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Predators that hunt by sight should be constrained by light levels. This problem should be especially acute in aerial predators such as common poorwills (Phalaenoptilus nuttallii: Caprimulgidae), which capture small insects at night. The purpose of our study was to test the hypothesis that poorwills are constrained to catching larger insect taxa due to limitations of visual orientation under low light. We assessed diet by analysing poorwill faecal samples collected at nest and roost sites in the Cypress Hills of Saskatchewan where prey density is relatively 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% coleopterans) and sticky trap samples (11% lepidopterans and 2% coleopterans). Based on measurements of femur length, all consumed coleopterans were > 5 mm long, even though 57% of those available were < 5 mm. Our results suggest that poorwills are constrained to taking prey above a certain size, but above this threshold, particular prey types and larger sizes may be selected. Selection for coleopterans may reflect the need to acquire polyunsaturated fat to facilitate the use of torpor.