SummaryStomach contents of 34 Purple MartinsProgne subisand 90 netted samples of flying insects were analysed taxonomically and quantitatively and compared with one another. Insect food sources tend to become more abundant, taxonomically variable, and larger in size late in the season (August). Maximal availability of food thus coincides with maximal energy requirements of Purple. Martins, which then are at peak numbers, initiating moult, accumulating fat and beginning migration.