Shell-Dropping Behavior of Western Gulls (Larus occidentalis)
1982; Oxford University Press; Volume: 99; Issue: 3 Linguagem: Inglês
ISSN
1938-4254
Autores Tópico(s)Marine and fisheries research
Resumo-Westem Gulls (Larus occidentalis) at Bodega Bay, California drop shelled prey items to break them. I presented Washington clams (Saxidomus nuttalli) of known weight to free-flying gulls to investigate factors affecting shell-dropping behavior. All adult gulls dropped clams, whereas only 55% of immature gulls did so. The other 45% of immature gulls that were given clams pecked at them on the ground instead. Gulls dropped clams on both hard and soft substrates. Flight distance and kleptoparasitism seemed important in influencing drop location. Adult Western Gulls dropped heavy clams from lower heights than they dropped light clams. Heavy clams, however, break less easily than light clams when dropped from the same height. Energetic constraints and/or kleptoparasitism could explain this apparent contradiction. Received 15 July 1981, accepted 12 December 1981. MANY gulls obtain food by dropping shells. Known cases include Kelp Gulls (Larus dominicanus), Mew Gulls (Larus canus), Herring Gulls (Larus argentatus), and Glaucous-winged Gulls (Larus glaucescens) (Oldham 1930, Tinbergen 1953, Barash et al. 1975, Siegfried 1977, Kent 1981). Northwestern Crows (Corvus caurinus) also drop shells (Zach 1978, 1979). Studies of shell dropping in the past have considered the character of the dropping surface (Barash et al. 1975; Siegfried 1977; Zach 1978, 1979; Kent 1981), age-related differences in dropping behavior (Barash et al. 1975, Siegfried 1977), size selection of dropped prey (Siegfried 1977; Zach 1978, 1979; Kent 1981), and the degree to which dropping height approached an energetic optimum (Siegfried 1977; Zach 1978, 1979). Despite these studies, we still know little about what factors govern the height from which clams are dropped. Siegfried (1977) suggested that the substrate at the drop site might influence dropping height. He failed to consider, however, how prey weight might affect dropping height. Prey weight could conceivably influence dropping height in the following three ways: (1) Gulls may drop heavy objects from lower heights than they drop light ones due to energetic considerations. (2) Heavy objects may be easier to break than light ones [Siegfried (1977) found this to be true for 1 Present address: Department of Biology, University of North Dakota, Grand Forks, North Dakota 58202 USA. mussels], and thus they might be dropped from lower heights. (3) Kleptoparasitism may increase with prey size, and this may influence dropping height. Do birds adjust dropping height based on prey weight? If so, which, if any, of the above factors might influence this? In this paper I analyze shell-dropping behavior in Western Gulls (Larus occidentalis) at Bodega Bay, California. In particular, I focus on the following questions: (1) What is the effect of shell weight on dropping height? (2) How does clam weight influence the probability of breakage at different heights? (3) Are shells dropped randomly or do substrate type, kleptoparasitism, and/or other ecological parameters affect where shells are dropped? (4) How does clam-dropping proficiency vary between immature and adult gulls?
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