THE FUNCTION OF EMBRYONIC VOCALIZATION IN THE LITTLE TERN (STERNA-ALBIFRONS)

Various functional explanations can be proposed for the evolution of b ird 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 b y 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, grou nd-nesting little tern. Parents that had their hatching eggs cross-fos tered with foreign eggs at the same hatching stage exhibited a parenta l behaviour similar to unmanipulated controls. Parents that incubated foreign eggs up to a stage in which embryos were not yet vocalizing, a nd were challenged with their own hatching eggs that had been incubate d in foster nests, performed less efficient parental cares than unmani pulated control pairs and pairs that had cross-fostered hatching eggs. The results do not support the hypothesis of early individual recogni tion and suggest that embryonic vocalizations in little terns have the function of promoting switching of parents from incubation to accepti ng and feeding hatchlings. Similar to other tern and gull species, the duration of incubation period in the little tern varies markedly amon g pairs and years. In these species, embryonic vocalizations can be ad aptive since they provide parents a cue to switch at a proper time fro m incubation to parental cares typical of the chick period.