Bird parents know which baby bird is theirs and show preferential bias toward them.
Parents ought to restrict costly parental care to their genetic offspring and, particularly when the risk of misdirecting care is high, parent-offspring recognition may evolve. I tested whether adult cave swallows, which nest in dense colonies and feed fledglings in mixed-family groups, discriminate against unrelated young, using temporary chick transfers at two nestling ages and a cross-fostering experiment. Temporary chick transfers indicated that parents bias feedings toward their own offspring near fledging (18 d) but not at about halfway through the nesting period (10 d). I also examined how parents learn to identify their offspring by cross-fostering young Ca 3 d after hatching and testing parental response 2 wks later. Adults did not favor their own offspring over unrelated nestlings when both were unfamiliar to the focal parents. However, when parents encountered two of their own offspring, one of which was reared by foster parents, they preferentially fed the familiar nestling. By recognizing young, cave swallow parents reduce some risks of misdirected parental investment (mobile fledglings) but not others (extra-pair young and intraspecific brood parasitism).