Song features and singing heights of American warblers: Maximization or optimization of distance?

By discriminant analysis we examined the multivariate relationships between quantifiable features of song and the usual rank of singing height in 19 species of warblers (Family Parulidae) resident in New Brunswick, Canada. We found a correlation of higher frequencies with higher singing heights, although the lowest singing group was not different from the second lowest in this respect. Associated with higher frequencies are repeated groupings of two phone types, low numbers of total phone types and fast frequency sweeps in the last phone of the song. Warblers with lower frequencies tend to have more phone types per song. Warblers singing at lowest levels, however, have large numbers of unrepeated phones or alternatively three to five separate phone types repeated in the series in their songs. Overall, the results indicate that in addition to frequency, structural characteristics of the songs are related to singing height. The results are not consistent with the view that warblers are maximizing the distance over which their songs are transmitted since low- frequency sounds should be preferred on an acoustical basis at most singing heights, if not the very lowest. Warblers may be producing songs which by their frequency and structure carry mainly to individuals of biological significance to the singers, probably individuals on adjacent territories. Other possible explanations are also considered.