UC Irvine UC Irvine Previously Published Works Title Reproductive performance of seabirds: the importance of population and colony size. Permalink https://escholarship.org/uc/item/2363j0gf Journal Auk, 103(2) ISSN 0004-8038 Authors Hunt, GL Eppley, ZA Schneider, DC Publication Date 1986 DOI 10.1093/auk/103.2.306 License https://creativecommons.org/licenses/by/4.0/ 4.0 Peer reviewed eScholarship.org Powered by the California Digital Library University of California REPRODUCTIVE PERFORMANCE OF SEABIRDS: THE IMPORTANCE OF POPULATION AND COLONY SIZE GEORGE L. HUNT, JR., ZOE A. EPPLEY, AND DAVID C. SCHNEIDERl Department of Ecology and Evolutionary Biology, University of California, Irvine, California 92717 USA ABSTRACT.-We compared reproductive performance of five species of seabirds at two colonies, St. George Island (2.5 million birds) and St. Paul Island (250,000 birds), in the southeastern Bering Sea. All species had lower chick growth rates at the larger colony, and the differences were statistically significant in four species. Fledge weights of Common Murres (Uria aalgt) on St. George Isla.nd were 84-88% of those on St. Paul. Average fledge weights of Thick-billed Murres (Uria lomvia) on St. George were only 74% of those for chicks from St. Paul. We found no significant differences in clutch size or breeding success between populations breeding at the two colonies. For three species, Black-legged Kittiwakes (Rissa tridactyla), Common Murres, and Thick-billed Murres, we extended our analysis to include published data from other colonies. We examined breeding performance as a function of colony size, population size (suggestive of intraspecific competition), and "effective colony size," the sum of the populations of species with considerable dietary overlap (suggestive of interspecific competition for food). We found consistently negative relationships between population size and several measures of breeding performance (clutch size, growth rate, fledge weight, and breeding success). In addition to the lower breeding success at colonies that support large populations, chic.ks from these colonies may be subject to higher postfledg­ ing mortality because of fledging at lower weights. Received 7 f anuary 1985, accepted 18 October 1985. THERE is now mounting evidence that repro­ bird species in Great Britain and found a ductive performance in seabird colonies may negative relationship between the size of the vary with population or colony size. Coulson population at a colony and the size of popula­ et al. (1982) documented changes in reproduc­ tions of the same species at nearby colonies. tive performance that could be related to intra­ They hypothesized that this relationship was specific competition for food. They found ear­ due to density-dependent depletion of prey lier breeding and increased egg size in a near colonies. In a review of recent work, Birk­ Herring Gull (Larus argentatus) colony after it head and Furness (1985) concluded that nega­ had been culled to one-quarter of its former tive relationships between population size and size. Hunt et al. (1981, 1982) suggested that col­ several aspects of reproductive performance ony size may affect reproductive performance support the hypothesis that intraspecific com­ in relatively stable colonies such as those of the petition for food near colonies may regulate Pribilof Islands. More recently, Gaston (in press) seabird populations. found a negative relationship between fledge Seabirds commonly breed in large, multispe­ weight and population size for Thick-billed cies colonies and may have considerable di­ Murres (Uria lomvia) and for Atlantic Puffins etary overlap with other species at the colony (Fratercula arctica) in data from seven colonies. (Belopol' skii 1957, Pearson 1968, Croxall and Gaston et al. (1983) measured fledge weights of Prince 1980, Hunt et al. 1982). Hence, both in­ murre chicks at three colonies within the same tra- and interspecific competition are possible. year and found a negative relationship be­ Belopol'skii (1957) attributed reduced breeding tween population size and fledge weight. Fur­ success and dietary changes in Black-legged ness and Birkhead (1984) examined four sea- Kittiwakes (Rissa tridactyla) to competition with Thick-billed Murres for food in years of low fish availability. Ashmole (1963) first suggested 1 Present address: Newfoundland Institute of Cold that competition for food around breeding col­ Ocean Science, Memorial University of Newfound­ onies may regulate seabird populations. land, St. Johns, Newfoundland AIB 3X7, Canada. If there are negative relationships between 306 The Auk 103: 306-317. April 1986 April 1986) Reproductive Performance of Seabirds 307 population or colony size and reproductive TABLE 1. Population sizes of seabirds breeding on performance, then the greatest effects should the Pribilof Islands (from Hickey and Craighead 1977). be seen in the largest populations or colonies. St. George Island supports a seabird colony of St. George St. Paul 2.5 million birds, of which 1.5 million are Thick­ Island Island billed Murres (Hickey and Craighead 1977). Northern Fulmar (Fulmarus This colony is one of the largest in the North­ glacialis) 70,000 700 ern Hemisphere and is the largest for which Red-faced Cormorant (Phala- reproductive information for several species is crocorax urile) 5,000 2,500 Black-legged Kittiwake (Rissa available. For 3 yr we compared seabird repro­ tridactyla) 72,000 31,000 duction at St. George Island and at a similar Red-legged Kittiwake (R . bre- but smaller colony on St. Paul Island. The latter virostris) 220,000 2,200 colony, consisting of 253,800 birds, is 63 km Common Murre (Uria aalge) 190,000 39,000 distant and supports the same suite of species Thick-billed Murre (U. lom- via) l,500,000 110,000 as found on St. George Island. Although the Parakeet Auklet (Cyclorrhyn- proportions of species change between the chus psittacu/a) 150,000 34,000 colonies (Schneider and Hunt 1984), all species Least Auklet (Aethia pusilla) 250,000 23,000 are more abundant on St. George Island (Table Crested Auklet (A. cristatella) 28,000 6,000 1). We compared reproductive performance of Horned Puffin (Fratercu/a cor- niculata) 28,000 4,400 five species of seabirds to look for evidence of Tufted Puffin (F. cirrhata) 6,000 1,000 density-related depression of reproduction. Total 2,519,000 253,800 In addition, we examined reports of kitti­ wake and murre reproductive performance in other colonies to determine whether a den­ fore egg-laying until chick departure or failure. Sam­ sity-dependent relation exists generally. To ple sizes are given in Table 2. Breeding success was partition the possible effects of intra- and in­ computed for each study area, using the ratio of chicks terspecific competition, we also examined re­ fledged to the number of breeding attempts (defined productive performance for negative relation­ as a defended territory or breeding site/nest con­ ships with population size (indicating struction). For comparison with published work on intraspecific competition) and "effective col­ kittiwakes, we used chicks fledged per completed nest ony size," the sum of species with considerable (= pair) as a measure of breeding success. Breeding dietary overlap (indicating interspecific com­ success in murres was estimated as the number of petition for food). The results of these compar­ chicks that departed successfully, divided by the av­ erage number of adults at each site during incubation isons suggest whether reproduction in North­ and brooding. We assumed missing murre chicks to ern Hemisphere seabirds generally shows have departed successfully if their age or plumage density-dependence and the relative impor­ on the previous visit indicated that they were at least tance of intra-and interspecific competition. 3 weeks old. We computed the number of murre chicks "fledged" per pair that laid egg(s} for com­ METHODS parison with other studies. Red-faced Cormorants and Black-legged Kittiwakes laid multiple-egg clutches at Breeding seabirds were studied at St. Paul and St. the Pribilof Islands. Clutch size was calculated using George islands, Pribilof Islands, southeastern Bering only nests containing eggs. Sea, during the summers of 1976, 1977, and 1978 (see Growth rates and fledge weights were obtained at Hunt et al. 1982 for a detailed account). We measured two sites accessible by ladder on St. George and at clutch size, growth rate, fledge weight, and breeding three sites on St. Paul. Study sites were visited at 3- success in Red-faced Cormorants (Phalacrocorax urile), 4-day intervals, except when weather prevented. Black-legged Kittiwakes, Red-legged Kittiwakes (Ris­ Growth rates were calculated as the difference be­ sa brevirostris), Common Murres (Uria aalge}, and Thick­ tween the initial weighing and the peak weight mea­ billed Murres at both colonies. Throughout the study, sured, divided by the number of days elapsed. Fledge methods and frequency of site visits by study teams weights were the last weights of chicks obtained be­ on the two islands were as similar as possible. fore fledging, or departure in the case of murre chicks. Clutch size and breeding success were estimated We used a one-way ANOVA (Sokal and Rohlf 1969) at three sites on St. George Island and at four sites to determine whether sites and years could be pooled on St. Paul Island. We visited each site at 3-4-day to compare islands. Years could not be pooled so we intervals to follow the progress of breeding from be- used I-tests