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Predation by a Brown Pelican at a Mixed-Species Heronry’

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Predation by a Brown Pelican at a Mixed-Species Heronry’ The Condor91:142-143 0 TheCooper Ornithological Society 1989 PREDATION BY A BROWN PELICAN AT A MIXED-SPECIES HERONRY’ MIGUELA. MORA Department of Wildrife and FisheriesBiology, Universityof California, Davis, CA 95616 Key words: BrownPelican: predation; mixed-species When the Brown Pelican (probably a male about 4 heronqqBaja California; Mexico. months postfledging, D. W. Anderson, pers. comm.; Schreiberet al. 1989) was first observedat the heronrv California Brown Pelicans(Pelecanus occidentalis cal- on 27 July 1988, a considerablenumber of birds hai ifornicus)are plunge-diving, fish-eating birds (Palmer left their nests,were flying in circles above the colony, 1962, Ashmole 1971). Jn some cases, they may be and were calling. The Brown Pelican was first observed largely dependent on only one speciesof fish at certain swallowingtwo eggsfrom a Cattle Egret nest, breaking times of the year. For example, about 90% of their diet them in its pouch, then regurgitatingthe shells. It then in southernCalifornia during the breeding seasoncon- moved to a mesquite between nests of a Cattle Egret sists of northern anchovies, Engraulix mordax (An- and a Great Egret and took one young from the Cattle derson et al. 1980). In other cases,however, the diet Egret and three young from the Great Egret nests, one is more diverse (consistingof more than 40 speciesof by one (Fig. 1). These young were about 1 week old fish and invertebrates in the Gulf of California, for and probably weighed near 100 g (Telfair 1983, p. 73). example; Anderson et al. 1982). Brown Pelicans have The Brown Pelican tried to simultaneouslyswallow all also been reportedas scavengersand as occasionalcan- four, but after several attempts was unsuccessful.It nibals on voung (Palmer 1962. D. 279). In extreme finally regurgitatedthree of the chicksand then ate and situations,~star&$ pelicans have also deen observed swallowed them singly. Both the Great Egret and the searchingfor fish scrapsand other waste food around Cattle Egret parentsstood nearby watching,but did not cities (Leek 1973). One recent scavengingcase was ob- deter the Brown Pelican. servedin December 1987in Monterey, California, when The Brown Pelican was again observed feeding on a juvenile Brown Pelican consumedthe fleshy pectoral chicksin the same area on 2 August 1988. Most of the portions and wings of an Eared Grebe (Podicepsnig- Cattle Egretsdid not leave their nestsas before; instead, ricollis) carcass(D. W. Anderson, pers. comm.). One they pecked at and injured the Brown Pelican’s bill as particular incident of predation by a Brown Pelican on it approachedtheir nests. I observed blood marks on living birds also occurredduring the fall in 1986 along the distal 10 cm of the Brown Pelican’s bill. Great the coastat Point Reyes,California, when a singleBrown Egrets, on the other hand, tended to leave their nests Pelican (young-of-the-year)preyed upon six live young when the Brown Pelican approached (Fig. 2). During Common Murres (Uris aalge) during a period of 3 hr this incident, the Brown Pelican displaced a pair of (A. Quintero, pers. comm.). Here I report a highly Cattle Egretsfrom their nest and ate two young about unusualcase of natural predation by a singleCalifornia 1 week old. It then took another young from another Brown Pelican which fed extensively on ardeid eggs Cattle Egret nest. The parents whose nests had been and nestlingsat a heronry in the Mexicali Valley, 42 preyed upon returned to their empty nests after the km southeastof the city of Mexicali, Baja California, Brown Pelican was gone, and a few stayed there for Mexico. several days following the predation. This heronry (32”15’N, 115”08’W) was located cen- One week after the first predation event by the Brown trally within an agriculturalvalley and was established Pelican was recorded, I observed fewer nests around on a small island (5,000 m2) formed by an agricultural the tree where the pelican loafed regularly.Some Cattle waste water canal that ran through cotton and aspar- Egrets had also dismantled previous nests (probably agusfields. A mixed-speciesbreeding colony here con- for nest materials but perhapsexacerbated by the pel- sisted mostly of Cattle Egrets, Bubulcus ibis (980/s), ican’s disturbances).In early August, about half of the Snowy Egrets, Egretta thula (lo/o), and Great Egrets, nestingpopulation of Cattle Egretshad young ready to Casmerodiusalbus (1%). The nesting sites were built fledge, and only 25-30% of the breeding adults re- on a densestand of salt cedar (Tamarix spp.) and some maining had young less than 2 weeks old or were still scattered mesquite (Prosopisspp.). No published in- incubating eggs.All predation seen occurred within a formation on breeding egretsexisted for the Mexicali short distanceof this Brown Pelican’s roostinglocation Valley, but interviews with farm workers and village near the middle of the ardeid colony. residents indicated that this colony had been estab- I saw the Brown Pelican at the heronry a total of 12 lished since 1972. times in the period 27 July-29 August. The Brown Pelican usually stood at the same perch on the top of a mesquite. My intermittent sightingsindicate that the I Received 13 January 1989. Final acceptance 20 Brown Pelican was probably at the colony for at least April 1989. 34 days during the breeding season. WI SHORT COMMUNICATIONS 743 FIGURE 2. A juvenile Brown Pelican preying on young Cattle Egretsand Great Egrets.One Great Egret observesfrom a distance. FIGURE 1. A juvenile Brown Pelican engagingin a ANDERSON,D. W., F. GRESS,K. F. MAIS, AND P. R. meal at a heronry in the Mexicali Valley, Mexico. The KELLY. 1980. Brown Pelicans as anchovy stock pouch contains four young egrets. indicators and their relationships to commercial fishing. CalCOFI Rep. 2 1:54-6 1. ASHMOLE,N. P. 1971. Seabird ecology and the ma- This manuscript has been improved by comments rine environment, p. 224-286. In D. S. Famer, J. from D. W. Anderson, D. E. Boellstorff,and two anon- R. King, and K. C. Parkes [eds.], Avian biology. ymous reviewers. I thank D. W. Anderson and M. E. Vol. I. Academic Press, New York. Mount for their help to obtain partial funding. I also LECK,C. F. 1973. Pelicansin the city of Lima, Peru. thank J. Coverly, R. Coverly, J. Mosqueda, and D. Condor 751357. Terry for help provided during the summer. Field re- PALMER,R. S. [ED.] 1962. Handbook of North Amer- search in the Mexicali Valley has been supported by ican birds. Vol. 1. Yale Univ. Press,New Haven, the U.C. MexUS nroaram. a fellowship from Conseio CT. National de Ciencia ; Tecnologia (CONACyT), a Jas- SCHREIBER,R. W., E. A. SHREIBER,D. W. ANDERSON, tro-Shields grant from the University of California Da- ANDD. W. BRADLEY. 1989. Plumanesand molts vis, and two researchassistantships from the Univer- of Brown Pelicans. Contrib. Sci. (Los Ang.) 402: sity of California Davis. l-43. TELFAIR,R. C., II. 1983. The Cattle Egret:Texas focus LITERATURE CITED and world view. The Caesar Kleberg Research ANDERSON,D. W., F. GRESS,AND K. F. MAIS. 1982. Program in Wildlife Ecology and Department of Brown Pelicans: influence of food supply on re- Wildlife and Fisheries Sciences,The Texas A&M production. Oikos 39:23-3 1. University System. .
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