Nest acoustics and begging call structure in nestling tree swallows

Acoustic signals are altered by the environment during transmission, and, as a result, many show features that overcome this constraint. This phenomenon is well documented for long-distance signals used for mate attraction and territorial defence, but is relatively unexplored for short-distance signals such as the begging calls of young animals. We used the cavity-nesting tree swallow, Tachycineta bicolor, to examine whether the acoustic environment of the nest affected the structure and transmission of begging calls. We found that begging calls were longer and minimum call frequency was lower in smooth-walled than rough-walled nests, and that minimum call frequency decreased with increasing cavity diameter. Begging call features did not, however, vary with reverberation and frequency response within nests, even though calls were more degraded in nests with a more variable frequency response. Our results show that begging call structure varies with the structure of nest cavities and that the acoustic environment of nest cavities affects call transmission. Thus, some of the variation in begging calls currently attributed to factors such as evolutionary conflicts of interest or predation might be partially attributable to the acoustic environment of the nest. More generally, selection for effective transmission deserves more attention as a factor affecting the structure of short-range acoustic signals.