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The Foraging Ecology of Peruvian Seabirds

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The Foraging Ecology of Peruvian Seabirds THE FORAGING ECOLOGY OF PERUVIAN SEABIRDS DAVID CAMERON DUFFY PercyFitzPatrick Institute of AfricanOrnithology, University of CapeTown, Rondebosch7700, South Africa ABSTRACT.--Mostfeeding by seabirdsin the Peruvian CoastalCurrent, an upwelling of high productivityoff the west coastof SouthAmerica, takes place in groups.The majorprey is an anchovy (Engraulisringens), which occursin large shoalsand is exploited mainly by three species:the Peruvian Booby (Sula variegata),Peruvian Brown Pelican (Pelecanusocci- dentalisthagus), and GuanayCormorant (Phalacrocorax bougainvillii). Other foragingsituations have different species'compositions, and these appear to be related to the size, depth, and duration of availability of prey. Dominance interactionsbetween speciesmay be important in structuringflocks that are scavengingor feeding on planktonswarms. Interspecific piracy seemsunimportant in flocks foraging on fish shoals.Certain speciesusually arrive first at new feeding situations.These species may be used as guidesby other speciesor merely may be faster and thus reach food sourcesfirst. Studies of foraging of seabirdsshould be a valuable addition to the study of the distribution of birds at sea. Received2 April 1982, resubmitted18 March 1983,accepted 8 July 1983. SEABIRDShave some of the largest foraging tation of patches of food (e.g. Hoffman et al. ranges of any vertebrates:birds with eggs or 1981)? young may travel 1,000 km from the nest to feed (Fisher and Lockley 1954, Harris 1977, METHODS Nelson 1979, Dunnet and Ollason 1982). De- spite the fact that they searchover such large The studyarea.--I gathered most of my observa- tions on Isla Mazorca (11ø23'S, 77ø45'W), Departa- areas,they forage over very small areason local mento de Lima, Peru. The island lies approximately concentrationsof prey (see Brown 1980 for a 15 km offshorein an areaof frequentupwelling (Zuta review). The study of these patchesand their and Urquizo 1972)and is the site of a large and long- use by birds should improve our understand- establishedcolony of guano birds, especially Peru- ing of the ecologyand distribution of seabirds vian Booby (Sulavariegata), Peruvian Brown Pelican at sea.For example,Erwin (1977) linked differ- ( Pelecanusoccidentalis thagus), and Guanay Cormorant ences in use of patchesby three larids to dif- (Phalacrocoraxbougainvillii) (Murphy 1925;Duffy 1981, ferencesin their nesting ecology. Hoffman et 1983a).Other observationswere madeon 12 boat trips al. (1981) showed that the presence of some between Isla Mazorca and the port of Huacho speciesmay affectthe foragingof others;flocks (11ø07'S,77ø44'W) and from the shore at La Puntilia, Callao Harbor (12ø05'S, 77ø44'W). are more than aggregationsof noninteracting The Peruvian Current is one of the most produc- birds attracted to a common food source. tive of marine ecosystems(Cushing 1971).It extends The structure of seabird flocks has never been from northern Chile to northern Peru and out to the investigated in low-altitude upwelling areas. GalapagosIslands, Ecuador(Murphy 1936). The up- The present paper reportson the numbersand welling is confinedto a relatively narrow band along behaviors of seabird speciespresent in differ- the coast(Cushing 1971). The dominant fish species ent types of feeding flocksin the Humboldt or of the upwelling is the Peruvian anchovyor ancho- Peruvian Coastal Current off the west coast of veta (Engraulisringens), which makesup about 90% of South America. Three general questionswere pelagic fish stocksin the current, basedon relative investigated:(1) what percentageof birds for- abtmdancesof eggsand larvae (Santander1981). The anchoveta is eaten by a wide variety of seabirds, age in flocksover patchesof prey, (2) doesthe predatory fish, and marine mammals (Coker 1920, species'composition of flocks differ in accor- Murphy 1936,Jordan and Fuentes1966, Paulik 1971). dance with the type of feeding situation, and A commercialfishmeal industry, which beganin 1955, (3) are flocksmerely aggregationsof indepen- took catchesof up to 12 million metric tons of an- dently attractedbirds or do the speciesinteract choveta, becoming the world's largest single-species to enhanceor inhibit the discoveryor exploi- fishery before the fish stockand industry collapsed 800 The Auk 100: 800-810. October 1983 October1983] Foragingof PeruvianSeabirds 801 TABLE1. Percentageuse of differentforaging situations by Peruvianseabirds (+ = <0.1). Percentagein foraging situation Zoo- Number Scav- Sea- plank- Species observed Shoal enging lions ton "Other" HumboldtPenguin (Spheniscus humboldti) 18 I1 - - - 88 Waved Albatross (Diomedeairrorata) 52 48 - - - 52 Wilson's Storm-Petrel(Oceanites oceanicus) 54 94 - 6 - - Elliot'sStorm-Petrel (Oceanites gracilis) 1 .... 100 Markham's Storm-Petrel (Oceanodromamarkhami) 1 .... 100 Unidentifiedstorm-petrels 21 5 - - - 95 CapePetrel (Daption capense) 2 .... I00 SootyShearwater (Puffinus griseus) 61,234 98 - + + 2 PeruvianDiving-Petrel (Pelecanoides garnoti) 61 7 - - - 93 MagnificentFrigatebird (Fregata magnificens) 1 .... 100 NeotropicalCormorant (Phalacrocorax olivaceus) 11 .... 100 GuanayCormorant (Ph. bougainvillii) 115,542 99 - - - 1 Red-leggedCormorant (Ph. gaimardi) 114 51 - - - 49 Blue-footedBooby (Sula nebouxii) 2 50 - - - 50 PeruvianBooby (S. variegata) 402,802 95 - + - 5 Peruvian Pelican (Pelecanusoccidentalis thagus) 3,341 69 + + - 30 Red-neckedPhalarope (Phalaropus 1obatus) 1 .... 100 PomarineJaeger (Stercorarius pomarinus) 11 27 - - - 72 Jaegersp. (Stercorariussp.) 7 71 - - - 28 GreyGull (Larusmodestus) 1,429 41 + 3 43 12 Band-tailedGull (L. belcheri) 554 4 49 3 16 28 Kelp Gull (L. dominicanus) 94 3 71 12 14 - Franklin'sGull (L. pipixcan) 743 36 11 I - 51 Sabine'sGull (L. sabini) 86 - - - 21 79 "Comic" Tern (Sternasp.) 1,328 I - + 44 55 ElegantTern (Sternaelegans) 5 60 - - - 40 Sandwich Tern (S. sandvicensis) 2 .... 100 Peruvian Tern (S. lorata) 2 .... IO0 Inca Tern (Larosternainca) 7,195 26 I 8 30 34 Percentageof total numberof birds (n = 561,605) 94.4 0.1 0.2 0.6 4.7 Number of observationsfor each foraging type 65 67 49 37 137 Number of speciesin each foraging type 20 6 I0 7 27 in 1972-1973(Idyll 1973;Duffy 1983b).A pilchard surface-diving.I added "terrestrial-seizing" for birds (Sardinopssagax) was thought to havereplaced the feedingon landor in theintertidal zone. For dipping anchovetaduring the 1970's(Walsh et al. ! 980),but species,I measuredduration of contactwith water, the data are equivocal. using a spring-operatedstopwatch and subtracting Observations.--Imade as many observationsof ma- 0.2 s for my reactiontime. For divingand plunging rine birds as possibleduring the courseof other species,I measured the duration of submergencefrom fieldworkbetween September 1977 and March 1978. first contact until the bird's head emerged. I mea- I recordedall foragingbirds encountered. Species are suredrates of dipping or surface-seizingby follow- listed in Table I. ing individualbirds until they completedI0 at- Most seabirds in the Peruvian Coastal Current are temptsor ceasedfeeding. I recordedinterspecific relativelyeasy to identify. The exceptionsare the interactions that included association with marine white "comic" terns (Sternaspp.) and the jaegers mammals,aggressive displacement from potential (Stercorariusspp.). Unidentified birds are treatedsep- sourcesof food,kleptoparasitism, and order of arriv- aratelybut makeup onlya minusculeproportion of al at food sources. Not all of these data could be col- birdsobserved. I determinednumbers in small groups lectedsimultaneously, so, in the results,the sample (lessthan 20-50)by directcounts of individuals.In sizesmay differ betweendifferent measurements. larger groups,I estimateda subgroupand then I assumedthat the foraging behaviorsand situa- countedthe number of such subgroups.I defined tions I encounteredwere proportional to their true feedingtechniques by followinga simplifiedversion abundances.I attempted to observeas wide a variety of Ashmole's (1971) terminology: piracy, dipping, of foragingsituations as possible by watchingfrom surface-seizing,plunging, pursuit-plunging,and both shipsand land; by attemptingobservations at 802 DAVID CAMERONDUFFY [Auk, Vol. 100 TABLE2. Species'occurrences over dense shoals of fish (+ = < 0.1). All birds Frequency Number Number/ at shoals at shoals Species present occurrence (%) (%) Peruvian Booby 381,957 6,160 68 95 Guanay Cormorant 114,380 2,288 20 77 Sooty Shearwater 60,036 7,504 ! 1 12 Peruvian Pelican 2,320 122 0.4 29 Inca Tern 1,888 118 0.3 25 Grey Gull 584 117 0.! 8 Franklin's Gull 270 39 + ! 1 Red-legged Cormorant 58 12 + 8 Wilson's Storm-Petrel 51 25 + 3 Waved Albatross 25 4 + 9 Band-tailed Gull 23 7 + 5 "Comic" tern 8 3 + 5 Jaegersp. 5 1.3 + 5 Peruvian Diving-Petrel 4 0.6 + 5 Pomarine Jaeger 3 1.0 + 5 Elegant Tern 3 3 + 1.5 Kelp Gull 3 3 + 1.5 Humboldt Penguin 2 1 + 3 Blue-footedBooby i 1 + 1.5 Storm-petrel sp. ! 1 + 1.5 night, using a full moon or bioluminescence;and by seen were feeding alone. After fieldwork, I combining observationsfrom before (September-Oc- found that the records could be divided into tober) and during (November-March) the breeding five arbitrary but recognizableforaging situa- season.The 7-month period should also have been tions:shoal-foraging, scavenging, foraging over long enough to sample any seasonalvariations in sealions,feeding on zooplankton swarms,and foraging. miscellaneous other situations. These are re- ferred to as shoal, scavenging, sealion, zoo- RESULTS plankton, and "other" foraging groups throughout
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