Predation of Nesting Larids by Peregrine Falcons at Rasa Island, Gulf of California, Mexico ’
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SHORT COMMUNICATIONS The Condor95:706-708 0 The Cooper Ornithological Society 1993 PREDATION OF NESTING LARIDS BY PEREGRINE FALCONS AT RASA ISLAND, GULF OF CALIFORNIA, MEXICO ’ ENRIQUETA VELARDE Institute de Biologia, Departamentode Zoologia, Apartado Postal 70-153, 04510 Mexico D.F., Mexico Key words: Falco peregrinus; Gulf of California; cholla), a few large columnar cacti (Pachycereusprin- Larus heermanni; nestingsynchrony: predation; sea- gleyi, Stenocereusgummosus, Lophocereus schottit) and birds;Sterna elegans;Sterna maxima. some halophyte patches (Heliotropium curassavicum, Salicorniapact&a, Sesuviumverrucosum) by the tidal Peregrine Falcon (Falco peregrinus)diets vary widely. lagoon’s shores. In 1982, about 120,000 Heermann’s On the Pacific Coast of North America, they feed on Gull pairs(about 95% ofthe world’s population), 15,000 petrels and alcids (Oceanodroma,Synthliboramphus, Elegant Tern uairs (Sterna elenans)(also about 95% of Ptychoramphus)(Green 1916, Brooks 1926, Bent 1938, the-world’s population) and 8,500 Royal Tern pairs (S. Beebe 1960), while in the Gulf of California they take maxima) bred on the island between March and July Eared Grebe (Podicepsnigricollis), Black Storm-Petrel (Velarde 1989). Colony establishment occurred be- (Oceanodromamelania), Least Storm-Petrel (0. mi- tween 15-20 March. crosoma),Red-necked Phalarope(Phalaropus lobatus), METHODS Bonaparte’sGull (Larusphiladelphia),Heermann ’s Gull (L. heermanni) and Craveri’s Murrelet (Synthlibo- I searched the entire island for prey remains once a rumphuscraven )’ (Porter and Jenkins 1988). week from 20 March through 20 May 1982. Searches Due to seabird coloniality, closeto exclusivedepen- were done systematicallyfollowing a circuit along the dence of individual Peregrine Falcons on particular island’s edge and along the ridges of all the hills of the prey speciesmay put their survival in an inverse re- island; this allowed me to view all of the hillsides and lation to the degreeof pollution of marine areaswhere valleys below. Therefore, I assumedthat all prev were their prey feed. In the caseof Rasa Island, 95% of the found. Prey remains of Peregrine Falcons are-easily population of the two main prey speciesnest synchro- identified (Crainhead and Crainhead 1969):,, .oectoral nously, providing close to 300,000 prey available for muscles and entrails are totally consumed and occa- three months. sionally the skull is opened and the brain consumed. In this study I determined: (1) the dietary compo- Usually, only the wings attached to the scapulargirdle, sition of a breedingpair of PeregrineFalcons that hunt- the vertebral column, the pelvic girdle, the legs and ed on a densely occupied seabird nesting island in the lower portion ofthe skull are left. The only other raptor Gulf of California. Mexico. (2) the relationshio be- presenton the island was a pair ofCommon Barn Owls tween number of individuals ‘of each seabird species (Tyto alba). Over a ten-year period, the owls’ diet never and their relative consumption by a Peregrine Falcon included adult gulls nor terns (Velarde, pers. observ.). and (3) the falcon’s effect on those seabird populations. There were no other predatorson adult seabirdson the island. Other Peregrine Falcons are found on closest neighboringislands:Salsipuedes and Partida, both about STUDY SITE 7 km (SE and NW, respectivelv) from Rasa Island. Rasa Island, a 1,000 by 700 m flat island with a max- They do not, however, hunt on Rasa Island. imum altitude of 33 m is in the northern half of the Average masses of potential prey were calculated Gulf of California (28’49’N. 112”59’W). It is volcanic using a sample of 64 Heermann’s Gulls, 43 Elegant in origin and composed of low rocky hills and valleys Terns and ten Royal Terns capturedon the island. The covered with guano. There are three tidal lagoons on number of nesting Heermann’s Gulls was estimated the northwest side. Vegetation is scant (less than 5% for 20 randomly distributed quadrats (10 x 10 m). cover), consistingmainly of coastal saltbush(Atriplex This value was multiplied times the surface area oc- barclayana), a few patches of dense cholla (Opuntiu cupied by the gulls. Numbers of nesting Elegant and Royal Terns was estimated measuringthe surfacearea occupied by their colonies, multiplied by the number of nests per unit area for each species,estimated from ’ Received 16 September 1992. Accepted 22 Feb- 20 randomly distributed quadrats (1 x 1 m). The re- ruary 1993. lationship of number of prey consumed to available [7061 SHORT COMMUNICATIONS 707 prey was calculated through a chi-square analysis. In TABLE 1. Relationship between the nesting popu- the analysisI assumedthat I found all prey (seeabove), lation size and total number of individuals of seabird Peregrine Falcons ate only Larids (no other prey were speciespreyed upon by a pair of Peregrine Falcons on found), and falcons ate nowhere else (due to the pres- Rasa Island, Gulf of California, Mexico. ence of other falcon pairs in nearest islands). No. of PRYed RESULTS AND DISCUSSION Prey N&iIlg individ- Total weight Remains of 50 adult larids were found in the 66-day species population II& hunted (kg) period: 40 Heermann’s Gulls, nine Elegant Terns and Law heermanni 240,000 40 16.00 one Royal Tern. The estimated average food require- Sterna elegans 30,000 9 2.25 ment for active adult Peregrine Falcons in Britain has Sterna maxima 17,000 1 0.45 been estimated as about 127 g/day, plus 20% of prey remains that are not consumed(Ratcliffe 1980). Lower x2=4.625,r=4,0.25 > P>O.l estimates for North American falcons have been ob- tained only from captive individuals, presumablywith lower energydemands. Therefore, the PeregrineFalcon reducesindividual probability of being attacked. This pair should have consumedabout 20 kg of prey in the predation may be one of the selective forces for syn- 66 sampling days (127 g/day x 66 days x 2 birds + chronous breeding, together with Yellow-footed Gull 20% of unconsumedprey) on Rasa Island. According (Larus livens) predation on Heermann’s Gull chicks to the prey remains collected on the island and the (Velarde 1989. 1992) and food distribution in time and averagemass of these birds, the following amounts of space(Velarde et al.‘). prey were consumed: 16 kg of Heermann’s Gulls (400 Many Peregrine Falcon populations are threatened, g/individual x 40), 2.7 kg of terns (250 g/individual ElenantTern x 9 + 450 a/individual Roval Tern). This from both illegal capture,and pollution (Anderson and tot& 18.7 kg of prey co&umed, which is 93.5%‘of the Hickey 1970). Gulf of California populations, partic- ularly those along the Baja California coast and Gulf estimated requirement for a pair of falcons. This rep- of California islands,apparently are in better condition resents 86% by mass for Heermann’s Gulls, 12% for (Porter and Jenkins 1988). Some seabirds on which ElegantTern, and 3% for Royal Tern. The proportion Gulf of California falcons feed may be moderately to in number of prey consumedofeach ofthe three species highly contaminated with pesticides, particularly or- in relation to those present(nesting population) did not ganochlorines(e.g., Heermann ’s Gull: DDE = 6.12 ppm; differ significantly (Table 1, x2 = 4.625, df = 2, 0.25 DDT i 0.1 ppm; PCBs = 1.67 ppm; Porter and Jenkins > P > 0. l), indicating that falconstook prey according 1988). Pairs of PeregrineFalcons in the gulf are prob- to their availability in the environment and apparently ably “specialists” on the particular prey combination not according to ease of capture or any other factor (Table 1). This is in agreement with the functional in their eyrie area. For example, falcons-onIsla Partida responsemodel (Hollings 1959, Oaten and Murdock (aonrox.~ . 7 km SE from Rasa Island). feed heavilv on 1975, Abrams 1982). Least and Black Storm-Petrels (Oceanodromamicro- soma and 0. melania, respectively), the major species The 40 Heermann’s Gulls consumed during the 66 nesting on that island (D. W. Anderson, pers. comm.) sampling days constitute less than 0.02% of the total and on gulls and terns on Rasa Island (pers. observ.). nesting population. The nine Elegant Terns represent Close to exclusive dependence of Gulf of California only 0.03% of the nesting population. PeregrineFalcons on seabirdsfor food placestheir sur- A Peregrine Falcon chick consumes an average of vival in an inverse relation to the degree of pollution 157g ofprey per day during its first year oflife (Ratcliffe of the ocean and in direct relation to the state of their 1980). Heermann’s Gulls are on the island for an av- erageof four months per year. Assuming that a falcon prey’s populations. pair in the Gulf of California produces an average of During this study I was supported by a graduate 1.6 chicks per year (Porter et al. 1988) the total mass fellowship program “Programa de Superacionde1 Per- of prey consumed should be 31 kg in four months. sonal Academico” through the Instituto de Biologia of Therefore, an averagefamily could consume, at most, the Universidad National Autonoma de Mexico. Field about 0.05% of the total adult Heermann’s Gull pop- work was supported by the Instituto de Biologia, the ulation. Consejo National de Ciencia y Tecnologia, Intema- Heermann’s Gulls spend several weeks before egg- tional Federation for the Conservation of Birds, Na- laying performing mass flights, at dusk when they re- tional Wildlife Federation and International Council turn to the nesting areas and at dawn when leaving for Bird Preservation, through the Arizona-Sonora (Velarde 1989 and unpubl. manuscript). These mass Desert Museum. I thank the following people for their performances,never reported for other gulls, undoubt- support and encouragement:B. VillaIR., J. San&h&n, edly facilitate nesting synchrony. Synchronousbreed- F. S. Todd, J. D. Hair, R.