We studied Black-legged Kittiwakes (Rissa tridactyla) in an Alaskan colony where movement of young among nests was possible because of moderate terrain and close nest spacing. Thirty-three percent of chicks in a focal group departed their nests prior to fledging, and seven of the vagrant chicks (58%) were adopted by foster parents. The overall frequency of adoption in three years was 8% of 88 chicks from 57 focal nests. Premature nest-departure occurred at different stages among first- and second-hatched chicks. Departing second-hatched chicks were usually expelled by their nest mates within a few days after hatching. First-hatched chicks left at all stages and usually were the sole nest occupant when they departed. The evidence for parent-offspring recognition was equivocal. Adults accepted alien chicks that appeared in the nest and also occasionally attacked their own young outside the nest. However, asymmetry in the response of parents and nonparents to vagrant chicks seeking access to a nest suggested that adults were often able to discriminate appropriately. Vagrant chicks appeared to have little control over their fate-most entered nests where they were smaller than the resident young and suffered nest-mate aggression. Reproductive error seems the likely explanation for the acceptance and foster caregiving observed in adult kittiwakes.