Song varies with latitude, climate, and species richness in a Neotropical bird

Animals can encode information within acoustic signals, particularly, bird
songs can be remarkably complex and can indicate individual identity and
quality. Two main sets of hypotheses attempt to explain the evolution of
increased birdsong complexity across large-scale geographic ranges: (1)
larger acoustic space availability, and (2) greater sexual selection
intensity, both of which would favor the evolution of more complex songs
at higher latitudes, more seasonal and/or species-poor environments.
However, few studies have assessed patterns of song complexity for birds
with broad geographic ranges. Here, we determined patterns of song
variation in the blue-black grassquit (Volatinia jacarina), considering
metrics of song complexity, structure and performance. This Neotropical
bird occurs from Mexico to Argentina and produces a monosyllabic song.
Using recordings from online databases, we calculated song metrics, such
as bandwidth, song rate, number of song components, and proportion of
vibratos of this signal. We found that song features varied with latitude,
climate seasonality, bird species richness and hemisphere. However,
contrary to theoretical predictions, complexity mostly decreased with
latitude and greater seasonality, while it was positively correlated with
bird species richness. Proportion of vibratos was positively correlated
with latitude and seasonality, and may be a feature under sexual selection
in this species. Overall, our results did not support the main hypotheses
proposed as explanations for song complexity. Our findings also highlight
that song complexity does not vary uniformly among songbirds and song
parameters, and future studies encompassing more species should clarify
patterns and drivers of song variation across broad geographic dimensions.