Some factors in the timing of parent-chick recognition in swallows

Abstract

Exchanged, sham-exchanged, and untouched clutches of eggs of tree swallows ( Iridoprocne bicolor) and of barn swallows ( Hirundo rustica) all hatched with equal incubation periods. Exchanged and sham-exchanged broods of tree swallows, exchanged at 3, 7, 12 and 17-days post-hatch showed no differences in overall or day-to-day weight changes, and barn-swallow groups also failed to show a difference. Nestlings exchanged and sham-exchanged on day 17 departed from the nest earlier than did untouched controls in both species, demonstrating a handling effect. Exchanged broods departed earlier than sham-exchanged broods. Observations of parental behaviour in the 30 min before and after exchanges and sham exchanges in the tree and barn swallows showed that handling decreased parental care similarly in both species. When own and alien young were tethered near the nest, alien young begged and struggled more and received more aggressive responses from the parents than did own young. Field observations of colour-marked birds showed that parent barn swallows restrict feeding and protective responses to their own young, and behave aggressively to alien juveniles. Juveniles vocalize to the sight or sound of approaching adults, and are not fed until they vocalize, so that exchange of signals could provide for the post-fledging discrimination. Thus, parent-chick recognition begins at about the time of fledging, after which it is very specific. This timing matches the factors of altricial nesting: parents can restrict parental care to their own eggs or nestlings by merely discriminating their nest from others, whereas after fledging parents must discriminate their own young.