ex i s t ing when the c lutch-s ize evolved. In the case of the Glaucous-winged Gull at Mandarte Island Lack (1965) has suggested that the added food resources provided by nearby garbage dumps may be an important factor . S im i l a r l y , the a b i l i t y of Bass Rock Gannets, Sula 3 bassana, to raise supernormal broods (Nelson, 1964) may be correlated with the recovery of th is colony from e a r l i e r depredations by man and the concomitant average annual growth rate of 3 per cent (Nelson, 1966b). In both cases there is the suggestion that the re lat ion between the numbers of breeding birds and the carrying capacity of the environment has been altered to the advantage of the present b i rds. Such an advantage might explain the successful ra i s ing of supernormal broods. An equally favorable s i tuat ion seems to be inf luencing the Double-crested Cormorant population at Mandarte Island and for th is reason i t was i n i t i a l l y chosen for study. This species has only been recorded in the Georgia Strait-Puget Sound area within the la s t f i f t y years (Drent et a l . , 1964). Recently, i t has grown from 140 occupied nests in 1959 to 440 in 1971. This amounts to an average annual increase of 10 per cent. When the study was o r i g i na l l y conceived i t was hoped that the Pelagic Cormorant, P. pelagicus, at Mandarte Island would provide the contrast of a stable population. However, recent census figures (1969 to 1971) indicate a 35 per cent increase in breeding pairs over the estimate of 370 to 380 pairs in I960.' Since much of th i s increase has taken place within the l a s t three years i t may be l inked with a recent 50 per cent decline in a nearby colony. Thus, part of the increase at Mandarte can probably be explained by immigration. The purpose of com-paring growing and more or less stable populations i s to test i f the corre lat ion between superior brood-rearing capab i l i t i e s and a growing population has any b io log ica l meaning. 4 To answer some of the questions raised above measurements were made of the fol lowing parameters of reproductive success: f ledging success, growth rate of the chicks, age at f ledg ing, post-f ledging sur-v iva l . f eed i ng rates , "and nest-s ite 'attendance by the parents at the various brood-sizes (1 to 8 in Double-crests, and 1 to 7 in Pelag ics ) . In add i t ion, some information on the egg-laying and incubation phases of the reproductive cycle is presented because i t i s new information for the two species and because i t re f lec t s on the general problem of c lu tch - s i ze . Egg-laying and Incubation Clutch-s ize may be influenced by egg-laying capacity, incubation capacity, f ledging success and post-f ledging su rv i va l . Although this study focuses i t s attention on the l a s t two inf luences, b r i e f considera-t ion i s given here to the egg-laying and incubation periods. The frequency of the c lu tch - s i ze s , the hatching success, and the frequency of the brood-sizes at hatching for both Pelagic and Double-crested Cormorants are shown in Table 1. These figures do not include eggs lo s t during the egg-laying period. This means that 5 or 6 eggs may be l a i d to complete a clutch of 3 or 4. The laying of 6 eggs consecu-t i v e l y , one egg every other day, is the maximum that I have recorded by any female for e i ther species. Bent (1922) claims that clutches of 7 eggs have been found in both species, but he does not state i f only one female l a i d these eggs. In two cases, 7 eggs and 8 eggs were l a i d con-secutively but in neither case was I pos i t ive that the same female was 5 Table 1 The Frequency of C lutch-s izes , the Hatching Success, and the Frequency of the Brood-sizes at Hatching in the Pelagic and Double-crested Cormorants Pe" agic Cormorant Dou Die--crested Cormorant 0 1 2 3 4 5 0 1 2 3 4 5 C lutch- s i ze * 14 33 5 17 81 11 % Hatching success 80.5 75.8 66.7 71.4 82.7 73.3 Freq. of brood-size at hatch 1 3 11 22 10 1 3 5 11 30 37 -The c lutch-s i ze i s the number of eggs present in the nest when egg-laying terminates, van Tets included the eggs los t during egg-laying in his tabulation of c lutch-s ize (Drent, van Tets et a l . , 1964). responsible for the ent i re c lu tch . Additional eggs may be l a i d i f the ent i re clutch i s lost.during incubation. Thus, a female may lay up to 11 eggs in one breeding season (van Tets in Drent et a l . , 1964). From th is there seems l i t t l e reason to'suppose that c lutch-s ize i s l imited by egg-laying capab i l i t i e s in e i ther species. A pecul iar feature o f ' the hatching success is the i n a b i l i t y of e i ther -species, pa r t i cu l a r l y the Double-crest, to hatch a l l eggs in a c lutch of 5. Double-crest parents were unable to produce any broods of 5 from the 11 clutches of 5. In contrast, of 58 clutches of 4 in which hatching success i s known, 28 pairs successful ly hatched a l l 4 eggs for a.difference which i s s t a t i s t i c a l l y s i gn i f i can t (P < .05). Why a clutch of 5 presents special problems for Double-crests and presumably for 6 Pelagics as well i s not known. It i s possible that the frequency of i n -f e r t i l e eggs r i ses markedly when more than 4 eggs are l a i d . In th is regard i t i s interest ing that the frequency of copulations f a l l s of f noticeably a f ter two eggs are l a i d (C. Inkster, pers. comm.). I t i s also possible that the f i f t h egg does not receive s u f f i c i en t warmth. Cormorants do not have the brood patches which might l i m i t incubation capacity in some species (Rice and Kenyon, 1962). Instead, they incubate the i r eggs between the top of the i r feet and the feathers of the i r breast and be l l y . The maximum number of eggs which can be successful ly incubated in this way is not known. In 1970 3 a r t i f i c i a l l y enlarged clutches of 6 were set up. In one nest a l l 6 eggs hatched-successfully,, but the other two nests hatched only' 2 and 3/eggs respect ively. Unfor-tunately, th is sample is too small to permit conclusions. However, the Gannet which incubates a s ingle egg"between i t s feet can successful ly incubate two eggs without any reduction in hatching success (Nelson, 1964). Although i t would seem a doubtful l im i t i n g fac to r , more informa-t ion is required to determine whether or not c lutch-s i ze in Cormorants could be l imited by incubating capacity. MATERIALS' AND: METHODS The study was conducted at Mandarte Island during the spring and summer of 1969 and 1970. A detai led description of Mandarte Island i s found in Drent et a l . (1964). Study areas were established where the reproductive a c t i v i t i e s of 7 the birds could be eas i l y observed. Brood manipulations were performed only when chicks were in the i r f i r s t week as parent birds might recog-nize strange chicks in the i r nest and neglect them. An attempt was made to have a l l chicks moved to foreign nests so that none would be raised in the i r .home nest. This was successful with the Pelagics, but several Double-crest chicks were raised in the i r home nest. This has no af fect on morta l i t y , as mortal i ty in each group was i d e n t i c a l . Growth studies 'were performed during the 1970 f i e l d season. Chicks were marked ind iv idua lTy ' to d ist inguish ' them both for weighings and for determining when each chick fledged. Spring-balances with ranges of 0 to 500, 0 to 1000, and 0 to .2500 were employed. The accuracy of each scale was w i th in 1 per cent. The weighings we re'-terrain a ted when -the^okiest ch icks . in a given colony reached 30 to 35 days o ld . By this age chicks are s u f f i c i e n t l y mobile to scramble away from the prospective weighers, but are vulnerable to getting lo s t in a strange part of the colony or to f a l l i n g down the c l i f f s . For th is reason very few f ledging weights were obtained. Direct measurement of food intake'was not attempted. Food i s s e l -dom observed since the chicks are fed while the i r head is thrust down the throat of the parent. With these l im i t a t i on s , feeding rates were estimated by making extended observations to record the number of feeding : t r i p s and indiv idual feedings per day for the various brood-sizes. During these observations I noticed that many feedings were not success-f u l , w e . , the chick received l i t t l e or no food. As a re su l t , an attempt was made to dist inguish; successful from unsuccessful feedings. 8 This seemed a worthwhile approach since the success in ra i s ing super-normal broods could be l imited by the d i f f i c u l t i e s posed by feeding a large brood as well as by the food supply. A successful feeding is i n -dicated i f any: of" the following" character ist ics^are" observed: a distended throat as food passes down the oesophagus of the chick; swallowing motions; and, cessation of high intens i ty begging. Usually a l l these phenomena are observed during and fol lowing a successful feeding. A feeding was scored as unsuccessful i f none of the above character i s t ic s was observed. RESULTS Fledging success and Chick" Production The number of nests studied in each season i s shown by Table 2. No differences in survival were apparent between the two seasons, .thus the data have been combined. Table 2 Number of Broods studied during the 1969 and 1970 F ie ld Seasons Pel agic Double--crest Normal (1-4) Supernormal (5-7) Normal (1-4) Supernormal (5-8) 1969 17 5 42 28 1970 20 14 36 13 9 In normal broods of both species the fledging success is very high (Figure 1). The 95.6 per cent fledging success of Double-crest chicks in these broods compares very favorably with the 95 per cent re-ported by van Tets ( in Drent et a l . , 1964).However, the 93.6 per cent (91.6 per cent " in 1969, 95.2 per cent in 1970) f ledging success in Pe l ag i c ch i ck s i s s i g n i f i c an t l y higher than van Tets 1 e a r l i e r f igure.of 76.2 per cent ( loc . c i t . ) . Snow (1960) has reported a s im i l a r yearly v a r i a b i l i t y in f ledging success in the Shag, Phalacrocorax a r i s t o t e l i s . The very high nest l ing surv ival within the normal-sized broods indicates that there i s v i r t u a l l y no difference whether a chick is raised in a brood of one or four. As the modal c lutch-s i ze in each species is four th i s i s not an unexpected'result; However, the boobies, Sula dactylatra and S_. leucogaster, commonly lay two eggs, yet very rarely raise more than one chick (Nelson, 1966a). Differences between the two species ar i se in the i r responses to supernormal broods (Figure 1). The a b i l i t y of parent Double-crests to raise supernormal broods increases chick production from a maximum of* 3.5 chicks fledged per pair at a brood-size of 4 (b/4) to approximately 5.5 chicks. Once th i s production level i s attained at b/6 the addition of s t i l l more chicks does not resu l t in increased production. Thus, b/6 appears to be the most productive brood-size in this species. Other sea-birds in which the most productive brood-size at f ledging i s greater than the most common c lutch-s i ze include the Glaucous-winged Gull (Vermeer, 1963), the Gannet (Nelson, 1964), and the Lesser Black-backed Gull (Har r i sand Plump, 1965). This phenomenon has also been recorded 10 Brood size Figure 1. The inf luence of brood-size on f ledging success and chick production in the Double-crested and Pelagic Cormorants. Sample s i ze (broods) of the two species i s ind icated. 11 in several passerine species (see Lack, 1949; von Haartman, 1954; Tutor, 1962; and Tyrvainen, 1969) and one raptor ia l species (Cave, 1968). The addition of extra chicks to Pelagic Cormorant broods does not resu l t in increased chick production as i t does in the Double-crest. Instead, chick production suffers a s l i gh t decrease (Figure 1). This i s a less dras t ic resu l t than that of other studies in which the addition of an extra chick frequently resulted in the loss of the ent i re brood (Rice and Kenyon, 1962; Har r i s , 1966; and Summers, 1970). Of the 19 supernormal Pelagic broods established only one brood suffered to ta l morta l i ty . Normally 3 or.4 chicks in these large broods were able to remain healthy throughout the nest l ing period and fledge successful ly. However, none of these supernormal broods was successful in ra i s ing more than 4 chicks. In contrast, Double-crests were able to raise up to 7 chicks "(2 nests out of 9). In two exceptional Pelagic nests 5 chicks fledged, but at least one chick in each nest had moved to a neighbouring nest at least three weeks pr io r to f ledging. In each case the adopted chicks were accepted and fed by the foster parents. Morta l i ty during the nest l ing period generally occurs during the f i r s t two" weeks (Table 3); Most chicks in normal broods which survive the f i r s t two weeks manage to fledge successful ly. One of the effects of supernormal broods is to increase this early morta l i ty . In Double-crest supernormal broods mortal i ty is generally re s t r i c ted to th i s early period, except for the two pecul iar instances ind icated, in Table 3. Howeverj the large Pelagic(broods ' suffer .a s i gn i f i can t mortal i ty un t i l the fourth and f i f t h weeks. This difference i s consistent with d i rect 12 Table 3 Chick Morta l i ty in Double-crested and Pelagic Cormorants related to Age and Brood-size Pelagic Cormorant Double- crested Cormorant Brood-s ize 1 2 3 4 5 6 7 .. 1 2 3 4 5 6 7 8 No. of chicks 8 28 15 40 25 72 14 10 50 72 76 55 102 63 32 Week 1 - 1 1 2 1 9 - - 1 2 2 3 5 1 3 2 - - - 3 8 1 - 1 - 1 3 4 5 7 3 - - - 1 1 11 3 - - 3 (8)* -4 - - - 2 2 4 - - - 1 -5 - - - 1 1 - - 1 - - - -6 - - - 1 - - - - - - - - -7 1 - - - - - -* The bracketed numbers indicate unusual morta l i ty : one chick in b/3 disappeared fol lowing a shooting inc ident; f i ve chicks in b/7 were k i l l e d by neighbouring adults fol lowing the death or desertion of one of the parents. observations made during the weighings that the degree of starvation is much greater in Pelagic-chicks than Double-crest chicks. Underweight Pelagic chicks generally d ied, whereas underweight Double-crest chicks managed to obtain s u f f i c i en t food to f ledge. A bias in Table 3 ought to be pointed out. . Cormorant-chicks are c losely brooded un t i l t he i r second week and are thus d i f f i c u l t to see. As a resu l t i t was not always possible to determine exactly when the. early mor ta l i t i e s took place. Therefore i t i s l i k e l y that 1some f i r s t week morta l i t ie s are shown as occurring in the second week. 13 In the section on growth rates further evidence i s presented on possible causes for chick morta l i ty . Growth Rates The pattern ofgrowth in Double-crested and Pelagic Cormorants, l i k e that of many b i rds , i s described by a sigmoid curve (Figure 2). This means that there are three phases in the growth period. From hatch-ing (day 0) to day 4 or 5 weight increase is slow. After t h i s , there is an increase in the growth rate so that at 8 days old (day 8) the maximum growth rate i s atta ined. For Double-crests and Pelagics these growth rates are approximately 100 grams per day and 60 grams per day respec-t i v e l y . This maximum growth rate is maintained in both species u n t i l , about day 25 when the rate of weight increase begins to decl ine. From day 25 to f ledging weight increment i s slow, and most of the energy i s taken up by feather growth, increased a c t i v i t y , and the higher costs of body maintenance. Weights of chicks' older than 30 days were d i f f i c u l t to obtain! This explains why f ledging weights are.shown only for the Double-crest (Figure 2). However, Snow (1960) includes weights of chicks up to 50 days old in the growth curve of the Shag. This growth curve has been included in Figure 2 since the Shag and the Pelagic Cormorant are c losely related and appear to atta in a s im i l a r weight. The nature of the Shag growth curve indicates that weighings of Pelagic chicks in th is study were terminated ju s t p r i o r to the asymptote. Considering the paucity of f ledging weights the best way to ure 2. Growth curves from normal broods of Double-crested Cormorant (upper l i ne ) and the Pelagic Cormorant (lower L ine) . Snow's (I960) data on the Shag are included. Curves f i t t e d by eye. 15 analyze the influence of brood-size on growth rate involves an analysis of the period of maximum o r a r i t h m e t i c growth (day 8 to day 25). This method i s s im i l a r to that employed by Spaans (1970). It provides an average growth rate for the per iod, and re f lect s the degree to which parents can f ind s u f f i c i en t food for the i r chicks. Thus, i t i s assumed that a lower .average growth rate for a par t i cu la r sample of chicks re-f l e c t s less food fed to those chicks. However, a slower growth rate need not necessari ly i n fe r a lower asymptote. It could be that the growth period i s prolonged in the slower growing chicks. In Double-crest broods the growth rates of the chicks at the var-ious brood-sizes are reasonably s im i l a r , considering the varying food requirements of these broods (Figure 3). Although the decline in growth at b/6 probably ref lects, the d i f f i c u l t i e s of feeding so many chicks, th i s decline represents only a 12 per cent reduction when compared to the average growth rates, in normal broods. Due to a small sample of birds weighed in b/1 (both chicks were probably males), th is estimate may be abnormally large. A contrasting picture i s presented by the Pelagic Cormorant broods. In th i s species growth rate declines rather sharply in broods greater than 4. This decline amounts to 36 per cent when the average growth rate in b/6 and b/7 i s compared to that of normal broods. Considering the poor f ledging success of chicks in these large broods, starvation must be considered an important contributor to chick morta l i ty . The role of starvation in supernormal broods i s easier to grasp when attention is directed to the indiv idual chicks rather than the 16 120-1 100-^ 80 5 60 £ 40 20 DCC o i l PEL ~I T I 1 1 1 1 1 1 2 3 - 4 5 6 7 8 Brood—- size Figure 3. The influence of brood-size on growth rate in Double-crested and Pelagic Cormorants. Ninety-five per cent confidence intervals are shown, except in two instances where only one sample was involved. 17 whole brood (Table 4). The growth rate of chicks ranked f i f t h or lower in supernormal broods is much less than in chicks from normal broods. For instance, Pelagic chicks ranked f i f t h and s ixth grow at a rate 60 per cent lower than normal chicks. In s im i l a r l y ranked Double-crest chicks the decline i s only 25 per cent. The l a t t e r decline i s not suf-f i c i e n t to produce a sharp r i se in morta l i t y , whereas the greater growth rate decline in Pelagics is correlated with increased morta l i ty . Unfortunately, the data are not s u f f i c i e n t l y comparable to give an ind icat ion whether e i ther species has greater a b i l i t y to withstand s tarvat ion. The influence of growth rate on mortal i ty in Pelagic chicks is indicated in Table 5. Comparing survivorship in chicks with a growth rate above 50 grams per day and those below, there is a difference which i s s t a t i s t i c a l l y s i gn i f i can t (P < .05). There are some inconsistencies in th i s re lat ionsh ip which demand some explanation. F i r s t , 5 of these chicks had an overal l growth rate in excess of 50 grams per day and s t i l l died. However, when the growth rates of these chicks in the two or three days p r io r to mortal i ty are analyzed 3 were experiencing l i t t l e or no growth. There were 2 chicks in one nest which did maintain a high growth rate. As a l l chicks in th is nest perished (b/7) i t i s possible that one of the parents e i ther died or deserted the nest exposing the brood to attack from neighbouring adults. The second apparent incons is-tency involves the survival of a few chicks whose overal l growth rate was very low. The chicks in th i s category grew slowly in supernormal broods un t i l one or two of the i r s ib l ings d ied, a f ter which time the i r own Table 4 Growth rates of Individual Chicks, measured in grams per day increase, ranked from Fastest to Slowest Chick Brood-size Pelagic Cormorant Double-crested Cormorant 1 2 3 4 5 6 7 1 2 3 4; 5 6 * 1 - 60.84 68.22 66.68 64.02 61.45 55.55 64 .88 108.30 106720 101.41 104.29 107.54• 99.07 * 2 , . 60.26 62473 62.53 56.13 50.28 55.68 92.32 93.03 93.67 99.98 95.30 * 3 57.32 58.15 49.74 46.57 55.30 87.87 89.18 96.06 93.32 * 4 55.45 47.68 36.47 43.18 82.86 86.90 91.35 * 5 30.90 27.72 29.27 73.991 74.97 * 6 18.49 15.13 67.33 * 7 13.01 Sample s i ze 4 6 4 5 5 4 2 2 3 6 3 5 2 Growth of indiv idual chicks, ranked from fastest to slowest. 19 growth rate increased. The use of overal l growth rates, l i k e those in the present analys i s , tend to mask such changes in the growth rate within the period studied. Table 5 Morta l i ty of Pelagic Cormorant chicks correlated with Growth Rate Growth rate (gms./day) Normal Broods Supernormal Broods No. of chicks Died No. of chicks Died 70.0 + 5 0 2 0 60.0 - 70.0 24 0 5 1 50.0 - 60.0 15 0 20 4 40.0 - 50.0 0 0 17 3 30.0 - 40.0 0 0 6 3 20.0 - 30.0 0 0 5 4 10.0 - 20.0 0 0 3 3 0.0 - 10.0 0 0 5 5 It i s also worthwhile to point out an unmistakable tendency for Pelagic chicks in even the f i r s t four positions in supernormal broods to experience a s l i g h t l y lower growth rate than normal. This means that the disadvantages accruing to supernormal broods in Pelagics extend to some degree at least to a l l chicks. The data avai lable shows no s im i la r ten-dency in the Double-crests. There is no ind icat ion in e i ther species that the younger chicks 20 were suffer ing the greater morta l i ty . The average age of chicks ranked f i f t h and lower averaged only one day younger than the oldest in each brood. Within-brood age differences of up to 4 days occur in nature. Length of Fledging Period Fledging behavior,in the two species is very s im i l a r , and the ob-vious differences seem related to t he i r d i f fe rent nesting habitats , i . e . , Pelagics nest on steep c l i f f s and Double-crests nest on broad rocky shoulders. Double-crest chicks s ta r t to explore and congregate into roosts on these shoulders about 10 days to one week before f ledging. The nests on steep c l i f f s preclude any extensive exploration or congre-gation of Pelagic chicks un t i l a few days before f ledging. At this time in both species f l y i n g is re s t r i c ted to short awkward hops and consider-able wing-f lapping. The f i r s t extended f l i g h t is usually simultaneous with the f i r s t t r i p to water.. At Mandarte the Double-crests accomplish th i s by making short hops down the gradual slope to the water 's edge. However, on two occasions f ledgl ings were observed to f l y d i r e c t l y to water for the i r f i r s t bathe. Fledgling Pelagics generally f l y to the water. Following t h e i r ' . f i r s t swim f ledgl ings of both species return to the nest and continue to be fed for several weeks, although they spend an increasing amount of time away from the nest. Since the feathers and feet of chicks in the nest are heavily stained with guano, f ledgl ings returning from the i r f i r s t bathe are clean, and thus are quickly i d e n t i f i e d . At th is time they are considered fledged, although f u l l independence is not achieved un t i l sometime l a te r . 21 60 _?55 Q> <0 <_ 50 CD 45 2 3 4 5 Brood — size Figure 4. The influence of brood-size on age at f ledging in Double-crested Cormorants. The range of results and 95 per cent confidence interva l s are included. . 60 •8 55-- 50i CD 45" 1 2 3 4 Brood — size 5+ Figure 5. The influence of brood-size on age at f ledging in Pelagic Cormorants. The range of results and 95 per cent confidence interva l s are included. 22 By making frequent t r ip s to the colonies, I was able to determine the exact time of f ledging of the i nd i v idua l l y marked chicks. The purpose of estimating the time of f ledging i s to f ind out i f chicks in larger broods take longer to f ledge, since they grow at a slower rate. The results show that they do, although the difference is only a matter of several days (Figures 4 and 5). Since no Pelagic broods fledged more than four chicks the f ledging age of the survivors of super-normal broods i s plotted separately. Although he was only dealing with two brood-sizes Nelson (1964) found a s i gn i f i can t di f ference in the age at f ledging between single and twinned broods of the Gannet. This i s consistent with, the growth rates of chicks both in th is study and Nelson's which show that there i s a tendency for growth to decline with increasing brood-size. It i s quite c lear then that .longer f ledging periods must re-f l e c t slower growth. However, there is no evidence that a s l i g h t l y longer f ledging period i s deleterious to survivorship. Only during the egg stage and the f i r s t week af ter hatching does predation influence survivorship. Post-f ledging Survival I t has already been stated that estimates of post-f ledging survival are c r i t i c a l for species which can successful ly raise supernormal broods, as the Double-crests do. S l i gh t l y slower growth rates and greater age at f ledging indicate that the supernormal Double-crest broods experience some d i f f i c u l t i e s , even though they fledge successful ly. I f th is i s the case, and there are few fledging ^eights^£^rMy.Jthisi a higher pbs't-f ledging mortal i ty in these broods should be expected. Lack (1948) and 23 Perrins (1965) have shown that heavier f ledgl ings have a higher chance of su rv i va l . Although study of the sub-adult phase in seabirds i s generally d i f f i c u l t i t i s fortunate that the year l ing (non-breeding) cormorants at Mandarte Island return and are seen in large numbers about the breeding colonies. Due mostly to differences i n habitat, accurate estimates of f i r s t year survivorship are possible only in the Double-crest. The c l i f f nesting habitat of the Pelagics makes observation d i f f i c u l t oyer exten-sive areas. Of 45 Pelagic chicks color banded in 1969 only 3 were seen the fol lowing year. Since a 93 per cent mortal i ty i s very un l ike ly in th is species, many surviving yearl ings were undoubtedly missed. In con-t r a s t , most of the roosting s i tes of the Double-crests are above the c l i f f s on more open and f l a t t e r r a i n , where they can be eas i l y observed. On these roosts the sub-adult birds can be c lea r l y distinguished from the black adults by the i r mottled brown plumage. The accuracy of the f i r s t year survivorship estimates ;is substan-t ia ted by the observations of van Tets (Robertson and van Tets, in prep.), who found that of the 90 banded Double-crest chicks of the 1957 cohort which were known to have survived at least one year, only 3 were missed during the summer of 1958. Two of these were found breeding in l a te r years and the other was found dead in June, 1958, about 25 miles from Mandarte Island. During the 1969 f i e l d season, over .100 Double-crest chicks from the various brood-sizes were banded with i nd i v idua l l y recognizable combin-ations of color bands. The fol lowing spring and summer was spent looking 24 for the survivors from this sample. The return of these yearl ings pro-vides an estimate of the influence of brood-size on post-f ledging sur-v iva l (Figure 6). With the exception of b/7, which represents the returns from only one nest, there is l i t t l e tendency for post-f ledging mortal i ty to increase with increasing brood-size. Thus,, the reproductive advantage of the large broods does not appear to be of fset by higher post-f ledging losses. Since the most c r i t i c a l mortal i ty occurs in the f i r s t few months a f ter f ledg ing, i t i s doubtful i f brood-size w i l l have any influence on second year morta l i ty . Thus, the survivorship in th i s sample ought to. be equivalent in each brood-size un t i l the indiv iduals j o i n the breeding population, when they are two or three years o ld . The conclusion from these results i s that supernormal broods in the Double-c res t , at least b/5 to b/7, can contribute more progeny to the breeding population. The bias of these results i s that they relate- 'to only one sample or cohort of chicks. Conditions a f fect ing the survivorship of th is cohort might have been unusually favorable. Potts (1969) has found that survivorship in the Shag may be highly variable from year to year. Two other attempts have been made to assess the influence of brood-s ize on post-f ledging survival and both have employed supernormal broods (Vermeer, 1963; and Ward, Ph.D. thesis in prep.). These studies of the Glaucous-winged Gull population from Mandarte Island have, involved obser-vations of f i r s t year birds at roosting s i tes and garbage dumps mostly in and around Vancouver, B .C . As these birds disperse to other areas as w e l l , neither estimate, represents a complete return of a l l surviving 25 Figure 6. F i r s t year survivorship (upper l i ne ) and progeny a l i ve at one year (lower l ine) in Double-crests as a function of brood-s ize at f ledg ing. Sample of chicks banded in 1969 i s shown. 26 year l ings. Vermeer found the percentage of yearl ings observed per brood-s ize increased with increasing brood-size, so that the recovery rate in normal broods (b/1 to b/3) was only 6 per cent, compared to 13 per cent in supernormal broods (b/4 to b/6). These results seem highly unusual, and Ward has found contradictory evidence. To date, the recovery rate from normal broods i s 30 per cent, and in supernormal broods 15 per cent. Feeding Rates There are several ways in which parent birds can respond to the food demands of a varying number of nestlings.. F i r s t , they can increase the da i l y number of feeding t r i p s as the number of nestl ings increases. Second, they may increase the quantity of food brought back to the nest with each feeding t r i p . Van Dobben (1952) has shown that the amount of food brought back to the nest by the European Cormorant, Phalacrocorax  carbo, may vary from 200 grams to 750 grams. Thus, there i s no reason to expect a l l feeding t r i p s to represent equivalent quantit ies of food. The th i rd p o s s i b i l i t y i s that parental feeding e f for t s w i l l increase with increasing brood-size only within the normal range of brood-sizes. Any increase in brood-size greater than this w i l l not be compensated for by greater feeding e f f o r t by the parents. Thus, the nestl ings w i l l be under-nourished. To test these and other p o s s i b i l i t i e s d i rect observations of broods were carr ied out to measure feeding frequency. Each unit of obser-vation was one complete day, approximately from 0430 hours to 2230 hours. In add i t ion, the influence of brood-size and age of brood on the success 27 of indiv idual feedings was measured. The quantity of food brought back on each feeding t r i p and the amount of food consumed by indiv idual nestl ings are d i f f i c u l t data to obtain in cormorants and are absent from th i s study. The manner in which cormorant chicks are fed i s not described here. This subject has been adequately covered by van Dobben (1952) and van Tets (1959). Feeding frequency in cormorants has two recognizable components: t r i p s to the feeding grounds and the feeding of indiv idual chicks. During incubation, the number of da i ly feeding t r i p s involves a normal pattern of 3 t r ip s in Double-crests (12 nests) and 2 t r ip s in Pelagics (4 nests). When the chicks hatch the frequency of these t r ip s increases un t i l a peak i s reached during the fourth week. I f the number of da i ly feeding t r i p s in the fourth week i s considered un i ty , then the frequency of t r i p s in the weeks thereafter i s generally within 10 per cent of th i s amount (Figure 7). Thus,; for purposes of analysis much of the feeding data from the fourth week on are combined. A r e l a t i v e l y constant feeding rate from the fourth week on i s consistent with the avai lable evidence on food requirements in chicks of related species. This evidence shows1 that food requirements reach a plateau by the fourth week, with the chance of a s l i g h t decline jus t pr io r to fledging (du P l e s s i s , 1957; and Kahl, 1962). An assumption of Lack's c lutch-s i ze hypothesis i s that feeding the most common brood-size represents the maximum e f fo r t which parent birds can summon. Assuming that th i s e f f o r t is proportional to the da i ly 28 o o 16 t Vi 100-5CH 0 o DCC A PEL 2 3 4 5 6 Age in weeks 8 Figure 7. Daily feeding t r i p s as a percentage of fourth week feeding t r i p s in Double-crested and Pelagic Cormorants. 6 00 i •S 300 t3 O I DCC — i 1 1— 3 4 5 Brood — size ~r 7 8 Figure 8. Dai ly brood growth as a function of brood-size in Double-crested and Pelagic Cormorants. 29 weight gain per brood, Pelagic.Cormorants do not increase the i r e f for t s for broods greater than 4 (Figure 8). Additional feeding e f for t s are character i s t i c of Double-crest parents faced with ra i s ing a supernormal brood. There i s a tendency for th i s e f f o r t to reach a maximum with a brood growth of 500 to 550 grams per day, which represents an increase of 35 per cent over the level at b/4. The one brood of 8 which survived did achieve a growth rate in excess of 600 grams per day in the two weeks pr io r to reaching 25 days o ld . However, th i s level of a c t i v i t y is un-l i k e l y to extend generally to a l l Double-crest parents. I t i s possible that part of the apparent increased e f f o r t re f lect s reduced food require-ments due to the energy conservation properties of a large brood (Royama, 1966; and Mertens, 1969). However, th i s phenomenon may be less app l i c -able to such a large species. As i t nests on rocks exposed to the sun, cooling i s probably a more important physiological problem for the chicks. I f the addit ional e f for t s of parent Double-crests i s to increase the number of feeding t r i p s then the re lat ionship between brood-size and da i ly number of feeding t r i p s should be proport ional. Figures 9 and 10, which include data from broods in t he i r fourth week and older, show that this is. not the case. A proportional response would mean that the number of da i ly t r ip s per chick would be constant, i . e . , a s t ra ight l i n e . Figure 10 shows that i t i s not. I t i s interest ing that Pelagic, adults make a s im i l a r number of feeding t r i p s , considering the poor survival of the i r supernormal broods. This less than proportional re lat ionship be-tween brood-size and da i l y feeding t r i p s i s s im i l a r to that found by Harris (1966) in his twinning experiments- with the Manx Shearwater, 30 Brood — size Figure 9. Dai ly number of feeding t r i p s as a function of brood-size in Double-crested and Pelagic Cormorants. Data taken from broods in t he i r fourth week and older. Sample s i ze (nest days of • . observation)--74 in Double-crests; 26 in Pelagics. CD LL C H 1 1 1 1 1 1 1 1 2 3 4 5 6 7 Brood — size Figure 10. Daily number of feeding t r i p s per chick as a function of brood-size in Double-crested and Pelagic Cormorants. Data from same sample as Figure 9. 31 Puffinus puff inus, and by other researchers in passerines (Lackand S i l v a , 1949; Kendeigh, 1952; von Haartman, 1954; and Seel , 1969). In both species the average maximum number of feeding t r ip s per day i s 8 or 9. However, da i l y feeding t r i p s of 10 or more were recorded in Double-crests on 6 separate occasions, 5 of them in supernormal broods. The absence of s im i l a r l y busy days in Pelagics is probably explained by the f a i l u r e of any broods of more than 4 to survive a f ter the fourth week. The results in both species indicate only a pa r t i a l adjustment in the frequency of feeding t r i p s to increasing brood-size. Since Double-crests successful ly raise supernormal broods, part of the i r increased e f f o r t must be expended by other means. One l i k e l y a l ternat ive is that the average amount of food brought back to the nest per feeding t r i p i s greater in the larger broods. While d i rec t evidence for th i s i s lacking I have frequently observed the parents of supernormal broods so f u l l of f i sh that the i r necks were v i s i b l y d i s -tended. Such adaptive behavior could conserve energy. I f during the course of a day parent birds took an extra 50 grams of food per feeding t r i p , one 300 to 400 gram feeding t r i p would be saved. Such a response to a large brood would be d i f fe rent to that described by Royama (1966). He found that in large broods (b/13) of the Great T i t , Parus majofr-the number of feeding t r i p s increased but that the s ize of the food items de-creased. Such d i f fe rent feeding strategies could be explained i f the Double-crests have an abundant food supply. This way, searching time and energy would be very low and i t would be advantageous for an adult Double-crest to return with a large load of f i s h . I f the food supply were 32 somewhat less than abundant as i t may be in the Great T i t , then searching time and energy might be higher. Thus, there might be an advantage to return to the nest with the f i r s t item captured even i f i t may be smaller. I f , as suggested above, the parents of larger broods return with more food, each feeding t r i p should give r i se to more successful feedings as brood-size increases. In Double-crests, the data avai lable supports th i s hypothesis (Figure 11). An alternate explanation could be that the greater number of feedings per feeding t r i p might r e f l e c t the parents tendency to feed a l l begging chicks. I t could do th i s -by reducing the rat ion in each feeding. In spite of the greater number of feedings per t r i p there i s a tendency for the da i l y feedings per chick in both species to decrease as brood-size increases (Figure 12). Since the decline i s of greater magni-tude than that for weight increment, there must be a tendency for parents in large broods to pass more food to the chicks at each feeding. This could resu l t from the rather intensive and competitive begging that con-fronts a parent returning to a large brood. In such conditions there is a tendency for parents to give a large feed to a few chicks, rather than attempt to feed the whole brood a smaller port ion. The success or f a i l u re of indiv idual feedings,was carefu l l y observed to f ind out i f brood-size or age influences the feeding of nest-l i ng s . In Double-crests the results give c lear ind icat ion that feeding becomes more successful as the chicks get older (Figure 13). Pelagic Cor-morants show a s im i l a r response but the sample of observations i s too small for inc lus ion here. (The high f a i l u r e rate in feeding may give 33 4-, »DCC \ •8 IS 2 -PEL 0 " T " 2 ~T~ 3 4 5 6 Figure 11. The influence of brood-size on the number of ind iv idual feedings per feeding t r i p in Double-crested and Pelagic Cormorants. Data from same sample as Figure 12. Figure 12. The influence of brood-size on the number of feedings per day per chick in Double-crested and Pelagic Cormorants. Sample s ize (nest-days of observation) in parentheses. 34 Figure 13. Frequency of unsuccessful feedings as a function of age in normal and supernormal broods of the Double-crested Cormorant. 35 some clue to the high nest l ing losses in the f i r s t two weeks, although i t does not explain the higher proportion of f a t a l i t i e s of supernormal chicks at th is stage.) There i s no evidence that larger broods inter fere with feeding success in the early weeks. In f a c t , at th is stage a s i g -n i f i c a n t l y higher proportion of f a i l e d feedings occurs in normal broods. Such results jnight be explained i f hunger i s a pos i t ive influence on feeding success. Growth data obtained during the f i r s t week reveals weight differences related to brood-size (Table 6). These differences would produce d i f fe rent hunger levels but the influence of hunger on feeding success remains-conjectural. Table 6 Weights of chicks from Normal and Supernormal Broods late in the i r F i r s t Week Age Weight ( in grams) ( in days) Normal Supernormal 5 125 (14)* 108 (3) 6 185 (18) .145 (12) 7 224 (15) 190 (12) Number of chicks in each group in brackets. Some general problems concerning the feeding of large broods are raised in the Discussion. Nest-s ite Attendance During the incubation period and the f i r s t week of brooding in both species the nest bowl i s so well hidden by the attending adult that 36 i t i s d i f f i c u l t to observe the eggs or newly hatched chicks. This pro-tect ive behavior of the brooding adult i s relaxed only during the second week of the nest l ing period. As the nest l ing period continues, other changes occur in the nest - s i te behavior of the adults. The time when both parents are on the nest - s i te together decl ines, and the occurrence of unattended nests goes up. The decline in nest - s i te attendance is re-lated to the increasing age of the chicks and the number of chicks in the nest (Figures 14 and 15). Although the data for the two species is pre-sented somewhat d i f f e ren t l y low nest - s i te attendance occurs at smaller broods in the Pelagics. But even at normal brood-sizes of the Double-c re s t j pa r t i cu l a r l y b/4, nests are frequently l e f t unattended. Unattended nests are vulnerable to occupation by strange adults. This ismuch more pronounced in the Double-crest. Only two occurrences of strange Pelagic adults occupying a nest were recorded. When a strange Double-crest adult occupies a nest i t fur ious ly pecks at the resident chicks so that they leave the nest or crouch within i t . Chicks which attempt the l a t t e r are ult imately driven o f f . In large supernormal broods the occupying b i rd i s occasionally driven off by the resident chicks. Potent ia l l y such occupations could contribute to mortal i ty i f the chicks were s t i l l f a i r l y young. However, th is has never occurred during this study. A few chicks have been k i l l e d by adults but in a l l cases one of the parents had e i ther died or deserted the nest. Such decreasing attentiveness on the part of parents i s an unmis-takable sign of the i r increasing burden. Nelson (1964) noted a s im i la r response in the"Gannet, but he did not state i f such unattended chicks 37 Figure 14. Nest -s i te attendance in the Double-crested Cormorant as a function of age and s ize of brood ( indicated at each l i n e ) . Nest -s i te attendance i s measured as some f rac t ion of both parents (2) on the nest. Sample indicated in tab le . 38 2i S V) 0"l 1 1 i 1 : — i 1 2 3 4 5 Brood size Figure 15. Nest -s i te attendance in the Pelagic Cormorant as a function of brood-size. Data taken from broods in t he i r fourth week and older. Sample s i ze : b/1-3; b/2-3; b/3-2; b/4-6; b/5-3. 39 were subject to attack by neighbouring adults. However, survivorship of twins was high, thus such attacks could not have had an influence on survivorship. DISCUSSION Although the Double-crested and Pelagic Cormorants commonly lay a clutch of 4, the addition of extra chicks creating supernormal broods has revealed d i f fe rent brood-rearing capab i l i t i e s in the two species. The most productive brood-size in the former species i s 6, whereas i t i s only 4 in the l a t t e r . Thus, brood-rearing capab i l i t i e s in Double-crests are at variance with the theory that the most productive brood-size corres-ponds to the most common c lutch-s i ze (Lack, 1954). Whtle the addit ion of extra chicks introduces the advantage of increased reproductive po ten t i a l , i t also results in a number of disad-vantages. In Pelagic Cormorants the most s t r i k i ng consequence of the large broods is reduced chick growth and increased nest l ing morta l i ty . These effects are more than s u f f i c i en t to of fset the i n i t i a l reproductive advantage of supernormal broods. In Double-crests the disadvantages of: supernormal broods are less apparent. Fledging success and growth rate are somewhat reduced but not so much as to cancel the or ig ina l increase. In add i t ion, post-f ledging mortal i ty from one sample is approximately equivalent for a l l brood-sizes up to 6. This indicates that broods of 5 and.6, and perhaps 7 as w e l l , can contribute more progeny to the breeding population than the normal broods do. While the results of post-fledging 40 mortal i ty are based on only one sample, i t i s noteworthy that preliminary returns of the 1970 cohort show that the highest survival rate i s found in birds from broods of 5. Two addit ional disadvantages of large broods are worth discussing. The f i r s t i s the influence of brood-rearing on the parents, pa r t i cu l a r l y those confronted with an a r t i f i c i a l l y enlarged brood. ^It seems obvious that i f parent birds make no increased e f f o r t to raise extra ch icks, the l a t t e r w i l l starve and the survivorship of the parents.wi l l not d i f f e r from those parents ra i s ing normal broods. When extra ef fort s are made, independent of chick s u r v i va l , there i s the chance that the future survivorship and reproductive potential of these parents w i l l be reduced (Will iams, 1966). When Harris (1966) weighed a sample of parents from twinned and control (single) broods of the Manx Shearwater he found no appreciable difference in the weights of the two groups, even though twinned broods were receiving approximately 20 per cent more feeding t r i p s . What appears to have happened in th is case is that extra e f for t s to feed the twins were l imited at that point when parental survivorship might be endangered. Harris concluded that th is strategy would be advantageous to a long- l ived species l i k e the Manx Shearwater, which can return to breed for many years thereafter. Presumably, such a strategy ought to apply to the Mandarte Island cormorants since they survive to 14 years at l eas t , and have an adult l i f e expectancy of 4 years (Robertson and van Tets, in prep.). Thus, i t is un l ike ly that parent Double-crests, which can raise large broods, would expend such an e f f o r t to feed chicks that the i r own future survivorship would be threatened. 41 On this pa r t i cu la r point the information from the Mandarte Island Double-crests is scanty. No adult weights were obtained, but the f i ve banded birds which raised supernormal broods in 1969 a l l returned to breed in 1970. This survivorship compares with the 81 per cent survival obtained for the rest of that sample. The second possible disadvantage of supernormal broods arises from observations of feeding frequency. When broods are small and the i r chicks are young the parent routinely feeds the begging chicks in turn. As the chicks get larger they s ta r t to pose a physical challenge to the parents, at least when they pester the parent for food. Also, as the brood-size gets larger i t becomes increasingly d i f f i c u l t for the adult a r r i v ing from a feeding t r i p to i so la te and feed in turn each chick from i t s brood, pa r t i cu l a r l y when this hungry brood i s mobbing i t . The term i so la te is used advisedly. Parent cormorants appear to respond to the begging of only one chick at a time. In large broods where this frequently i s not poss ib le, the feeding of indiv idual chicks becomes a d i f f i c u l t procedure. From observation on at least two supernormal broods (1 of b/6; 1 of b/8) I suspect that these d i f f i c u l t i e s in ter fere with the successful d i s t r i bu t i on of the food the parents have ava i lab le. Not su rpr i s ing ly , the one brood of 8 which survived was fed under these conditions from the age of 3 weeks on. Interest ing ly, th is brood experienced v i r t u a l l y no weight gain from day 25 to day 45, and had a fledging weight some 500 to 800 grams below the normal weight. For th is and other reasons mentioned above, broods greater than 6 appear to o f fer l i t t l e reproductive advantage to Double-crests. 42 I t remains to explain the possible reasons for the d i s s im i l a r brood-rearing capab i l i t i e s of the two species and to ask i f the Double-crest results const itute a va l id contradict ion of Lack's c lutch- s i ze hypothesis. The d i f f i c u l t y involves postulating an explanation for the high Double-crest product iv i ty which excludes the Pelagics from the same inf luence. It i s worthwhile to state that the present results are un-l i k e l y to r e f l e c t a seasonal v a r i a b i l i t y in the resources u t i l i z e d by the two species. Including supplementary data from the 1971 f i e l d season, f ledging success in both species has remained remarkably constant for three consecutive years. Although our knowledge of the two populations is l im i t ed , one difference i s f a i r l y s t r i k i n g . The Double-crest population has been growing rather rap id l y , whereas the Pelagic population has been growing slowly or not at a l l excluding immigration. For th i s reason, one can assume that the Double-crest population has not reached i t s asymptote. I f so, the combination of factors which w i l l eventually l i m i t this popu-la t ion i s not yet f u l l y operative. This means that there is probably an excess of avai lable resources, and this could explain the r e l a t i v e l y high success ;of supernormal broods in Double-crests. Such an explanation has already been proposed by Lack (1965, 1966) as a modification to his theory. In view of the starvation experienced in supernormal broods of Pelagics, food appears to be the resource which might explain the d i f f e r -ences between the two species. However, too l i t t l e i s understood about the feeding ecology of the two species to permit a more conclusive answer. There i s evidence that factors other than food might l i m i t 43 reproductive output in the two species. I t i s c lear that food a v a i l a b i l -i t y cannot explain the high losses which occur during the f i r s t two weeks .of the nest l ing period. V i r t u a l l y a l l of the nest l ing losses in normal broods take place at th is stage. Whatever the factors are which contribute to th i s mor ta l i t y , the i r influence i s exacerbated in super-normal broods. This i s why most of the supernormal broods of 7 and 8 were frequently reduced by 1 or 2 chicks before the end of the second week. In Double-crests th is morta l i ty does not continue, but in Pelagics i t does in the large broods. Snow (1960) found the same early mortal i ty in the Shag and b r i e f l y reviewed the possible factors . She mentioned the mechanical d i f f i c u l t i e s ,of brooding and feeding a chick so much smaller than the adult as the most l i k e l y cause. The s ign i f icance of th i s early morta l i ty is, that i t l im i t s the number of large chicks which the parents w i l l have to feed. This means that i t i s fa r better for some chicks to die ear ly in the nest l ing period i f food i s l im i t i n g than for the parents to feed an ent i re brood inade-quately. This i s the argument used by Lack (1954) to explain the s i g n i -ficance of the asynchronous hatch in b i rds\ and why the youngest in these broods frequently d ie . From this i t seems that brood-rearing success can only be p a r t i a l l y explained in terms of food a v a i l a b i l i t y . Clear ly the factors causing the early mortal i ty play a s i gn i f i can t ro le . LITERATURE CITED 45 LITERATURE CITED Bent, A. C. 1922. L i f e h i s tor ies of north american petrels and pelicans and the i r a l l i e s . U. S. Nat. Mus. B u l l . 121. Cave, A. J . 1968. The breeding of the Kes t re l , Falco tinnunculus L., in the reclaimed area Oostel i jk Flevoland. Neth. J . Zool. 18:313-407. Dobben, W. H. van. 1952. The food of the Cormorant in the Netherlands. Ardea 40:1-63. Drent, R. H., G. F. van Tets, F. Tompa, and K. Vermeer. 1964. The breeding birds of Mandarte Is land, B r i t i s h Columbia. Canad. F i e l d -Nat. 78:208-263. du P l e s s i s , S. S. 1957. Growth and da i ly food intake of the White-breasted Cormorant (Phalacrocorax carbo) in c ap t i v i t y . Ostrich 28: 197-201. Haartman, L. von. 1954. Der Trauerfliegenschnapper. I I I . Die Nahrungsbiologie. Acta. Zool. Fenn. 83:1-96. Harr i s , M. P. 1966. Breeding biology of the Manx Shearwater, Puffinus  puff inus. Ibis 108:17-40. • and W. J . Plump.. 1965. Experiments on the a b i l i t y of Herring Gulls and Lesser Black-backed Gulls to raise larger than normal broods. Ibis 107:256-257. Kahl, M. P h i l i p , J r . 1962. Bioenergetics of growth in nest l ing Wood Storks. Condor 64:169-183. Kendeigh, S. C. 1952. Parental care and i t s evolution in b i rds . 111. B i o l . Mon. 22:1-342. Lack, D. 1948. Natura l - se lect ion and family s ize in the Star l ing.-Evolution 2:95-110. _ 1954. The natural regulation of santmal numbers,."" Tfl'arendon Press* Oxford. . 1965. Evolutionary Ecology. J . Anim. Ec. 34:223-231. . 1966. Population studies of b i rds. Univ. Press, Oxford. , J . Gibb, and D. F. Owen. 1957. Survival in re lat ion to brood-s ize in t i t s . Pro. Zool. SocLondon 128:313-326. 46 Lack, D. and E. T. Si lva.. 1949. The weight of nest l ing Robins. Ibis 91:64-78. Mertens, J . A. L. 1969. The influence of brood-size on the energy metabolism and water loss of nest l ing Great T i t s , Parus major major. l"i Ibis 111:11-16. Nelson, J . B. 1964. Factors inf luencing c lutch-s i ze and chick growth in the North A t l an t i c Gannet, Sula bassana. Ibis 106:63-77. . 1966a. Clutch-s ize in the Sulidae. Nature 210:435-436. . 1966b. Population dynamics of the Gannet, Sula bassana, at the Bass Rock, with comparative information from other Sulidae. J . Anim. Ec. 35:443-470. Perr ins , C. M. 1965. Population f luctuat ions and c lutch-s i ze in the Great T i t , Parus major L. J . Anim. Ec. 34:601-647. Potts, G. R. 1969. The influence of eruptive movements, age, popula-t ion s i ze and other factors on the survival of the Shag, Phalacrocorax  a r i s t o t e l i s L. J . Anim. Ec. 38:53-102. Rice, D. W. and K. W. Kenyon. 1962. Breeding cycles and behavior of Laysan and Black-footed Albatrosses. Auk 79:517-567. Robertson, I. and G. F. van Tets. 1971. Aspects of the population dynamics of the Double-crested Cormorant, Phalacrocorax auritus at Mandarte Is land, B. C. In prep. Royama, T. 1966. Factors governing feeding rate, food requirement and brood-size of nest l ing Great T i t s , Parus major. Ibis 108:313-347. Seel, D. C. 1969. Food, feeding rates and body temperature in the nest-l i n g House Sparrow, Passer domesticus at Oxford. Ibis 111:36-47. Snow, B. 1960. The breeding biology of the Shag, Phalacrocorax  a r i s t o t e l i s , on the is land of Lundy, B r i s t o l Channel. Ibis 102: 554-575. Spaans, A. L. 1970. On the feeding ecology of the Herring Gull (Larus  argentatus Pont.) in the northern part of the Netherlands. Unpub-l i shed Ph.D. Thesis, Univ. of Leiden. Summers, K. 1970. Brood-rearing studies on the Rhinoceros Auklet, Cerorhinca monocerata, at Cleland Is land, B. C. Unpublished Ms. Tets, G. F. van. 1959. A comparative study of the reproductive behavior and natural history of three sympatric species of Cormorant (Phala- crocorax aur i tus , P_. pen i c j l l a t u s , and P_. pelagicus) at Mandarte Is land, B. C. Unpublished M.A. Thesis, Univ. B r i t . Col . 47 Tutor, B. M. 1962. Nesting studies of the Boat-ta i led Grackle. Auk 79:77-84. Tyrvainen, H. 1969. The breeding biology of the Redwing (Turdus i l i a cu s L.). Ann. Zool. Fenn. 6:1-46. Vermeer, K. 1963. The breeding ecology of the Glaucous-winged Gull (Larus glaucescens) on Mandarte Is land, B. C. Occas. Papers B r i t . Col. Prov. Museum B:1-104. Ward, J . G. 1971. Studies on the feeding ecology and breeding success of the Glaucous-winged Gull (Larus glaucescens) at Mandarte Is land, B. C. Ph.D. Thesis, in prep., Univ. B r i t . Col . Wil l iams, G. C. 1966. Natural se lec t ion , the costs of reproduction, and a refinement of Lack's p r i n c i p l e . Am. Nat. 100:687-690. 

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