| Western Sandpipers (Calidris mauri) migrate between breeding areas in Alaska and a broad nonbreeding range that stretches over Pacific and Caribbean coastlines. I studied the relationships between migratory behaviour and feather condition, moult timing, body size, age and sex of Western Sandpipers at three nonbreeding locations (México, Panamá, Ecuador).; Within all sex and age classes, individuals at more southerly nonbreeding areas were larger (based on culmen and wing length), and had disproportionately longer wings (relative to culmen). Wing wear also increased with migratory distance. In Panamá, most yearlings (juveniles become yearlings at the start of the spring migration period) deferred migration in their first spring, and remained on the nonbreeding area (‘oversummered’). Some adults also oversummered. In contrast, at sites further north (e.g., México), most yearlings migrate in their first year. In Panamá, oversummering birds completed pre-basic wing moult by late August, about one month earlier than migrant birds. The advanced timing of moult may give survival benefits, as the earlier period seems safer with regard to predation danger.; Juvenile Western Sandpipers arrive in Panamá from Alaska about one month later than adults, and do not undergo a wing moult in their first winter. As a consequence, juveniles that migrate as yearlings accumulate three long migrations (southward, northward, and southward again). The associated wear is assumed to have detrimental effects on flight performance, and I hypothesized that wing wear is a factor selecting for deferred migration from more distant nonbreeding locales. To test the hypothesis that sandpipers make migratory decisions contingent on the condition of their feathers, I simulated wing wear by clipping 2–4 mm from the wing tips of Western Sandpipers in Panamá prior to northward migration. Clipped individuals showed lower pre-migratory mass gain than non-clipped control birds. Consistent with the hypothesis, clipped adults were resighted more often, and had a later mean last-date-of-detection than controls. Since female Western Sandpipers generally migrate further than males, the effects of feather wear may differ between sexes. I conclude that feather wear is a significant factor in the proximal regulation and evolution of migratory and life history strategies of Western Sandpipers. |
