Various functional explanations can be proposed for the evolution of bird embryonic vocalizations during the pre-hatching period, namely: 1. To elicit switching of parents from incubation to parental behaviour typical of the chick period; 2. To allow thermoregulation of embryos by soliciting parents to incubate; or 3. To establish parent—offspring individual recognition. In this paper, we present the results of field experiments designed to test hypotheses 1 and 3 in the colonial, ground-nesting little tern. Parents that had their hatching eggs cross-fostered with foreign eggs at the same hatching stage exhibited a parental behaviour similar to unmanipulated controls. Parents that incubated foreign eggs up to a stage in which embryos were not yet vocalizing, and were challenged with their own hatching eggs that had been incubated in foster nests, performed less efficient parental cares than unmanipulated control pairs and pairs that had cross-fostered hatching eggs. The results do not support the hypothesis of early individual recognition and suggest that embryonic vocalizations in little terns have the function of promoting switching of parents from incubation to accepting and feeding hatchlings. Similar to other tern and gull species, the duration of incubation period in the little tern varies markedly among pairs and years. In these species, embryonic vocalizations can be adaptive since they provide parents a cue to switch at a proper time from incubation to parental cares typical of the chick period.