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Selection and Dropping of Whelks By Northwestern Crows

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image of Behaviour

Prey selection, dropping behaviour and dropping site selection was investigated in Northwestern crows (Corvus caurinus) feeding on whelks (Thais lamellosa). Crows selected only the largest whelks available in the intertidal zone. Although equally palatable, smaller whelks were ignored. Crows assessed size of whelks first by sight and then by weight. Thus, some were picked up with the bill and laid down again before a final selection was made. Rejects were longer and heavier than available whelks and shorter and lighter than selected ones. Usually, crows carried and dropped only one whelk at a time. While dropping, crows typically flew almost vertically up, released the whelk and then dove after it. The mean number of drops required to break a whelk was 4.36. Crows dropped whelks until they could obtain most or all of the soft parts. Usually crows did not give up dropping a whelk until it broke. The mean height of drop was 5.23 m. Crows had a tendency to increase height of drop over successive attempts. During most flights, crows lost some height before releasing a whelk. Presumably, this allowed them to watch whelks fall and bounce. All whelks were dropped onto rock. None were released over water or grass. Crows had specific dropping sites. This was because they selected hard substrate and safe sites for dropping. On dropping sites whelks were unlikely to bounce into water. Results were compared with those of other studies of dropping behaviour in gulls and hawks. Dropping behaviour in crows seems to be much more adaptable than in gulls.

Affiliations: 1: (Department of Zoology, University of British Columbia, Vancouver, B.C., Canada)


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