To investigate adaptations for long-range acoustic communication in birds,
I analyzed associations between broad categories of habitats and
properties of territorial songs for eastern North American oscines.
From published recordings (Cornell Laboratory of Ornithology 1975), I
obtained three frequency properties (maximal, minimal and dominant
frequency) and three temporal properties of songs (presence of sidebands,
presence of buzzes, minimal period of repeated elements). Sidebands and
buzzes indicated rapid amplitude modulation of a carrier frequency.
Habitats occupied by territorial males were classifed into six categories
(broad-leaved or mixed forest, coniferous forest, and parkland or forest
edge, shrubland, grassland, and marshes).
Frequencies in songs correlated strongly with body size, which varied
among habitats. Analysis of covariance and phylogenetic regression
(Grafen 1989), controlling for body size, revealed an association of
maximal, but not dominant or minimal, frequencies with habitat.
In contrast, the temporal properties of song were all strongly associated
with habitat, even within phylogenetic groupings. These results suggest
that the temporal properties of songs of many oscines have evolved to
reduce the effects of reverberation in forested habitats. Exceptional
species might have retained features of song subject to degradation in
order to permit listeners to judge distances to singers. In addition,
adaptations for acoustic communication in different habitats might include
differences in the perception of songs.