The importance of understanding diet composition and ecological correlates of an insectivorous bird in decline

Aerial insectivores are birds that prey exclusively on insects collected during flight. These bird species populations have been declining more rapidly than other groups. A proposed cause for this decline is a decline in the availability of insect prey. We collected 54 bolus samples during summer 2015 from adult Purple Martins (Progne subis) as they returned to nests to feed offspring. Insects were identified to family, and the length and width of each insect was measured to estimate mass. Samples contained 34 insect families and 2 arachnid families. The most frequently collected insect orders were Odonata and Diptera. The average length of all insects, the average number of insects by order per bolus, and the average mass of insects by order per bolus follow patterns relating to developmental stages of the chicks. Although Purple Martins appear to be opportunistic feeders, aquatic insects made up 55% of the total diet. If Malathion causes a decline in the population numbers of non-target aquatic insects, it may contribute to the decline of Purple Martins as well as other aerial insectivores.