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Shell Dropping: Decision-Making and Optimal Foraging in Northwestern Crows

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Decision-making and optimal foraging was investigated in Northwestern crows (Corvus caurinus) feeding on whelks (Thais lamellosa). Crows foraged on whelks by dropping them from a height for breaking. This foraging pattern is a stochastic process; the probability of breaking depends on height of drop, size of whelk and type of substrate. Large whelks selected by crows broke more readily than medium and small ones. They also had a higher caloric content. Crows minimized total amount of ascending flight required for breaking whelks when choosing height of drop. This was advantageous because ascending flight was energetically expensive. Crows dropped individual whelks repeatedly until they broke. This was profitable because percentage of whelks breaking over successive drops remained relatively stable. Thus, it was not beneficial to go in search of another whelk if a given one did not immediately break. All whelks were dropped on rock, the best substrate available for breaking. Crows made a net gain of 1.49 kcal per whelk dropped, sufficient to break an additional 1.5 whelks. Dropping medium and small whelks would have resulted in a loss of energy because these whelks broke less readily and also had a lower caloric content than large ones. Achieved foraging efficiency was 3.71. This is close to the maximum value attainable for crows. Foraging efficiency was relatively insensitive to length of foraging bout and crows took from one to three whelks per bout.

Affiliations: 1: (Department of Zoology, U.B.C., Vancouver, B. C., Canada


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