Egg Size and Early Nestling Growth in the Snow Petrel ’

The Condor91:345-35 I 0 The CooperOrnithological Society 1995

EGG SIZE AND EARLY NESTLING GROWTH IN THE SNOW PETREL ’

TROND AMUNDSEN Department of Zoology, Universityof Trondheim, N-7055, Dragvoll, Norway

Abstract. I studiedegg size variation, and the influenceof eggsize on early nestling growth,in SnowPetrels Pagodroma nivea breedingat Svarthamaren,Dronning Maud Iand, Antarctica(71 ”53’S, 5”lO’E). Egg sizes ranged from 36.4 to 52.1 cm3, with a mean of 44.9 cm). Hatching occurredduring 16-24 January,with a median hatching date of 20 January. Egg size had a significant effect on the body mass of hatchlings, explaining 30% of the variation in body massof nestlingshatched within the last 24 hr, and 581 of the body mass variation of nestlingsweighed while still slightly wet. An experiment, which included swapping of eggsbetween nests, together with analysesof non-manipulated nests, revealed an effect of eggsize on nestlingbody massesat agesof two and four days. From the experiment, no effect of maternal quality as expressedby her egg size could be found. At an age of four days, 40% of the nestlingswere left alone in the nest by their parents. Nestlingsnot attended by a parent at this age were significantly lighter than were those with parental company. Parents that had left their young by the time these were four days old may have been poor quality birds, as indicated by the tendency for such birds to have laid smaller eggsthan had those still tending their young at the same nestling age. Key words: Antarctica;breeding synchrony; egg size; hatchling size: nestlinggrowth; Pa- godroma nivea; parental quality.

INTRODUCTION kirch 1990, Thompson and Hale 199 1, Sydeman Within individual bird species,egg size may vary and Emslie 1992), supporting the idea that egg considerably (Grant 1982, Greig-Smith et al. size reflects parental quality. Recently, a few 1988). This variation has both a genetic (Boag experimental studies aimed at separating effects and van Noordwijk 1987, Lessells et al. 1989) of egg size per se, and of parental quality, have and an environmental component (Price et al. found an effect of egg size at early nestling ages 1988). A number of studies have indicated that (Amundsen and Stokland 1990, Reid and Boers- a benefit is gained by laying large eggs. Firstly, ma 1990, Magrath 1992). These studiesalso have large eggs tend to result in large hatchlings, as demonstrated growth (Amundsen and Stokland has been demonstrated for many species (e.g., 1990, Magrath 1992) and survival (Bolton 199 1, Parsons 1970, Nolan and Thompson 1978, Fur- but see Meathrel et al. 1993) to be influenced by ness 1983, Stokland and Amundsen 1988, Grant parental quality (as expressedby the size of eggs 199 1, review by Williams 1994). Secondly, nest- laid). Taking the results of thesestudies together, lings hatching from large eggstend to grow faster (1) egg size seems in general to reflect parental (Schifferli 1973, Williams 1980) and/or to have quality, and (2) parental quality seems to be a a higher survival rate (Davis 1975, Howe 1976, more important determinant of nestling devel- Thomas 1983) than small-egg nestlings. Since opment than is egg size in itself. these general trends have been based primarily The aim of the present study was to analyze on descriptive studies, the apparent effect of egg the variation in egg size of Snow Petrels Pago- size could either be due to egg size per se, or to drama nivea, and its relationship to nestling parental quality, the latter being reflected in the growth during the early nestling phase. This was size of eggslaid (Amundsen and Stokland 1990, done primarily by analyzing descriptive data, but Williams 1994). Several studies have demon- was also based on a small scale, egg-swapping strated egg size to be positively related to ma- experiment. Snow Petrels brood and guard their ternal age (Wiggins 1990, Croxall et al. 1992, chicks during the first few days after hatching Weimerskirch 1992) or experience (Weimers- (Mougin 1968, 1975) probably until the chicks have developed a thermoregulatory capacity. Clearly, a trade-off exists between brooding/ I Received23 August1994. Accepted 9 January1995. guarding the chick, and foraging at sea. This is

13451 346 TROND AMUNDSEN particularly pronounced at inland colonies. Here, of breeding synchrony. In four nests, however, I present results on the relationship between the nestlings were so small when found (< 40 g) nestling growth and parental attendance,and also that they almost certainly had hatched on that explore potential determinants of the attendance day. These were included in analyses of nestling patterns. growth relative to egg size. In a few cases,nests were not visited on the STUDY AREA AND METHODS exact nestling ages (two and four days old), but The study was carried out at Svarthamaren on the preceding and following days. In such (71”53’S, 5”10’E), Dronning Maud Land, Ant- cases I have estimated the body mass on days arctica, during 1O-24 January 1994. At Svartha- two and four, respectively, as the mean of the maren, about 500 pairs (Mehlum et al. 1988), or masses when one day younger, and when one more (own observations), of Snow Petrels breed, day older (i.e., assuming linear growth). mostly underneath boulders, or in crevices. The I further carried out a small-scale experiment colony is situated in the Miihlig-Hoffmann aimed at separating the effectsof egg size per se, mountain range, about 200 km from the conti- and of parental quality, on nestling growth. Based nental ice shelf. Snow Petrels breeding at Svar- on the first 66 nests found, I divided the nests thamaren thus have to fly at least 400 km during into three egg size categories.Eggs belonging to each feeding trip (Mehlum et al. 1988). the lower 33 percentile were termed “small,” During 10, 11 and 17 January, I searchedfor those in the 33-67 percentile “middle-sized,” and accessible nests within a sub-plot of the Snow thoseabove the 67 percentilewere termed “large.” Petrel breeding area. I measured the length and I then swappedlarge and small eggsbetween nests. breadth of each egg found (to the nearest 0.01 Hatching and subsequent growth were recorded mm, using a digital caliper). Egg volume was in four nests with small eggsswapped for other calculated from the formula: Volume (cm3) = small eggs(S/S), four with small eggssubstituted 0.0005 1 x Length x Breadth2(Hoyt 1979). Nests with large ones (S/L), five with large eggsswapped with eggs showing signs of hatching (egg shell for other large eggs (L/L), and four with large cracking or punctured) were visited daily, mostly eggs swapped for small ones (L/S). Nests with in the evening (20:00-24:00 hours). The hatching middle-sized eggs,and also some with large or date could therefore be recorded, and any nest- small eggs,were not manipulated. lings hatched since the previous visit weighed (to Statistical tests are two-tailed except when the nearest 0.1 g, using a Pesola spring balance). stated otherwise. Means are reported + one stan- I noted whether the hatchlings were dry, or dard deviation (SD). whether they were slightly wet or scruffy; the RESULTS latter indicating that they had only hatched within the last few hours. I visited the nests once EGG SIZE more when the nestlings were two days old, and, The mean length of the 113 eggsmeasured was for a smaller sample of nests,also when the nest- 56.5 -t 2.2 mm (range 5 1.0-62.3) and the mean lings were four days old. The smaller sample at breadth of the eggswas 39.5 -t 1.0 mm (range agefour days was due to the fact that, as members 36.5-4 1.8). Eggvolume, as calculated from mea- of the 1993-l 994 Norwegian Antarctic Research surements of breadth and length of the eggs(see Expedition, we had to leave Svarthamaren on 24 the Material and Methods section), had a mean January. of 44.9 + 2.9 cm3 (range 36.4-52.1). I found no During the period when I was recording hatch- correlation between the length and breadth of ling body mass and early nestling growth (18-24 individual eggs(r = 0.008, n = 113, NS). January), I also searchedfor additional nests to include in the study. Most nests found during TIMING OF HATCHING this period still had unhatched eggs, and these Hatching occurred between 16 and 24 January were measured in the same way as were the eggs (n = 64), with a median hatching date of 20 in nests found earlier. Some nests, however, did January. Additionally, five nestscontained intact contain hatched chicks at the time of the first eggs that were either infertile, or hatched later visit, with body massesranging from 32.8 to 79.1 than 24 January. Hatching was highly synchro- g. The hatching dates of these chicks were esti- nous: 84% of the eggshatched during the five- mated from growth data for chicks of known age. day period 18-22 January. Including only nests These data were primarily used in the analysis with directly observed (not estimated) hatching SNOW PETREL EGG SIZE 341

50 , (a)r=0.55 0 - 45- N=37 p < 0.001 00 40.

t 36 38 40 42 44 46 48 50

.-F

41 42 43 44 45 46 47 48

Egg volume (cm3) FIGURE 1. Relationship betweenegg volume and hatchlingbody massof Snow Petrelsbreeding at Svartha- maren, Dronning Maud Land, Antarctica: (a) hatched within the last 24 hr, (b) wet or scruffy, i.e., hatched within the last few hours. dates, I found no relationship between egg vol- data for recently hatched nestlings (wet or scruf- ume and hatching date (r = 0.05, n = 37, NS). fy) only, revealed that the variation explained by Nor did I find any relationship between egg size egg volume increased to 58% (r = 0.76, n = 8, and the probability of successfulhatching (t = P < 0.05; Fig. lb). 0.08, df = 98, NS). EARLY NESTLING GROWTH HATCHLING BODY WEIGHT The mean mass of nestlings aged two days was The mean mass of hatchlings (including only 47.6 f 6.6 g (n =49). The nestlings showed a those nestlings known to have been hatched dur- mean daily mass increase of 7.0 f 2.8 g (n = 42; ing the last 24 hours) was 33.9 f 4.5 g (n = 41; Fig. 2) from hatching until two days old. Nest- range 26.0-47.7). When making the same anal- lings aged four days had a mean mass of 57.7 -t ysis with the data only for the nestlings that were 13.3 g (n = 33), and had grown by 5.2 k 6.5 g wet or scruflj~ at weighing time (i.e., hatched (n = 33; Fig. 2) per day during these two days. within the previous few hours), mean hatchling Note, however, that the mass development be- body mass was 30.6 f 3.0 g (n = 11; range 26.0- tween days two and four proved to be strongly 37.1). influenced by the presenceor absenceof the par- Hatchling body mass was significantly related ents (see below). to eggvolume (r = 0.55, n = 37, P < 0.001; Fig. Including only the data for the nestlingshatched la); i.e., egg volume explained 30% of the vari- from manipulated eggs(i.e., reared by foster par- ation in hatchling body mass. An analysis of the ents), I found a significant influence of egg size 348 TROND AMUNDSEN 65r 60 - 5 55 - $ g 50-

g 45-

z 40-

i LMS L MS L MS I 0 2 4 Nestling age (d) FIGURE 2. Mean nestlingbody masses(2 1 SE) at agesof 0, 2 and 4 days,of nestlingshatching from small (S. filled circles).medium-sized (M. onen circles),or large(L, filled squares)eggs. See Methods for definitions. Numbersabove error bars indicke ’samplessizes:

(categories small vs. large) on hatching mass attended by a parent (mean 45.2 k 6.4 g, n = (ANOVA, F,,,, = 13.86, P < O.Ol), and on mass 10; t = 3.98, P < 0.001; Fig. 3). The two groups when two days old (F,,,, = 5.94, P < 0.05). The did not differ in regard to the mean volume of original egg size of the foster parents did not the eggsfrom which they had hatched (attended: influence nestling body massat any of thesestages 45.2 + 1.6 cm’, n = 6; unattended: 44.3 f 1.7 (data not shown). Too few data were obtained cm3, n = 7; t = 0.96, P = 0.18, one-tailed), mean for any analysis to be made of the body mass at hatchling mass (32.6 f 3.1 g, n = 13 vs. 34.0 + day four for manipulated clutches.Since no effect 3.2 g, n = 10, respectively; t = - 1.06, P = 0.30; of parental quality (as expressedthrough mater- Fig. 3), or mean mass at age two days (46.3 + nal eggsize) was apparent, an analysis was made 6.8 g, n = 15 vs. 49.4 f 4.3 g, n = 10, respec- to investigate the potential influence of egg size tively; t = - 1.28, P = 0.21; Fig. 3). However, on nestling growth, including the data for both the females of attending pairs (day 4) tended to the manipulated and the non-manipulated nests have laid larger eggsthan had the females of non- (but only for those with known, not estimated, attending pairs (45.4 + 1.3 g, n = 6 vs. 43.8 k hatching dates). This analysis revealed a signif- 1.3 g, n = 7; t = 1.57, P = 0.07, one-tailed). The icant effect of egg size on nestling growth during difference in body mass at age four days seemed days 2-4 (r = 0.59, n = 14, P < 0.05), and, in to be due to a difference in the mean daily growth consequence, an effect of egg size on the body rate during the preceding two days. While nest- mass of the nestlings on day 4 (r = 0.67, n = 14, lings still attended by a parent on day four had P < 0.01). a mean daily growth rate of 8.6 f 5.3 g (n = 15), those not attended by a parent at the same stage TERMINATION OF BROODING/GUARDING had a mean daily body mass loss of 2.1 k 2.6 g At the age of four days, 10 of 25 nestlings (40%) (n = lo), during the 24 day period (Fig. 3). had been left alone by their parents. The body mass on day 4 of the nestlings still attended by DISCUSSION a parent (mean 63.5 + 13.5 g, n = 15) was The eggs of Snow Petrels breeding at Svartha- significantly higher than that of the nestlings un- maren were similar in size to those of birds SNOW PETREL EGG SIZE 349

2 4 Nestling age (d) FIGURE 3. Mean nestling body masses(+ 1 SE) at ages0, 2 and 4 days, of nestlingsattended (A, filled circles) by their parents when four days old, and those not (IV, filled squares)attended at the same age. Numbers above error bars indicate sample sizes. breeding at Davis (Brown 1966) and at Cape Marchant and Higgins (1990, p. 407) who re- Denison (Falla 1937) but were smaller than eggs ported laying to be “well synchronized.” Breed- of birds breeding at Dumont d’urville (Isen- ing synchrony could either be a reflection of the mann 1970). The latter colony is considered to time span available for breeding being restricted, be a pure colony of the larger subspeciesPago- or it could be a means of predator swamping. droma nivea confusa, whereasthe colonies at Da- Concerning the first of these points, the breeding vis and Cape Denison, as also that at Svartha- areas probably have to be relatively free of snow maren, comprise birds of the smaller nominate for egg-laying to take place, while at the end of subspeciesP. n. nivea (Marchant and Higgins the breeding season the young need to become 1990). In fact, the adult size of Snow Petrels at fledged before the sea-ice starts to freeze (the Svarthamaren seems to be among the smallest latter resulting in an extraordinarily long flying recordedin any colony (Haftom et al. 1988, Mar- distance when leaving the nest, as well as a re- chant and Higgins 1990). The mean size of eggs duction in suitable feeding areas). at Svarthamaren in 1994 (this study) was very I found the mean body massof recently hatched similar to that of a small sample measured at the nestlings to be 30.6 g, and the mean mass of same site in 1985 (Mehlum et al. 1986). nestlings hatched within the last 24 hr to be 33.9 Warham (1990, p. 283) has analyzed the gen- g. This is about 10 g, or 25%, lower than the eral relationship between femaie body mass and hatching body mass recorded at Dumont d’Ur- egg size in procellariiform birds. The mean egg ville (Mougin 1968, Isenmann 1970) which are size recorded at Svarthamaren was 22% greater the only other data for hatching body mass of than that expected from Warham’s (1990) gen- this species that I know of. These body mass eral function, showing that Snow Petrels lay rel- values, however, are not directly comparable, atively large eggsfor their body size. since the birds at Dumont d’Urville belong to Hatching of the chicks was highly synchro- the larger subspeciesconfusa (Marchant and Hig- nous, with 84% hatching during a five day period gins 1990). The differencein mean hatching body (18-22 January). Synchrony in breeding seems mass for the two colonies (25%) roughly reflects to be a general feature of the Snow Petrel, cf. the corresponding difference in egg size (18%). 350 TROND AMUNDSEN

In the present study, egg volume explained from the present study allow no firm conclusion about 30% of the variation in body massof chicks to be drawn as to whether the low body masses hatched within the last 24 hr. When the analysis were a consequenceof the parents having already was restricted to nestlings that were still slightly left the nest, or their cause,but the former seems wet or scruffy when found, the value increased more plausible. Those parents that had left their to almost 60%. I have no evidence that might chicks alone at or before a nestling age of four indicate how long it takes for a Snow Petrel days tended to have laid small eggsthemselves, hatchling to expand its down completely, but I suggestingthat these may have been low quality assume that it is a question of only a few hours. birds. They were thus perhaps less able to with- Perhaps the average age of these nestlings may stand long fasting periods, and were forced to be l-2 hr, whereasthe averageage of all nestlings leave their young while at an early age, in order hatched within the last 24 hr would be approx- to collect food for themselves and for the chick. imately 12 hr. This suggeststhat nestling mass Despite no observed effect of parental quality on increases by about three grams during the first nestling growth, these findings suggestthat egg 10 hr, or by about eight grams during the first size, to some extent, reflects parental quality in day of life. This is similar to the mass gain ob- Snow Petrels. served during the immediately following days. Since the proportion of the variation in body ACKNOWLEDGMENTS massexplained by eggsize decreasesrapidly with The study was carried out during the 199311994Nor- nestling age, it is evident that the initial growth wegian Antarctic Research Expedition (NARE), and rate is not related to eggsize to any considerable was financed by a grant from the Norwegian Research Council. I thank Svein Haftom, Bemt-Erik Saether, extent. Philip A. Tallantire, and two anonymous referees for I found that eggvolume had a significant effect comments on previous drafts. This is publication No. on nestling body mass at ages of two and four 135 of the Norwegian Antarctic ResearchExpeditions days, respectively. Probably, this is primarily a (199311994). reflection of differences in hatchling body mass. LITERATURE CITED It is noteworthy, however, that egg volume sig- -EN, T., ANDJ. N. STOKL,AND. nificantly influenced the growth rate between days 1990. Egg size and parental quality influence nestling growth in two and four (but not between days zero and the shag. Auk 107:410-413. two). BOAG,P. T., ANDA. J. VANNOORDWUK. 1987. Quan- In my attempt to separate the effects of egg titative genetics,p. 45-78. In F. Cooke and P. A. size per se, and of parental quality (as expressed Buckley [eds.], Avian genetics. Academic Press. London. by the size of eggslaid), I did not find any evi- BOLTON,M. 1991. Determinants of chick survival in dence of an effect of parental quality on growth the Lesser Black-backed Gull: relative contribu- up to an age of four days. However, this result tions of egg size and parental quality. J. Anim. is not very conclusive, since the sample sizes in Ecol. 60:949-960. my egg swapping experiment were very small. BROWN,D. A. 1966. Breeding biology of the Snow Petrel Pagodroma niveu (Forster). Aust. Natn. At an age of four days, nearly half of all the Antarct. Res. Exped. Scient. Rep. Ser. B Zool. 89: nestlings had been left alone in their nests. This l-63. is a relatively younfr age for termination of ~~OXW, J. P., P. ROTHERY,AND A. CRISP. 1992. The brooding and-guarding, Compared with results effectof maternalage and experienceon egg-size and hatchingsuccess in Wandering Albatrosses from Davis (Brown 1966), Robertskollen (Ryan Diomedea exulans.Ibis 134~219-228. and Watkins 1989), and Dumont d’Urville D AVIS, J.W.F. 1975. Age, egg-sizeand breeding suc- (Mougin 1968, 1975). Robertskollen is an inland cessin the Herring Gull Larus argentatus.Ibis 117: colony in westernDronning Maud Land (71”27’S, 460-473. 3”15’W). Like Svarthamaren and Davis, it is in- FALLA,R. A. 1937. Birds. B.A.N.Z. Antarctic Res. Exped.1929-1931 Rep. Ser.B. 2:1-288. habited by birds of the smaller subspecies.Chicks F URNESS,R. W. 1983. Variation in sizeand growth at these three colonies would, at equivalent ages, of Great SkuaCutharacta skua chicks in relation be more vulnerable to chilling that those ofthe to adult age, hatchingdate, eggvolume, brood size larger subspecies breeding at Dumont d’Urville. and hatching sequence.J. Zool., Lond. 199:101- 116. Nestlings found to be alone at the age of four GRANT,M. C. 1991. Relationships between egg size, days were significantly lighter than were those chick size at hatching, and chick survival in the that were still attended by a parent. The data Whimbrel Numeniusphaeopus.Ibis 133:127-133. SNOW PETREL EGG SIZE 351

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