Flies were collected during 1954 from 5 spp. of passerine birds on Fair Isle, Shetland. First flies appeared with 1st young birds in mid-June, and up to 70% of juveniles were infested during July. Thereafter flies decreased rapidly and were very scarce by Sept. Large body-size and habit of nesting in holes are conducive to heavy infestation. Adult birds were less heavily infested than juveniles in hole-nesting spp., but not in those building open nests. Change of host was studied by marking flies with cellulose paint and releasing them on birds. After a mean recapture period of 5.5 days 75% of those recovered were on the same bird; 18.5% had changed to other birds of the same sp.; 6.5% had changed host-species. Males were rather shorter-lived and changed hosts more frequently than females. Puparia are laid at intervals of 4 days or less while the fly is on a bird. They fall to the ground and do not produce imagines until the following summer.