Prey selection in relation to insect availability by the common poorwill (Phalaenoptilus nuttallii)

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Abstract
We determined the diet of common poorwills (Phalaenoptilus nuttallii) by analysing fecal samples collected from roost and nest sites in the Okanagan Valley of British Columbia. The two major insect orders present in the diet in 1989 were Coleoptera (47% by volume) and Lepidoptera (49%), which comprised only 15 and 3%, respectively, of the insects sampled using sticky traps in 1989. Based on the femur length of coleopterans consumed, all coleopterans eaten were longer than 6.7 mm (excluding the head), whereas 76% of the coleopterans caught on sticky traps were less than 6.7 mm long. We found no evidence to support the prediction, based on optimal foraging theory, that poorwills would broaden their diet to include other insect taxa or size classes during 1990, when wet conditions likely depressed prey availability and foraging opportunities. There was no difference in the proportions of various taxa consumed or the size of coleopterans eaten between 1989 and 1990. Acknowledging the bias of our insect sampling regime, we conclude that the apparent prey selection by poorwills results passively from the constraints imposed by detecting prey under conditions of low light.

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