Partial Migration, Oversummering, and Intermittent Breeding by Shorebirds.

AbstractIndividuals of some long-distance migrant shorebird species may remain on or near nonbreeding areas in place of making a breeding migration. Existing hypotheses associate oversummering with factors that impair successful migration and breeding. The hypothesis developed here takes a life history trade-off perspective. Based on the idea that survival while oversummering is higher than that of migration and breeding, it predicts that oversummering evolves when the survival advantage compensates in fitness terms for the reproduction foregone by doing so. Adults have higher reproductive success and so oversummer less readily than do yearlings. If the oversummering survival gain is similar to the threshold level of compensation required, interindividual variation in condition may place some individuals above and others below the threshold for oversummering. Partial oversummering can result. This theory accurately predicts the strong contrast in oversummering patterns observed in Peru for both adult and yearling semipalmated Calidris pusilla and western sandpipers C. mauri, otherwise very similar species. Delayed maturity (i.e., oversummering by yearlings) and intermittent breeding (partial oversummering by adults) strongly affect population productivity. These behaviors may have increased over recent decades and hence could be contributing to the steep declines being reported for some shorebird species.