Although there seems to be a minor role for sexual selection as well.
Different bird species produce sounds across a wide range of frequencies. From the low-frequency booming calls of the Eurasian Bittern (Botaurus stellaris) to the high-pitched sounds of the Common Firecrest (Regulus ignicapilla). Ornithologists have formulated several hypotheses to explain the evolution of these frequency differences. The Acoustic Adaptation Hypothesis, for instance, suggests that acoustic signals are adapted to maximize the effectiveness of their transmission in a particular environment. Because low-frequency sounds travel better in dense vegetation than high-frequency sounds, bird species in forests are expected to produce calls at lower frequencies because of physical constraints. The Morphological and Phylogenetic Constraint Hypothesis, however, states that body size and/or evolutionary history limit the range of frequencies that a species can reach. Larger species tend to produce lower-frequency sounds. Finally, the Sexual Selection Hypothesis posits that frequency might act as an indicator of an individual’s size, dominance or fighting ability. Low-frequency sounds give the impression of a larger individual and might thus be selected for in the context of male-male competition.
To discriminate between these possible explanations, a recent study in the journal Ecology Letters took a closer look at the songs of more than 5000 songbirds. The researchers used the public database xeno-canto to collect information on the highest frequencies in different bird songs. Next, they correlated these peak frequencies with several evolutionary and ecological variables. The analyses revealed that evolutionary history (43-56%) and body mass (10-15%) explained the largest proportion of variation in peak song frequency. Heavier birds sang at lower frequencies. After accounting for these two factors, there was also a significant association with sexual size dimorphism (although the explained variation was very low: 1-3%). Peak frequencies were lower in species where males are larger than females, suggesting strong sexual selection. Song frequency might thus be a proxy for the competitive ability of males.
These patterns indicate that phylogenetic history and body size constrain the range of frequencies that a species can produce. Within the consequently limited range of frequencies, sexual selection appears to influence the peak frequency in particular species.
No Acoustic Adaptation?
Interestingly, there was no support for the Acoustic Adaptation Hypothesis. Although this hypothesis makes intuitive sense, there was no clear effect of tree cover (a proxy for habitat density) in the large dataset. The researchers propose several explanations for this negative result:
- The analyses lacked information about bird behavior or habitat at the recording location. These factors could influence the produced frequency.
- The estimates of habitat density were quite crude. Better measurements might pick up subtle signals.
- Other biotic and abiotic factors that might influence background noise were not included in the analyses.
More research is thus needed to explore how important acoustic adaptation is in the evolution of bird song. Nonetheless, phylogenetic constraints seem to restrict the available evolutionary pathways (similar to genomic constraints in hybrid species). Evolution is a powerful process, but that doesn’t mean that anything is possible.
Mikula, P., Valcu, M., Brumm, H., Bulla, M., Forstmeier, W., Petrusková, T., Kempenaers, B. & Albrecht, T. (2021). A global analysis of song frequency in passerines provides no support for the acoustic adaptation hypothesis but suggests a role for sexual selection. Ecology letters, 24(3), 477-486.
Featured image: European Robin (Erithacus rubecula) © Charles J. Sharpe | Wikimedia Commons