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Feeding Ecology of the Dark-Rumped Petrel in the Galapagos Islands

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Feeding Ecology of the Dark-Rumped Petrel in the Galapagos Islands The Condor 94~437-441 0 The Cooper Ornithological Society 1992 FEEDING ECOLOGY OF THE DARK-RUMPED PETREL IN THE GALAPAGOS ISLANDS M. J. IMBER Department of Conservation,P.O. Box 10420, Wellington, New Zealand J. B. CRUZ School of the Environment, Duke University,Durham, NC 27706 J. S. GROVE, R. J. LAVENBERG AND C. C. SWIFT Los AngelesCounty Museum of Natural History, Los Angeles,CA 90007 F. CRUZ Charles Darwin ResearchStation, Isla Santa Cruz, Galapagos,Ecuador Abstract. During a study of Dark-rumped Petrel (Pterodromaphaeopygia) breeding bi- ology in the GalapagosIslands, we collected over 80 samplesof food regurgitatedby petrel chicks. We identified the prey to the lowest possible taxonomic level and determined the relative importance of each food class in the petrel’s diet. We also monitored the daily changesin massof 14 chicks on Floreana Island to determine the quantity of food delivered as well as the food delivery rate. Dark-rumped Petrel hatchlings were fed a mixture of stomach oils, semi-digested fish, cephalopods and crustaceans. On the basis of mass composition, about 46% of the diet was cephalopods, about 31% fish, and about 17% crustaceans.Cephalopods, of at least 16 families, occurredwith the greatestregularity. Fish were present in the samples more frequently than Crustacea:seven and five families, re- spectively.Feeding rateswere highestin the month after hatchingand lowest before fledging. Estimated food loads averaged 68 g and did not increase in size over the growth period. Dark-rumped Petrels fed on mainly mesopelagicprey, taken presumably at night, while ranging up to 2,000 km from the colonies, largely southwards. Key words: Pterodroma phaeopygia;nestling diet; feeding rate;feeding habits; Galapagos Islands. INTRODUCTION The endangered Dark-rumped Petrel (P. p. The life-history characteristics of gadfly petrels phaeopygia) of the Galapagos Islands is a me- Pterodroma spp. may be adaptations enabling dium-sized gadfly petrel of about 420 g (Cruz them to make efficient use of food resourcesthat and Cruz 1987). Little is known of its feeding are distant from the breeding colony, patchily biology, although the food of the Hawaiian sub- distributed, and of relatively low nutritive value species has been documented summarily (Si- (Imber 1973, Croxall and Prince 1982). The diet mons 1985). We report here the findings of a of only a few Pterodroma specieshas been ex- three-year study, amplifying what little is known amined in any detail: Phoenix Petrel P. alba about the diet of this petrel. (Ashmole and Ashmole 1967, Ricklefs 1984) MATERIALS AND METHODS Grey-faced Petrel P. macroptera gouldi (Imber 1973), Great-winged Petrel P. m. macroptera and FIELD METHODS Soft-plumaged Petrel P. mollis (Schramm 1983), Data were collected from three locations in the Bonin Petrel P. hypoleuca (Harrison et al. 1983) Galapagos archipelago during 1984-l 986 (Fig. and Hawaiian Dark-rumped Petrel P. phaeopy- 1): Cerro Pajas, Floreana Island (1”13’S, gia sandwichensis (Simons 1985). Prey species 90”22’W); Media Luna, Santa Cruz Island (0”29’S, are mostly found in surface waters of the ocean 90”2 1’W) and Santiago Island (0”8’S, 90”32’W), at night. all of which are within the Galapagos National Park. Samples of petrel prey were collected ca- sually from chick regurgitates over three years 1Received 23 August 1991. Accepted 21 January fromFloreana(1984,n=3; 1985,n= 13; 1986, 1992. II = 35) two years from Santiago (1985, n = 10, [4371 438 M. J. IMBER ET AL. 1986, n = 4) and one year (1985, n = 17) from We assumed that increments in chick body Santa Cruz. These were stored in formalin or mass during the growth period reflected food de- 70% ethanol for up to seven months before anal- livery rates. Although some food packets were ysis. undoubtedly small, we assigned mass gains of The samples were initially sorted into food 5 5 g to measuring error. We partitioned the 120- classesand the total number of items in each day nestling period into lo-day segments and classwas recorded.We identified cephalopodand counted the number of feedings (mass incre- crustaceanprey to the lowest possibletaxonomic ments) and number of masslosses in each period. level using beaks and exoskeletons,respectively, Each increment of more than 5 g was counted and for fish prey using otoliths and skeletons. as one feeding. To more accurately assessthe Estimated weights of cephalopods taken were mass of meals, mean daily mass loss of chicks calculated from beak measurements, using for- during the same period was added to the daily mulae given by Wolff (1984) and Clarke (1986). mass gain when feeding occurred. Ten-day pe- Noting the findings of Imber (1973) and Furness riods with less than eight observations were et al. (1984) on the persistenceof larger mature omitted from analysis. We calculated mean cephalopod beaks in seabirds’ stomachs, we ex- number of feedingsper 1O-day period, and mean cluded from calculations of the cephalopod mass number of feedings per month. Due to a few component samples containing only beaks and/ missing observations, the number of feedings or eye lenses. during the chick period is underestimated and, On Floreana Island in 1986 we followed 20 therefore, the mean interval between feedings is petrel nests from the pre-laying season through slightly less than we estimate. fledging. We weighed the chicks daily at these We averaged the mass gained or lost daily per nests from hatching to fledging to estimate feed- chick during the previously assigned 1O-day pe- ing rate and meal size. A recurring problem in riods and also by months. Only increments or collection and analysis of the regurgitates in- decreasesfrom the previous day’s mass were in- volved the portion of oil in the sample. Several cluded in analysis. Both mass gains and losses samples were composed largely of oil and, in were analyzed using a repeated measuresdesign some cases,much of this was lost on the ground to clarify patterns of change in mass over time. and was impossible to quantify. Consequently, no attempt was made to measure the amount of RESULTS oil in each sample. We did not obtain the total PREY stomach or proventriculus contents from either chicks or adults due to the birds’ status as an Initially, parents feed stomach oil to their chicks endangered species.Nor was it possible to sam- by regurgitation. The oil’s primary function is ple directly the food organisms from the marine nutritive (Boersma et al. 1980) but it may also environment in the area of the Galapagos ar- be used as a defense against predators (Clarke chipelago. 1977). As chicks age, partially digestedfish, crus- taceans and cephalopodsare combined with the DATA ANALYSIS oil. Later in the nesting season, larger pieces or Data were analyzed using the SAS statistical whole animals are fed to the chicks. packageon an IBM VM370 computer at the Uni- Regurgitates consisted of oil, fish parts (ver- versity of Connecticut, Storm, CT. We separated tebrae, otoliths, scales, heads, jaws, clithrums, the petrel’s prey into three food classes and operculums, fin elements, flesh and mush), crus- counted the number of items in each sample as taceans (whole or carapaces, other exoskeletal an indication of the relative importance of each parts and bits), and partially digested cephalo- food classin the petrel’s diet. The Kruskal-Wallis pods (lenses, beaks, mantles, gladii, arms, sper- test was used for interisland comparisons of the matophores, hectocotyles, buccal bulbs, and proportion of each food classpresent in the sam- mush). Thirty-six prey items of the Dark-rumped ples. A frequency-of-occurrence analysis based Petrel were identified in three classesincluding on the number of samples containing each food 2 1 species in 16 families of the Cephalopoda class indicated whether a prey was consistently (Table l), seven families plus one order ofPisces, eaten. However, these data were not suitable for and five genera in five families of Crustacea(Ta- statistical tests of significance. ble 2). Becausethere were no differencesbetween DARK-RUMPED PETREL FEEDING ECOLOGY 91. 90. I , CULPEPPER i 0.30_.’ .WENMAN i PINTA l i cl GENOVESA G3 $JORTH SEYMOUR 2, FlAbA FERNANMNA aSANTA FE I *-. c7SAN CRISTOBAL 0 50km .* ESPAkLA FLOREANA Q si* 9o* FIGURE 1. Map of the Galapagosarchipelago with its location near the South American continent indicated in the upper right inset. The Dark-rumped Petrel colonies we studied are located on Floreana, Santa Cruz, and SantiagoIslands; their approximate locations are indicated by solid squares.Dark-rumped Petrel colonies also exist on San Cristobal Island and possibly on the southern volcanoesof Isabela Island. islands in the proportions of prey from each food ropus) and Decapods (Pasiphaeidae, Oplophor- class (Kntskal-Wallis test, x2 = 2.14, df = 2, P idae) were the most important crustacean prey. < 0.34) the data were pooled for further analysis However, the Cephalopoda were the most (Fig. 2). important prey category;the most frequently en- Cephalopods occurred in 86% of the samples, countered specieswere Sthenoteuthisoualanien- fish in 59% and crustaceansin 46%. Seven sam- sisjuveniles, Onychoteuthisbanksii, Mastigoteu- ples contained only beaks and/or eye lenses of this dentata,Pholidoteuthis boschmai juveniles cephalopods, but 45% of samples containing and Chiroteuthisveranyi. cephalopod remains had no beaks, reflecting the prevalence of juvenile and small mature ceph- FEEDING FREQUENCY AND alopods in this petrel’s diet. By estimating the MEAL SIZES percentage by mass of the three prey classesin The intervals between feedings for some chicks each sample, excluding residual items (beaks,eye were quite long and variable. Feeding frequency lenses) included in Figure 2, then pooling the of 14 chicks on Floreana in 1986decreased from data, we calculated the diet to comprise 46% a high of 3.75 f 1.03 feeds during the first 10 Cephalopoda, 37% Pisces and 17% Crustacea.
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