Body size correlates negatively with the frequency of distress calls and songs of Neotropical birds

The allometric relationship between body size and song frequency has been established in previous studies of temperate and tropical bird communities. However, the relationship between body size and the frequency of distress calls has been examined in only one study of temperate birds. We examined size-frequency relationships in the distress calls and songs of a Neotropical bird community in northwestern Costa Rica. In 2008 and 2009, we recorded distress calls and determined the body mass of 54 mist-netted birds representing 38 species, 35 genera, and 14 families. We obtained songs for these same species from sound libraries and commercially available compact discs. For each vocalization, we measured minimum frequency and frequency of maximum amplitude. Larger birds produced lower-frequency distress calls and songs than smaller birds. Phylogenetically controlled analyses revealed that the frequency of maximum amplitude was negatively correlated with body mass for both distress calls and songs. Minimum frequency was negatively correlated with mass for distress calls, but not songs. Our analyses suggest that the influence of phylogeny on the relationship between frequency characteristics and body size is modest. Pair-wise comparisons across 37 species revealed that distress calls and songs had similar minimum frequencies, but songs had significantly lower frequencies of maximum amplitude than distress calls. This difference may arise from differences in signal function. Lower-frequency sounds should transmit farther through forest habitats and songs must often transmit longer distances to reach their intended audience than distress calls. Our results support the general theory that body size is negatively correlated with the frequency of acoustic signals by demonstrating that this pattern holds true for both distress calls and songs in a Neotropical bird community.