Hatching asynchrony, chick care, and adoption in the common tern: can disadvantaged chicks win?

In 1987 and 1988, common tern (Sterna hirundo) chicks at a colony near Port Colborne, Ontario, were individually colour banded according to known hatch order. Intraclutch hatch intervals produced size disparities among chicks at brood completion; third-hatched chicks were significantly lighter and at a significant survival disadvantage compared with their earlier-hatched siblings. There were differences in feeding rates according to hatch order and many third-hatched chicks obtained fewer or no feedings during our periods of observation. Sixty-five chicks known to have abandoned their home broods gained acceptance into foreign broods. Chicks that remained in the foreign brood for more than 2 days (average residency 11.9 ± 5.3 days; n = 26) were fed and brooded by the foster parents, were on average older than the youngest resident chick, but were not always the last hatched in their home brood. Conversely, chicks that were in a foreign brood for less than 2 days were no different in age from the youngest resident chick. Survival and fledging success was highest for chicks accepted into two chick broods in which they were older than the resident second chick; in effect, the adoptee became the second chick. Parents that accepted a foreign chick for more than 2 days experienced a seasonal fitness loss compared with nonadopting parents. As the only viable option available to them, selection favours movement away from home broods by chicks that may be disadvantaged there.