Montezuma oropendolas modify a component of song constrained by body size during vocal contests

Physical constraints on animal signals can influence how challenging they are to produce, which in turn can provide potentially important cues about the condition or quality of a signaller. Some components of a signal are often more challenging to produce than others, however, and we expect animals to modify those elements that are especially costly so as to give high-intensity and demanding displays only when necessary. We found that Montezuma oropendolas, Psarocolius montezuma, modify a particular component of their songs during vocal contests, the lowest peak frequency (LPF). Songs that overlapped other males' songs during interactions at nesting colonies had slightly but significantly lower LPFs than did the same song types produced when singing alone. Other components of song, in contrast, did not change. We also provide evidence that the production of loud, low-frequency notes in this species is constrained by body size, based on a negative bounded relationship between mean measurements of LPF and tarsus length across 102 species and subspecies of New World blackbirds (Icteridae), the songbird family to which the Montezuma oropendola belongs. Measures of LPF in the songs of Montezuma oropendolas fall near the lower limit of what members of the family produce. Taken together, our results suggest that low-frequency notes in these birds might constitute an honest signal of competitive ability during male–male interactions.