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Reproductive Performance of Seabirds: the Importance of Population and Colony Size

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Reproductive Performance of Seabirds: the Importance of Population and Colony Size REPRODUCTIVE PERFORMANCE OF SEABIRDS: THE IMPORTANCE OF POPULATION AND COLONY SIZE GEORGEL. HUNT, JR., ZOE A. EPPLEY,AND DAVID C. SCHNEIDER• Departmentof Ecologyand Evolutionary Biology, University of California, Irvine,California 92717 USA ABSTRACT.--Wecompared reproductive performance of five speciesof seabirdsat two colonies,St. George Island (2.5 million birds) and St. Paul Island (250,000birds), in the southeasternBering Sea.All specieshad lower chick growth ratesat the larger colony,and the differenceswere statisticallysignificant in four species.Fledge weights of Common Murres (Uria aalge)on St. GeorgeIsland were 84-88% of thoseon St. Paul. Averagefledge weights of Thick-billed Murres (Uria lomvia)on St. Georgewere only 74%of thosefor chicks from St. Paul. We found no significantdifferences in clutchsize or breedingsuccess between populationsbreeding at the two colonies.For three species,Black-legged Kittiwakes (Rissa tridactyla),Common Murres, and Thick-billed Murres, we extendedour analysisto include publisheddata from other colonies.We examinedbreeding performanceas a function of colony size, populationsize (suggestiveof intraspecificcompetition), and "effectivecolony size," the sum of the populationsof specieswith considerabledietary overlap (suggestive of interspecificcompetition for food). We found consistentlynegative relationships between population size and several measuresof breeding performance(clutch size, growth rate, fledgeweight, and breedingsuccess). In addition to the lower breeding successat colonies that supportlarge populations,chicks from thesecolonies may be subjectto higher postfledg- ing mortalitybecause of fledgingat lower weights.Received 7 January 1985, accepted 18 October 1985. THEREis now mounting evidence that repro- bird species in Great Britain and found a ductive performance in seabird colonies may negative relationshipbetween the size of the vary with population or colony size. Coulson population at a colony and the size of popula- et al. (1982) documentedchanges in reproduc- tions of the same species at nearby colonies. tive performancethat could be related to intra- They hypothesized that this relationship was specificcompetition 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 (Larusargentatus) colony after it head and Furness (1985) concluded that nega- had been culled to one-quarter of its former tive relationshipsbetween population size and size. Hunt et al. (1981, 1982)suggested that col- several aspects of reproductive performance ony size may affect reproductiveperformance support the hypothesisthat intraspecificcom- in relatively stablecolonies such as thoseof the petition for food near coloniesmay regulate Pribilof Islands.More recently,Gaston (in press) seabird populations. found a negativerelationship between fledge Seabirdscommonly breed in large, multispe- weight and population size for Thick-billed cies colonies and may have considerable di- Murres (Uria lornvia)and for Atlantic Puffins etary overlap with other speciesat the colony (Fraterculaarctica) in data from seven colonies. (Belopol'skii 1957, Pearson 1968, Croxall and Gastonet al. (1983) measuredfledge 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- successand dietary changesin Black-legged ness and Birkhead (1984) examined four sea- Kittiwakes (Rissatridactyla) to competition with Thick-billed Murres for food in years of low fish availability. Ashmole (1963) first suggested • present address: Newfoundland Institute of Cold that competition for food around breeding col- Ocean Science,Memorial University of Newfound- onies may regulate seabirdpopulations. land, St. Johns,Newfoundland A1B 3X7, Canada. If there are negative relationshipsbetween 306 The Auk 103: 306-317. April 1986 April1986] ReproductivePerformance ofSeabirds 307 population or colony size and reproductive T^BLE1. Population sizes of seabirdsbreeding on performance, then the greatest effects should the Pribilof Islands (from Hickey and Craighead 1977). be seen in the largest populationsor colonies. St. George Island supportsa 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- reproductiveinformation for severalspecies is crocoraxurile) 5,000 2,500 available. For 3 yr we comparedseabird repro- Black-leggedKittiwake (Rissa 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, consistingof 253,800 birds, is 63 km Common Murre (Uria aalge) 190,000 39,000 distant and supportsthe samesuite of species Thick-billed Murre (U. lorn- via) 1,500,000 110,000 as found on St. George Island. Although the ParakeetAuklet (Cyclorrhyn- proportions of species change between the chuspsittacula) 150,000 34,000 colonies(Schneider and Hunt 1984),all species LeastAuklet (Aethiapusilla) 250,000 23,000 are more abundant on St. GeorgeIsland (Table Crested Auklet (A. cristatella) 28,000 6,000 1). We comparedreproductive performanceof Horned Puffin (Fraterculacor- niculata) 28,000 4,400 five speciesof seabirdsto look for evidenceof Tufted Puffin (F. cirrhata) 6,000 1,000 density-relateddepression of reproduction. Total 2,519,000 253,800 In addition, we examined reports of kitti- wake and murre reproductiveperformance in other colonies to determine whether a den- sity-dependent relation exists generally. To fore egg-layinguntil chickdeparture or failure. Sam- pie sizesare given in Table 2. Breeding successwas partition the possibleeffects of intra- and in- computedfor eachstudy area, using the ratio of chicks terspecificcompetition, we also examined re- fledged to the number of breeding attempts(defined productive performancefor negative relation- as a defended territory or breeding site/nest con- ships with population size (indicating struction).For comparisonwith publishedwork on intraspecificcompetition) and "effective col- kittiwakes,we usedchicks fledged per completednest ony size," the sum of specieswith considerable (= pair) as a measureof breeding success.Breeding dietary overlap (indicating interspecificcom- success in murres was estimated as the number of petition for food). The resultsof thesecompar- chicksthat departedsuccessfully, divided by the av- isons suggestwhether reproduction in North- eragenumber of adultsat eachsite during incubation and brooding. We assumedmissing murre chicks to ern Hemisphere seabirds generally shows have departedsuccessfully if their age or plumage density-dependenceand the relative impor- on the previousvisit indicatedthat they were at least tance of intra-and interspecificcompetition. 3 weeks old. We computedthe number of murre chicks"fledged" per pair that laid egg(s) for com- METHODS parisonwith other studies.Red-faced Cormorants and Black-leggedKittiwakes laid multiple-eggclutches at Breeding seabirdswere studied at St. Paul and St. the Pribilof Islands.Clutch size was calculatedusing Georgeislands, Pribilof Islands,southeastern Bering only nestscontaining eggs. Sea,during the summersof 1976, 1977,and 1978 (see Growth ratesand fledge weights were obtained at Hunt et al. 1982 for a detailed account). We measured two sitesaccessible by ladder on St. George and at clutchsize, growth rate, fledge weight, and breeding three sites on St. Paul. Study sites were visited at 3- successin Red-facedCormorants (Phalacrocoraxurile), 4-day intervals, except when weather prevented. Black-leggedKittiwakes, Red-legged Kittiwakes (Ris- Growth rates were calculated as the difference be- sabrevirostris), Common Murres (Uria aalge), and Thick- tween the initial weighing and the peakweight mea- billed Murresat both colonies.Throughout the study, sured,divided by the number of dayselapsed. Fledge methodsand frequencyof site visits by study teams weights were the last weights of chicksobtained be- on the two islandswere as similaras possible. fore fledging,or departurein the caseof murrechicks. Clutch size and breeding successwere estimated We useda one-way ANOVA (Sokaland Rohlf 1969) at three sites on St. George Island and at four sites to determinewhether sitesand yearscould be pooled on St. Paul Island. We visited each site at 3-4-day to compareislands. Years could not be pooled so we intervalsto follow the progressof breedingfrom be- used t-teststo comparecolonies within each year. If 308 HUNT, EPPLEY,AND SCHNEIDER [Auk, Vol. 103 TABLE2. Sample sizes used in comparisonof reproductiveperformance of seabirdsat St. Paul (STP) and St. George (STG) islands.a Red-faced Black-legged Red-legged Thick-billed Cormorant Kittiwake Kittiwake Common Murre Murre STP STG STP STG STP STG STP STG STP STG Clutch size nv nv nv nv nv nv 1976 13 10 70 19 1977 49 17 102 70 1978 84 37 110 39 Growth rate 1976 17 11 33 24 4 12 5 4 16 23 1977 4 14 22 20 3 42 9 3 17 34 1978 8 16 16 16 nd 13 nd 12 16 25 Fledge weight 1976 nd nd 14 8 4 28 5 4 16 23 1977 nd nd 21 20 3 29 11 3 17 34 1978 nd nd 15 16 nd 11 nd 12 16 25 Breeding success
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