PREY SELECTION AND FORAGING CONSTRAINTS IN COMMON POORWILLS (PHALAENOPTILUS-NUTTALLII, AVES, CAPRIMULGIDAE)

Predators that hunt by sight should be constrained by light levels. Th is problem should be especially acute in aerial predators such as comm on poorwills (Phalaenoptilus nuttallii: Caprimulgidae), which capture small insects at night. The purpose of our study was to test the hypot hesis that poorwills are constrained to catching larger insect taxa du e to limitations of visual orientation under low light. We assessed di et by analysing poorwill faecal samples collected at nest and roost si tes in the Cypress Hills of Saskatchewan where prey density is relativ ely low. Coleopterans (44% and 39% in 1991 and 1992, respectively) and lepidopterans (46% and 55%) dominated the diet, yet these two insect orders were rare in light-suction trap (22% lepidopterans and 1% coleo pterans) and sticky trap samples (11% lepidopterans and 2% coleopteran s). Based on measurements of femur length, all consumed coleopterans w ere > 5 mm long, even though 57% of those available were < 5 mm. Our r esults suggest that poorwills are constrained to taking prey above a c ertain size, but above this threshold, particular prey types and large r sizes may be selected. Selection for coleopterans may reflect the ne ed to acquire polyunsaturated fat to facilitate the use of torpor.