Leaving on migration: estimating departure dates of Barn Swallows Hirundo rustica from summer roosts using a capture–mark–recapture approach

Abstract

Capsule Capture–mark–recapture data can be used to predict departure dates of Barn Swallows Hirundo rustica from summer roosts.Aims To investigate how long Barn Swallows remain at their breeding grounds before migration by estimating departure dates.Methods A capture–mark–recapture approach was applied to an extensive data set (65 303 ringed and 710 recaptured birds) from two summer roosts. Multiple-day constancy models were used to estimate apparent survival, which was subsequently translated to residence time.Results The longest intervals between ringing and recapturing were between 66 and 67 days; estimated mean minimum durations were between 16.3 and 18.5 days. Apparent survival was high over most of the summer, indicating that there is little emigration during this period and was followed by a sharp departure-related decline in late August and September. Expected residence time, derived from apparent survival estimates, declined linearly from about 80 days in early July to less than 10 days in September. Estimated departure dates were highly consistent between years and occurred in late September.Conclusions Barn Swallows stay much longer at post-breeding roost sites than is necessary to build up their fat reserves for migration. We suggest that the birds are likely to derive a number of benefits from such a prolonged stay that are not directly related to preparation for migration; for example, minimizing predation risk by foraging in familiar areas, and gathering information on the quality of future breeding sites.