Perinatal Acoustic Communication in Birds: Why Do Birds Vocalize in theEgg?

In this review the development of acoustic communication between embryos or between embryos and chicks as well as between embryos/chicks and the breeding parents will be addressed. Special emphasis is given to the impact of embryonic acoustic signals for hatching synchronization. In the Muscovy duck (Cairina moschata L. f. domestica) clicking sound communication is absolutely essential for a synchronized hatching. The mechanism underlying this special case of communication is a synchronization of clicking rates. In a clutch, embryos adapt their clicking rates to each other. Clicking rates of less developed embryos rose faster (acceleration) than clicking rates of more developed embryos (retardation). No evidence was found that vocalization of embryos, chicks and parents improve hatching synchronization. Although, many authors assume that prenatal acoustic interaction by vocalization (an exchange of acoustic signals) exists, in the Muscovy duck it was shown that an acoustic interaction started when the first embryo had hatched. Specific call types serve as communication-releasing signals. Acoustic mother-duckling interaction developed later and gradually during the process of nest-leaving also based on specific call types. Measurements on sound transmission indicate that all embryos within a clutch are in mutual acoustic contact.