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Adoption seeking by semi-precocial chicks of some bird species can be adaptive since it provides an alternative tactic to permit poorly cared-for young to survive despite their neglectful parents’ behaviour. Moreover, own-nest desertion may enhance inclusive fitness of fugitive chicks by increasing survival prospects of siblings. On the other hand, adoption by breeding adults can be detrimental to foster parents’ fitness if they invest resources in promoting survival of others’ offspring at the expense of their own. In this study we report on the proximate 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 albifrons) and the common tern (Sterna hirundo). Adoptions were frequent in little tern and, notably, in common tern colonies. Chicks that deserted their original brood were poorly fed compared with resident chicks, but 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 higher mortality than resident ones. Common tern chicks deserted their original nest significantly more often than little tern chicks. Adoptees of both species benefited by the same survival prospects as resident chicks. A negative correlation existed between the proportion of food given to strange chicks and seasonal fitness or chick survival rare of breeding pairs. This was nor predictable because: (1) foster parents could have increased their parental efforts, thus ensuring the same survival prospects to their own chicks, and (2) chicks of foster parents could have been, in turn, adopted thus obliterating the negative fitness 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 reduction in seasonal fitness in both species. Possible alternative explanations for nest desertion and adoption behaviour are also discussed.