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 swap- ping 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 with- in 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).