Age, Resource Availability, and Breeding Effort in Brandt's Cormorant
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AGE, RESOURCE AVAILABILITY, AND BREEDING EFFORT IN BRANDT'S CORMORANT ROBERTJ. BOEKELHEIDEAND DAVID G. AINLEY PointReyes Bird Observatory, 4990 Shoreline Highway, Stinson Beach, California 94924 USA ABSTRACT.--Wegathered life-history data on banded Brandt'sCormorants (Phalacrocorax penicillatus)at SoutheastFarallon Island, California, from 1972 to 1984, and documented breedingperformance as affectedby age and annual variation in food availability. Femalesbred at a youngerage than males,but did not live as long. Birdsof both sexes that bred at least once bred the samenumber of years.Mate fidelity was low (9%) because of poor synchronyin the arrival of matesand low site fidelity by females.Prior breeding experiencehad little influence on reproductivesuccess. The most successfulindividuals fledged10-20 chicksover their lifetime, and averaged2.5 chicks/breedingyear over four to eight years. Cormorantsexperienced significant interannual differencesin food availability. "Poor" food yearsoccurred frequently and, consequently,all adultsskipped breeding at leastone seasonduring their reproductivelifetimes. Food availability also affected age at firstbreeding, as well as age-relatedbreeding phenology, reproductiveeffort and success,and return rates of banded juveniles and adults. The reproductive traits of Brandt's Cormorant allow it to exploitan unpredictableenvironment; previously proposed life-history models relegate cor- morantsto stableenvironments. Received 5 May 1986,accepted 24 January1989. MANYvertebrates can adjustreproductive ef- wind-driven transportof surfacewaters for nu- fort to the amountof energyavailable each year trient input (through upwelling and advec- (Giesel1976). Schaffer (1974) hypothesized that tion). Considerable interannual fluctuation in life-history characteristicsin variable environ- oceanographicclimate, biologicalproductivity, ments depend on when significant mortality and developmentof the food web is character- occursduring the life cycle. If variable condi- istic of these systems(Glantz and Thompson tions lead to variable success,then breeding 1981, Chelton et al. 1982, McLain and Thomas effort should be adjustedto insure survival until 1983, McLain et al. 1985). The most dramatic of the next favorable year. If conditionsinstead the periodicanomalies in productionare known lead to variable adult survival, then breeding as E1Nifio-Southern Oscillations (ENSO). Dur- effortshould be increasedin orderto reproduce ing ENSO, biological productivity decreases immediately, before death occurs. radically and the food web collapses(Glantz In seabirds,which are long-lived and thus and Thompson1981, McLain et al. 1985,Ainley shouldhave ample opportunityto adjustbreed- and Boekelheide 1990). ing effort, many life-history traits are strongly The seabirdsof easternboundary currents are influencedby ageand this relationshiphas been abundantand unique,with a high incidenceof the subjectof much research.Age-influenced endemismin respectiveavifaunas (Brown 1976, traitsinclude the proportionof individualsthat 1980). Brandt's Cormorant (Phalacrocoraxpeni- breed, nest-site characteristics, mate selection, cillatus)is unique to the California Current, but phenology, clutch size, and various compo- is less abundant than its ecological counter- nents of reproductivesuccess (see review by parts,the GuanayCormorant (P. bougainvillii)of Ryder 1980, and more recent studies:e.g. Pu- the Peru Current and the Cape Cormorant (P. gesek 1983, Pugesekand Diem 1983, Ainley et capensis)of the BenguelaCurrent (Ainley and al. 1983, Shaw 1986). Yet, interannual variations Boekelheide1990). The latter two speciesare in breeding behaviorand success,as influenced important commercially becauseof the guano by age, have receivedscant attention. they produceand they have been the subjectof The California Current is one of the world's appreciablestudy. Little work hasbeen directed five major eastern boundary-current systems. toward Brandt's Cormorant. We collected in- These oceansystems are the most biologically formationon the reproductivesuccess of known- productive in the world, and depend upon age Brandt'sCormorants from 1972to 1984,and 389 The Auk 106: 389-401. July 1989 390 BOEKELHEIDœANDAINLœY [Auk,Vol. 106 analyzed patterns in relation to food availabil- SEFI nestedwithin 300 m of the natal colony,but a ity to determine the validity of Schaffer'shy- few nestedas much as 1 km away. On the basisof pothesiswith regard to this species. sitefidelity information,we assumedthat oncea bird nestedwithin a colony it was unlikely to emigrate. In supportof this assumption,Potts (1969) found that STUDY ARF• AND METHODS only 8% of Shags(P. aristotelis)breeding for the first We studied Brandt's Cormorants on Southeast Far- time nestedoutside their natalcolony, and that < 1% allon Island (SEFI), 48 km west-southwestof San Fran- movedafter havingbred once.Consequently, return cisco,California, at 37ø42'N,123ø00'W, the largestof ratesof establishedbreeders more accuratelyreflect the South Farallon Islands. Between 1971 and 1983, true survival rates. cormorantnumbers averaged 16,000 and ranged be- We usedfour principal criteria to rate yearson the tween 4,300 and 23,800individuals (Ainley and Boe- basisof food availability:(1) tendencyof cormorants kelheide 1990). to feed within 5 km of SEFI (vs. as much as 80 km We observedknown-age individuals from a blind away),(2) proportionof juvenilerockfish (Sebastes spp.) overlookinga colony of about500 pairson the north- in the diet of Farallon piscivorousbirds (including west shoreof SEFI (for details,see Ainley and Boe- Brandt'sCormorants), (3) breeding populationsize of kelheide 1990). Bet-ween1970 and 1983,workers from Brandt'sCormorants relative to the 13-yrmean, 1970- Point ReyesBird Observatory(PRBO) banded 5,866 1983,and (4) the occurrenceof significantnest aban- cormorantchicks in the studycolony. From 1971on, donmentduring the breedingperiod (see Ainley and each chick was given two bands:a USFWS individ- Boekelheide1990 for moredetails). Subsequent direct ually numberedmetal band on one leg and a colored, assessmentof prey abundanceand distributionin wrap-around plastic band on the opposite leg. Leg 1985-1988 indicates that these criteria were accurate position, band number, and band color designated (unpubl. data, PRBO and National Marine Fisheries year class.We banded chicks at night to minimize Service,Tiburon Laboratory).Years of "good" food gull predation on nestlingsand eggs.In 1970-1978 availabilitywere 1977,1979, and 1981;1980 and 1982 (except 1971) we used aluminum bands; those used were "moderate"years; and 1976,1978, and 1983(all in 1971 were monel and all were soon lost. After 1978 yearsof oceanographicanomaly; McLain et al. 1985) we usedstainless-steel (incoloy) bands, and through were "poor" years. 1985 none had fallen off. We corrected for loss of RESULTS aluminum bands from estimates made on the double- banding system. We monitored banded birds daily from April to Age of first sightingand first breeding.--Onav- August in 1972-1985. We read band numberswith a erage,female and male Brandt'sCormorants first 20-60x spotting scope, mapped the location and returned to the breeding colony at the sameage movement of each bird, recorded the condition or (female 2.7 + 1.3 yr, n = 96; male 2.9 + 1.2 yr, absenceof metal and colorbands, described plumage, n = 212; t = 0.91, df = 306, P > 0.2; Fig. 1). The and noted sexuallydistinct behaviors. We noted the modal age of first sighting for both sexeswas date when each bird began to occupy a site contin- 2 yr, but < 5% of birds of either sexreturned as uously, date of pairing, egg-layingand chick-hatch- 1-yr-olds. Among all birds eventually sighted, ing dates,and nest condition. From 1979 to 1983,we about90% returned by their fourth year. alsorecorded date of first arrival in spring. Despitesimilar agesof return, malesand fe- We calculatedage at firstbreeding, effects of breed- ing experience,and lifetime breeding recordsonly malesdiffered in the averageage of first breed- for individuals with completeannual records.Vari- ing (Fig. 1). Femalesfirst bred at 3.5 __1.5 yr (n ation aboutmeans are given as + 1 standarddeviation. = 40), and males at 4.2 + 1.3 yr (n = 70; t = Breeding chronology, clutch size, and reproductive 2.61, df = 108, P < 0.02). The modal age of first successwere analyzedfor similarityof variance(AN- breeding for females was 2 yr, but only 2 out OVA) and between-agedifferences were testedusing of 131 two-yr-old males bred. The modal peak the Student-Newman-Keuls multiple comparison for males occurredat 4 yr. (SNK) (Sokal and Rohlf 1969). The averageage of birds sightedor breeding We sexed cormorants according to criteria dis- for the first time varied annually; many young cussedby Williams(1942). We excludedfrom analyses birds appearedand attempted to breed in years birds that we were unableto sex,visited only a few times,or nestedout of sight within the colony. of good food availability but few or none did We determined return rates of banded birds after soin poor years.For example,in the 1983ENSO correctingfor band loss.Because of an unknown rate year, no 2-yr-olds (out of 600 chicksbanded in of emigration,return rateswere consideredminimal. 1981) were observed. This contrastswith 1979, The majorityof bandedbirds observed elsewhere on an exceptionallygood year, when we observed July1989] BiologyofBrandt' sCormorant 391 60 of the 592 birds banded as chicks in 1977. In 1979, 64% (n = 11) of 1977-cohort females that eventually bred