Differential degradation of antbird songs in a neotropical rainforest: adaptation to perch height?

Habitat characteristics that affect transmission and degradation of acoustical signals should influence strongly the evolution of bird songs. In this study propagation properties of songs of five antbird species were measured in a rainforest in southern Venezuela. The investigated species (Myrmothera campanisona, Thamnophilus aethiops, Thamnophilus amazonicus, Myrmotherula axillaris, and Herpsilochmus dorsimaculatus) use different song post heights at all levels of the rainforest. Because there is a height-specific pattern in degradation, it is hypothesized that their songs are adapted to species-specific transmission paths. To test this assumption, transmission parameters (excess attenuation, signal-to-noise ratio, and blur ratio) were measured for the songs at five different heights and at three different distances. In three of the five species, the results indicate a strong influence of environmental conditions on the design of the vocalizations. Degradation was minimized by the concentration of the signal to a narrower frequency range, the usage of lower frequencies, or a slower time structure for the songs near the ground. The results are discussed in relation to acoustical models of sound propagation and physiology, and it is suggested that height-dependent degradation within a forest is an important selection pressure for transmissibility in avian communication.