The Condor 94~437-441 0 The Cooper Ornithological Society 1992

FEEDING ECOLOGY OF THE DARK-RUMPED 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 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: 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 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 ’ 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 ’ 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 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

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aSANTA FE

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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. days after hatching (that is, about once every 2.6 The most frequently captured fish were Sterno- days) to a low of less than one feeding every 10 ptychidae (hatchet fish), Exocoetidae(flying fish), days over the last 20 days of the nestling period Stomiatoid dragonfish and Macrouridae (rat- (Fig. 3). Feeding rates were highest during the tails). Amphipods (Eurythenes), Isopods (Anu- first month after hatching (3.33 + 1.Ol feeds per 440 M. J. IMBER ET AL.

TABLE 1. Cephalopoda identified from beaks in 83 food samples from Dark-rumped Petrels of Galapagos Islands. Mass (g) determined from beak measurements:lower rostra1length (mm) for squids, lower hood length (mm) for octopuses.

Squids Ancistrocheiruslesueuri 1 Sub-ad. 3.8 96 96 2 Octopoteuthisnielseni 5 various 3.4-7.0 52 262 5 Tun&gia danae 1 Juv. 2.3 9 9 Onvchoteuthisban&i 19 Ad. 2.1-2.6 44 845 1: ’ Discoteuthissp. 2 Juv. 3.8 89 178 4 Gonatusantarcticus 1 Ad. 6.7 291 291 6 Pholidoteuthisboschmai 11 Juv. 1.7-3.3 35 383 8 Histioteuthisheteropsis Ad. 3.0-4.2 109 548 Histioteuthisspp. : Ad. 2.5-2.9 53 160 Ctenopterjx sicula 5 2.0-2.1 200 Eucleoteuthisluminosa 2:: 4.1 %: 99 Sthenoteuthisoualaniensis 3: Juv. 1.1-2.2 13 424 Chiroteuthisveranyi various 1.5-4.3 17 117 Mastigoteuthisdentata 12 Ad. (1J) 2.0-3.8 44 528 Leachia sp. Ad. 1.2 3 3 tr Galiteuthispaczjica 6 various 2.3-4.8 56 333 7 Unidentified squid 1 Ad. - 40 40 Octopuses Amphitretuspelagicus 1 Ad. 3.3 69 69 Vitreledonellarichardi 1 Ad.? 2.0 33 33 Argonautasp. 2 Ad.? 2.7 51 102 Alloposusmollis 1 Sub-ad. 6.5 184 184 i Totals and mean mass 119 41 4,904 100

10 days or about 10 feeds per month) dropping the second (74.9 + 32.6 g, n = 69) lessening to 2.88 + 0.74 and 2.87 f 1.13 feedings per 10 during the third (70.2 f 26.4 g, n = 52) and days (about 8.5 feeds per month) during the sec- fourth month (68.4 + 33.6 g, n = 28). Combining ond and third months, and falling further to an this information with frequency of feedings per average of 1.15 f 1.O 1 feeds per 10 days (about month, we found that chicks received on average 3.5 feeds per month) during the last month in an estimated 565 g of food during days O-30, the nest. Intervals between feedingsaveraged 3.3 647 g in days 31-60, 605 g in days 61-90, and days for the first three months of the develop- 236 g in days 90-120. This indicates that chicks ment period but increased to 8.5 days in the received at least 2,053 g of food: 59% of the total month before fledging (Fig. 4). However, the food was received during the first half of devel- number of feedings is probably underestimated opment, and 88% of the food was delivered by in the last month, when small feeds are most the end of the third month of the nestling period. likely (Imber 1976) causingthe feeding intervals A repeated-measures analysis of the mass to be somewhat overestimated. gained by chicks indicated that individuals re- Estimated food loads, which included consid- ceived similar meal sizes by mass throughout erable proportions of stomach oil, ranged from development with the exception of days 60-70 less than 30 g to over 150 g per feeding and (F,2,,5= 3.47, P < 0.01) and 70-80 (FI ,,,, = 3.57, averaged around 68 g. The estimated average P < 0.02) when chick mass gain was more vari- mass of cephalopodstaken (4 1 g) was not much able. Similarly, individual chicksappeared to lose less, indicating that these petrels prefer cepha- massbetween feedingsin a constant manner with lopods as large as they can consume. Chicks the exception of the 20-30 day (F,,,l, = 3.55, P gained the least mass per feeding during the first < 0.004) and 80-90 day periods (F6,,0 = 6.30, P month after hatching (56.5 f 24.0 g, n = 80) < 0.006) when mass loss was more variable. and gained the greatest mass per feeding during Estimated loss of chick mass between feedings DARK-RUMPED PETREL FEEDING ECOLOGY 441

TABLE 2. Fish and Crustaceain regurgitationsor stomachcontents from 83 Dark-rumped Petrelson Galapagos Islands.

Food ,tem Frequency in identification n 83 samples

Fish Total 62 Clupeidae Harengulacf. peruvana 1 1 Stemoptychidae Argyripnussp. 1 1 unidentified 14 12 Stomiatoidea unidentified 3 3 Stomiatidae unidentified 3 3 Myctophidae Electronacf. paucirastra 1 unidentified : 3 Gadiformes unidentified 5 4 Macrouridae unidentified 2 2 Exocoetidae Hirundichthyssp. or 6 6 Fodiatorsp. unidentified 1 1 Bramidae unidentified 4 4 Unidentified 18 17

Crustacea Total 52 Mysidacea Lophogastridae Gnathophausiacf. zoea 2 2 Amphipoda Lysianassidae Eurythenessp./spp. 18 16 Isopoda Anuropodidae Anuropussp. 6 5 Decapoda Oplophoridae Acanthephyrasp. 5 5 cf. Notostomussp. 1 1 Pasiphaeidae unidentified 5 4 unidentified decapods 7 7 Unidentified 8 8 indicated that the greatestloss of mass (F, ,,347= mastrephidae (some Sthenoteuthisoualaniensis), 3.74, P < 0.001) occurred during the secondand Histioteuthidae, Octopoteuthidae, Chiroteu- third months of the development period (26.3 thidae, Onychoteuthidae (Onychoteuthis sp. or ? 16.8 g per day and 27.7 + 16.4 g per day, Moroteuthis sp.), and Pholidoteuthidae (Pholido- respectively). Least mass was lost during the teuthis sp.). Exocoetidae fish and the isopod An- month after hatching (18.1 f 14.4 g per day) and uropuspacijicawere also identified. There is much the month before fledging (2 1.1 + 17.9 g per day) overlap in the prey speciestaken by the Waved (Fig. 5). Albatross and the Dark-rumped Petrel. Harris’ data do not indicate the size of prey taken by the DISCUSSION by the albatross, so the amount of overlap re- mains unknown. However, theseprey speciesmay PREY be the most common or most easily obtainable In common with the Galapagos subspecies,Si- for both species.Seasonal variation in avail- mons (1985) found that the Hawaiian Dark- ability of these prey to seabirdsof the Galapagos rumped Petrel took fish of the families Exocoe- is unknown, but Banse (1964) noted that at low tidae, Stemoptychidae and Myctophidae, squid latitudes massive nightly migrations to the sur- of the Ommastrephidae, and also Anuropus iso- face occur throughout the year. However, the pods. Loomis (1918) reported pteropods and distribution of prey organisms around the Ga- coelenteratesin Galapagos Dark-rumped Petrel lapagosis not well documented. Many prey taken stomachsbut we found no trace of these. by Dark-rumped Petrels are mesopelagicspecies Harris (1973) found that squid beaks regur- that migrate vertically from the deeper ocean gitated by chicks of the Waved Albatross (Di- layers (200 m-1,000 m) to feed at the surface at omedea irrorata) in the Galapagos included Om- night and then return to depths during day. Im- 442 M. J. IMBER ET AL.

3 13 10 17 35 4 83

80

Island and Year FIGURE 2. The percent of each prey class fed to chicks by Dark-rumped Petrel adults on three Galapagos Islands in 1984 to 1986. Sampleswere analyzed individually and then pooled for this comparison. In descending order: cephalopods,fish, crustaceans.Sample sizes at top. ber (1973) discusses this aspect of Grey-faced large fragments of prey, such as eye lenses, came Petrel feeding ecology. Of the cephalopods taken from large cephalopodsthat the petrels probably by Dark-rumped Petrels, 82% by number and scavenged.However, the composition of regur- 75% by mass are bioluminescent, which may aid gitatesof Dark-rumped Petrelsindicates that most the birds in detecting prey at night. As well as foraging is nocturnal. Many of the fish prey, in- flying fish, a frequent squid prey (Onychoteuthis) cluding unidentified ones, had less robust oste- is a “flying squid” (Barnes 1980). Both might be ology indicating their deep sea origin. But the captured on the wing by this petrel, as its close sizeof fish taken is also surprising.Standard length relative the Juan Femandez Petrel (Pterodroma of one myctophid was 175 mm while the pectoral externa) catches flying fish (Ainley and Boekel- fin of one flying fish was at least 160 mm long. heide 1983). As these prey were consumed entire, the Dark- Argonautids, ommastrephids and flying fish rumped Petrel is capable of capturing prey larger may be caughtduring the day (Ashmole and Ash- than previously supposed. Most cephalopods mole 1967) particularly crepuscularly;and some taken were one-meal-sized. DARK-RUMPED PETREL FEEDING ECOLOGY 443

6

30 50 70 90 110 Age of Chick (days) FIGURE 3. Rate of feedingsdelivered to Dark-rumped Petrel chickson Floreana Island in 1986. Bars represent means per lo-day period, sample sizes are indicated over the error bars.

Among the problems in analysis of regurgitates squid in the diet while underemphasizing the im- is the danger of overestimating some types and portance of fish and crustaceans. sizes of prey while underestimating others. Fur- STOMACH OILS ness et al. (1984) report that squid beaks can remain and collect for at least 50 days in the Cummins (1967) Clarke and Prince (1980) stomach of the Shy Albatross (Diomedeacauta) Croxall and Prince (1982) and Simons and Whit- whereasfish remains are quickly digested.Small tow (1984) found calorific values for some ceph- cephalopod beaks may also be completely di- alopods,crustaceans and fish to range from 1.07- gested (Imber 1973). Fish identifications were 1 1.53 kJ per g of wet mass. Simons and Whittow very difficult due to several factors including: (1) analyzed several food samples from the Hawai- the material’s advanced state of digestion, (2) ian Dark-rumped Petrel and found that fish and scarcity of information on deep sea fish in Ga- squid yielded 4.5-5.0 kJ per g, while stomach oil lapagoswaters; and were complicated by (3) the yielded 41.74 kJ per g. Energy content of oils normal changesin fish morphology with age. Be- from 12 speciesof analyzed by causeof these considerations, our analysis of the Warham et al. (1976) ranged from 39.23-42.17 importance of prey types by their frequency of kJ per g. Stomach oils can therefore represent a occurrence may overestimate the proportion of 3.5 to 35 times more concentrated energy source 444 M. J. IMBER ET AL.

7s 104 T 1 I35 I

O-30 31-60 61-90 91-120 I-30 31-60 61-90 91-120 Age of Chick (days) Age of Chick (days) FIGURE 4. Average intervals between feedings of FIGURE 5. Estimated averageloss of mass between Dark-rumped Petrel chicks on Floreana in 1986. Error feedingsfor Dark-rumped Petrel chicks on Floreana in bars and sample sizesare given for each 30-day period. 1986. Sample sizes are given above the error bars. than prey at the time of capture. Oils may be the boschmaiare taken by sperm whales (Clarke main sourceof energy and water for small chicks 1980). (Warham et al. 1976) as they form the bulk of From Pitman’s data on petrel distribution material regurgitatedby young Dark-rumped Pe- around the Galapagos, we suggestthat Dark- trel chicks. Simons and Whittow (1984) sug- rumped Petrels take the more abundant prey gested that 30-50% of the energy requirements found within 15” of the equator (Ring and Iver- of Dark-rumped Petrel chicks in Hawaii were sen 1962). Because the upwelling currents tend met through stomach oil fed to them. to vary in strength seasonally there may be ac- companying shifts in petrel foraging zones. FORAGING RANGE FEEDING FREQUENCY AND Systematic sightingsfrom ships have been made MEAL SIZES by several observers, and observations by R. L. The infrequent feedings in the period before Pitman (1986) on Dark-rumped Petrel density fledging suggestthat, like Leach’s Storm-petrel and distribution at sea are presentedin Figure 6. (Oceanodromaleucorhoa) (Ricklefs et al. 1987) Petrel density is greatest in and around the is- and the Hawaiian Dark-rumped Petrel (Simons lands, and to the southeast,south, and southwest and Whittow 1984) the chicks metabolize and of the archipelago (from 2”N to 13”S, a range of store much fat some time before they fledge (Lill 300-l ,700 km). The northern and southern areas and Baldwin 1983). Because meal size is fairly coincide with the productive watersresulting from constant over the nestling period and because the upwelling and deflection of the Equatorial feeding rate decreasesduring the last month be- Undercurrent. Concentrations of petrels to the fore fledging, a decreasein chick mass during the southwest of the archipelago also coincide with last month is not surprising. We found no pro- the productive waters where cetaceansare prin- nounced starvation period in thesebirds, but the cipally located (Whitehead 1987). Juvenile decreasedfrequency of food delivery was marked. Pholidoteuthisboschmai are a component of The averagemeal size of about 68 g (16% of adult Dark-rumped Petrel regurgitate, while adult P. mass) is similar to that reported for the Grey-

446 M. J. IMBER ET AL. faced Petrel (97 g, 18%) (Imber 1973) Great- munities of the South Pacific Ocean. Stud. Avian winged Petrel (86 g, 14%) (Schramm 1983), Soft- Biol., No. 8:2-23. ASHMOLE,N. P., AND M. J. ASHMOLE. 1967. Com- plumaged Petrel (74 g, 20%) (Schramm 1983) parative feeding ecology of seabirdsof a tropical and Phoenix Petrel (49.4 g, 18.2%) (Ricklefs oceanic island. Bull. Peabody Mus. Nat. Hist. 24: 1984). 1-131. The mass-specificmetabolic rate and the en- BANSE, K. 1964. On the vertical distribution of zoo- ergy requirement for growth in Hawaiian Dark- plankton in the sea, p. 53-125. In M. Sears [ed.], Progressin Oceanography,Vol. 2. Macmillan, New rumped Petrels are highest in very young chicks York. (Simons and Whittow 1984). This pattern has BARNES,R. D. 1980. Invertebrate zoology. Saunders also been found in three burrow-nesting petrels College, Philadelphia. on Marion Island (Brown 1988). This suggests BOERSMA,P. D., N. T. WHEELWRIGHT,M. K. NERINI, AND E. S. WHEELWRIGHT.1980. The breeding that much energy in the early nestling period is biology of the Fork-tailed Storm-Petrel (Ocean- devoted to developing the thermoregulatory odroma furcata). Auk 97:268-282. ability just after hatching. The increased mass BROWN, C. R. 1988. Energy requirementsfor growth loss during the 30-90 day period in the Gala- of Salvin’s Prions Pachvotila vittata salvini. Blue pagosDark-rumped Petrel chick is consistentwith Petrels Halobaena caeruieaand Great-winged Pe- trels Pterodromamacroptera. Ibis 130:52%534. the highest total energy expenditures found dur- CLARKE. A. 1977. Contamination of Perearine Fal- ing this same period in the Hawaiian Dark- cons (Falco pere,q-inus)with fulmar stomach oil. rumped Petrel. During this period the chick may J. Zool. 181.11-20. be more sensitive to limitations of food delivery. CLARKE.A.. ANDP. A. PRINCE. 1980. Chemical com- position and calorific value of food fed to molly- Variable food resourcesmay reduce the energy mauk chicks Diomedea melanophris and 0. diverted to growth and development during this chrvsostomaat Bird Island. South Georgia. Ibis period, thus extending the total time to fledging. 122:488-494. CLARKE,M. R. 1980. Cephalopoda in the diet of ACKNOWLEDGMENTS sperm whales of the Southern Hemisphere and their bearing on sperm whale biology.-. Discovery We are gratefulto Dr. Malcolm Coulter and Dr. Friede- Rep. 37:1-324. mann Kiister and to WWF-International, which sup- CLARKE.M. R. 1986. A handbook for the identifi- ported the project in the field (1982-1986), while a cation of cephalopodbeaks. Clarendon Press,Ox- grant from the Hawaiian Audubon Society (1988) aid- ford, England. _ ed with data analysis. The GalapagosNational Park, CROXALL.J. P.. AND P. A. PRINCE. 1982. Calorific under the leadership of Lcdo. Miguel Cifuentes and content of squid (Mollusca:Cephalopoda). Br. Lcdo. Faust0 Cepeda, allowed accessto petrel nesting Antarct. Surv~Bull. .55:27-3 1. areas, and supported our work with the loan of Park CRUZ.J. B.. ANDF. CRUZ. 1987. Conservation of the personnelon severaloccasions. The staff of the Charles Dark-rumped Petrel Pterodromaphaeopygia in the Darwin ResearchStation and its Director, Dr. Gunther GalapagosIslands, Ecuador. Biol. Cons. 42:303- Reck, were exceedinglyhelpful, especiallywith the lo- 311. gisticsand administration of the project. Many workers CUMMINS,K. W. 1967. Calorific equivalentsfor stud- contributed their time and energy in the field and we ies in ecological energetics. Pymatuning Lab. of thank them all, especially Maria Herminia Comejo, Ecology, Univ. of Pittsburgh, Pittsburgh. Claudio and Eliecer Cruz, Greg Estes,Monica Fabara, FURNESS,B. L., R. C. LAUGKXH, AND D. C. DUFFY. Mirella Pozo, and Hugo Sanchez.Several others have 1984. Cephalopod beaks and studies of contributed freely of their time and advice including diets. Auk 101:619-620. Drs. Brian Bell, GeorgeClark, Lyn Fiedler, Mac Given, HARRIS,M. P. 1973. The biology of the Waved Al- Jim Keith, Fred Streams,and Kent Wells. We are par- batrossDiomedea irroruta of Hood Island, Gala- ticularly grateful to Dr. Uwe Koehn for his help and pagos.Ibis 115:483-510. advice with statistical analysis, and to Chris Lewis, HARRISON,C. S., T. S. HIDA, AND M. P. SEKI. 1983. BrookeNodden, and Mary Jane Spring for drawing the Hawaiian seabird feedingecology. Wildl. Monogr. figures. Robert Pitman contributed unselfishly of his 85:1-71. information on the distribution of Dark-rumped Pe- IMBER,M. J. 1973. The food of Grey-faced Petrels trels at sea. We are very grateful to the Los Angeles (Pterodroma macroptera gouldi (Hutton)), with County Museum of Natural History for the use of special reference to diurnal vertical migration of equipment and facilities. This is contribution No. 468 their prey. J. Anim. Ecol. 42:645-662. of the Charles Darwin Foundation for the Galapagos IMBER,M. J. 1976. Breeding biology of the Grey- Islands. faced Petrel Pterodroma macropteragouldi. Ibis 118:51-64. LITERATURE CITED KING, J. E., AND R.T.B. IVERSEN. 1962. Midwater trawling for forageorganisms in the Central Pacific AINLEY, D. G., AND R. J. BOEKELHEIDE.1983. An 1951-1956. U.S. Fish and Wildlife Serv. Fish. Bull. ecological comparison of oceanic seabird com- 62:271-321. DARK-RUMPED PETREL FEEDING ECOLOGY 447

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