ADOPTION BEHAVIOR IN LITTLE AND COMMON TERNS (AVES, STERNIDAE) - CHICK BENEFITS AND PARENTS FITNESS COSTS

Adoption seeking by semi-precocial chicks of some bird species can be adaptive since it provides an alternative tactic to permit poorly care d-for young to survive despite their neglectful parents' behaviour. Mo reover, own-nest desertion may enhance inclusive fitness of fugitive c hicks by increasing survival prospects of siblings. On the other hand, adoption by breeding adults can be detrimental to foster parents' fit ness if they invest resources in promoting survival of others' offspri ng at the expense of their own. In this study we report on the proxima te causes and survival consequences of adoption seeking by chicks, and on the presumed costs of adoption sustained by foster parents in two colonial, ground-nesting tern species: the little tern (Sterna albifro ns) and the common tern (Sterna hirundo). Adoptions were frequent in l ittle tern and, notably, in common tern colonies. Chicks that deserted their original brood were poorly fed compared with resident chicks, b ut crowding in the brood and age rank relative to nest companions did nor influence the chances of desertion. Deserting their original brood was risky for little tern chicks since stray chicks experienced highe r mortality than resident ones. Common tern chicks deserted their orig inal nest significantly more often than little tern chicks. Adoptees o f both species benefited by the same survival prospects as resident ch icks. A negative correlation existed between the proportion of food gi ven to strange chicks and seasonal fitness or chick survival rare of b reeding pairs. This was nor predictable because: (1) foster parents co uld have increased their parental efforts, thus ensuring the same surv ival prospects to their own chicks, and (2) chicks of foster parents c ould have been, in turn, adopted thus obliterating the negative fitnes s effects of adoption. The sexes were identical with respect to their proneness to provide food to strange chicks. We suggest that adoption seeking by little and common tern chicks has evolved as an alternative tactic that is pursued to counter the effects of poor parental care. Adoption behaviour seems maladaptive since it is accompanied by a redu ction in seasonal fitness in both species. Possible alternative explan ations for nest desertion and adoption behaviour are also discussed.