Robin W. Baird
Colonial Waterbirds 19:260-261. 1996.
Abstract
I observed a Yellow-footed Gull (Larus livens) capture an apparently healthy Black Storm-Petrel (Oceanodroma melania) near a breeding colony of petrels in the Bay of La Paz, Baja California Sur, Mexico. The petrel was caught after it left the colony, approximately 75 m from shore, when it was forced to the water's surface by the gull, and apparently consumed. This represents the first report of a Yellow-footed Gull killing a storm-petrel, as well as the first record of larid predation on Black Storm-Petrels. It is unusual also, because the petrel was captured at sea, rather than directly on a breeding colony.
Key words. - Yellow-footed Gull, Larus livens, Black Storm-Petrel, Oceanodroma melania, predation.
Predation on storm-petrels at seabird colonies has been recorded on numerous occasions. However, actual observations of predation are rare. Most records of predation are indirect, usually obtained through analysis of stomach contents, pellets, or remains collected from predators (e.g., gulls, owls and falcons - French 1979, Trapp 1979, Vermeer et al. 1988, but see Watanuki 1986, Ryan and Moloney 1991). Predation by diurnal predators is believed to exert selection influencing the development of nocturnal behaviour in storm-petrels at colonies, with birds usually approaching and departing from colonies at night (cf. Lockley 1932, Harris 1974, Watanuki 1986). Here I describe an incident of Yellow-footed Gull (Larus livens) predation on a Black Storm-Petrel (Oceanodroma melania).
On February 14, 1996, at approximately 0840 hrs local time, a Black Storm-Petrel was seen leaving Los Islotes, a small pair of islands in the Bay of La Paz, Baja California Sur, Mexico. This site is a breeding colony of both Black Storm-Petrels and Yellow-footed Gulls. As at other storm-petrel colonies, petrels are rarely seen close to shore in daylight hours at this site (pers. obs.). The storm-petrel was observed at close range (within 5 m) flying about 2-3 m above the water's surface quickly away from shore, and was first seen approximately 20 m from shore. Within several seconds, two Yellow-footed Gulls flew out from the island about 3 m above the water's surface, and began chasing the petrel, catching up with it and quickly forcing it to the water's surface when about 75 m from shore. One of the gulls grabbed the petrel in its mouth, almost completely engulfing it. Both gulls then flew back to the islands, landing out of sight behind several large boulders. Although actual ingestion was not observed, I presume that the gull swallowed the storm-petrel.
This observation is of interest for several reasons. First, Yellow-footed Gull predation on storm-petrels does not appear to have previously been reported in the literature, although this species has been recorded capturing and killing other bird species (Dunning 1988, Verlarde 1992). Similarly, predation by any larids on Black Storm-Petrels does not appear to have been previously reported. Secondly, this capture occurred over water and away from a colony, demonstrating that gulls are, at least under some circumstances, able to capture free-flying storm- petrels. According to Harrison (1983) it is not possible to discriminate juvenile from adult Black Storm-Petrels. As the storm-petrel left the island it flew quickly and directly away, suggesting a healthy individual. The possibility remains, however, that this particular individual was a juvenile and not experienced with avoidance of predators and/or that it not healthy and was therefore unable to easily avoid capture by the gulls. Observations of potential avoidance behavior by storm- petrels at sea, when storm-petrels and gulls are in the same vicinity, would help clarify whether such possibilities were factors in this incident.
ACKNOWLEDGMENTS
This observation was made while working with Special Expeditions, Seattle, WA, on board a tender from the MV Sea Lion. I would like to thank D. Anderson, A. Burger, G. McChesney and R. Podolsky for helpful suggestions or leads to references, as well as D. Anderson and an anonymous reviewer for comments on the manuscript.
LITERATURE CITED
Dunning, J.B. 1988. Yellow-footed Gull kills Eared Grebe. Colonial Waterbirds 11:117-118.
French, T.W. 1979. Great Horned Owl predation on Leach's Storm-petrels in Maine. Auk 96:202.
Harris, S.W. 1974. Status, chronology, and ecology of nesting storm petrels in northwestern California. Condor 76:249-261.
Harrison, P. 1983. Seabirds - an identification guide. Houghton Mifflin Company, Boston.
Lockley, R.M. 1932. On the breeding habits of the storm-petrel, with special reference to its incubation and fledging-periods. British Birds 25:206-211.
Ryan, P.G., and C.L. Moloney. 1991. Tristan Thrushes kill adult White-bellied Storm-petrels. Wilson Bulletin 103:130-132.
Trapp, J.L. 1979.Variation in summer diet of Glaucous-winged Gulls in the western Aleutian Islands: an ecological interpretation. Wilson Bulletin 91:412-419.
Verlarde, E. 1992. Predation of Heermann's Gull (Larus heermanni) chicks by Yellow-footed Gulls (Larus livens) in dense and scattered nesting sites. Colonial Waterbirds 15:8-13.
Vermeer, K., K. Devito and L. Rankin. 1988. Comparison of nesting biology of Fork-tailed and Leach's Storm-petrels. Colonial Waterbirds 11:46-57.
Watanuki, Y. 1986. Moonlight avoidance behavior in Leach's Storm-petrels as a defense against Slaty-backed Gulls. Auk 103:14-22. Return to homepage of Robin W. Baird