We used geolocators to track Common Terns Sterna hirundo from a breeding colony in the northeastern USA. Seven females started autumn migration between 1 and 22 August (mean 9 August), whereas four males started between 12 August and 5 October (mean 6 September). Common Terns have prolonged post-fledging care and our findings support earlier inferences that males perform more of this care than females. This would result in higher costs for males.

Most terns that nest in the temperate zones have a prolonged period of 1–3 months between completing breeding and starting autumn migration, during which they disperse within the breeding range, undergo a partial moult, and attend and feed juveniles (Nisbet 1976, 2002; Burger 1980; Gochfeld et al. 1998; Watson & Hatch 1999). In at least some species, post-fledging care continues for several months after autumn migration (Ashmole & Tovar 1968; Feare 1975; Barlow 1998; Gochfeld et al. 1998). Male and female terns perform parental care more or less equally until late in the chick-rearing period (Gochfeld et al. 1998; Nisbet 2002), but the relative contributions of the sexes after fledging are not known. Nisbet et al. (2010) described an event at Bermuda in which a hurricane on 5 September 2003 eliminated all the male Common Terns Sterna hirundo: most or all females survived and appear to have left the island early, while the males had continued caring for juveniles until the day before the hurricane. Here we report direct measurements of autumn departure dates of male and female Common Terns from a post- breeding staging area around Cape Cod, Massachusetts, USA.

This study was supported by grants from the Blake Fund of the Nuttall Ornithological Club and the Goldenrod Foundation. Logistical support was provided by the New Bedford Harbor Trustee Council. We thank J. Spendelow, S. Luecke Flaherty, E. Lencer, N. French, R. Veit, J. Hatt and S. Mitra for help in the field. We thank James Fenton for the photograph. Peter Becker and an anonymous referee made helpful comments on an earlier draft.

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