A new study, published in The Auk: Ornithological Advances, has recommended that as many as 21 populations of Neotropical birds should be recognised as distinct species.
Birds often choose their mates based on song, making it a key factor in separating species. However, analysing spectrograms can only tell us so much, and the characteristics that birds hone in on when identifying potential mates are not necessarily the same ones that scientists notice in audio recordings.
In the newly published research, scientists studied 72 pairs of related but geographically separate bird populations in Costa Rica, Panama and Ecuador. In addition to analysing more than 1,000 song recordings for seven variables, they used playback experiments to judge the reactions of wild birds with recordings of their relatives, observing whether or not they responded and approached the speaker.
The results show that when the divergence between the characteristics of the recordings is high, birds consistently fail to recognise recordings of their relatives in the field, but when divergence is low, birds’ discrimination is much less consistent. In other words, analysing recordings can’t accurately predict how birds will act when presented with songs just slightly different from their own.
Many pairs that failed to recognise each other are currently categorised as members of the same species, suggesting that current taxonomy does not reflect actual bird behaviour when it comes to song. The scientists propose that 21 such pairs should be recognised as separate species based on song discrimination and that playback experiments should be the standard for assessing whether song divergence between populations is a barrier to interbreeding.
Benjamin Freeman, one of the researchers, explained: “It is abundantly clear to anyone familiar with the amazing diversity of Neotropical birds that there are many cases where populations that sing very different songs are classified as the same species. These populations look the same – they have similar plumage and are similar in size and shape – but assuming that populations that sing differently tend not to interbreed, this means that species-level diversity in the Neotropics is underestimated.”
J V Remsen, an expert on Neotropical birds, added: “Playback experiments between geographically isolated taxa provide key data on how populations might perceive each other in terms of ‘same’ or ‘different’ if they were in actual contact. Hopefully, this pioneering study will catalyse a wave of similar studies around the globe as a way to approach the always-thorny problem of species limits in these birds.”
Freeman B G & Montgomery G A. 2017. Using song playback experiments to measure species recognition between geographically isolated populations: A comparison with acoustic trait analyses. The Auk 134(4):857-870. https://doi.org/10.1642/AUK-17-63.1