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Selection and dropping of whelks by northwestern crows


Behaviour 67(1-2): 134-148
Selection and dropping of whelks by northwestern crows
Prey selection, dropping behavior and dropping site selection were 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. 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. Crows usually carried and dropped only one whelk at a time. While dropping, crows typically flew almost vertically, 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. This probably 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 substrates 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 behavior in gulls and hawks. Dropping behavior in crows seems to be much more adaptable than in gulls.


Accession: 006380227

DOI: 10.2307/4533924



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