Maximum frequency of songs reflects body size among male dusky warblers Phylloscopus fuscatus (Passeriformes: Phylloscopidae)

Abstract

Song plays a vital role in communication in songbirds, primarily for territorial defense and mate attraction. Larger animals, having larger vocal organs, produce low sound frequencies more efficiently. Accordingly, the frequency of vocalizations is often negatively correlated with body size across species, and also among individuals of many species, including several non-songbirds. However, little is known about whether song frequency reveals information about body size among males. We tested for the predicted positive relationship between body size and song traits in male dusky warblers (Phylloscopus fuscatus). In many animal species, some vocalizations are difficult to produce and can therefore indicate quality of signalers. Male dusky warblers produce songs with trilled sequences that are limited in their timing and frequency structure by physiological constraints on vocal performance. We investigated whether there was a relationship between body size and trilled vocalizations in the dusky warbler. We recorded songs of free-living male dusky warblers at dawn, and captured birds with mist nets for morphometric measurements (tarsus length, as an indicator of body size). We conducted correlation analyses between tarsus length and a composite measure of songs. In dusky warblers, body size was not significantly related to the average frequency of their overall songs, low frequency, bandwidth frequency, average note number of songs, or average song duration. In contrast, the maximum frequency of song was found to correlate positively with tarsus length. For trilled vocalizations, we found most trill traits were not significantly related to body size, and only the maximum frequency of trills was positively related to body size. Additionally, the maximal value of frequency bandwidth decreased with increasing trill rates. Overall, these results suggest that in male dusky warblers, the maximum frequency of the entire song is a reliable indicator of body size.

Keywords

• Bird song
• body size
• dusky warbler
• maximum frequency
• trill traits