Song Battles With Other Species Can Change Your Tune

Many animals defend territories from members of their own species for mating, breeding, and finding food and they often use species-specific vocalizations to do this. Defending a territory can be risky and costly in both energy and time, so even territorial animals generally don't waste this effort on other species that do not share their same food and breeding needs. But what do you do if you live around another very similar species that has the same needs that you do? Can two species learn to speak each other's languages to live in territorial harmony?

Both the common nightingale and the thrush nightingale have species-specific songs and they learn their songs as young chicks. However, thrush nightingales that live in sympatric populations with common nightingales learn to incorporate common nightingale elements into their songs. This has caused a convergence, or an increase in similarities between the songs of the two species over multiple generations in this zone of overlap. Because birdsong is also generally used to attract mates, this convergence of songs would seem to be maladaptive, increasing the risk of breeding with the wrong species. A research team from the Czech Republic and Poland recently set out to explore whether the convergence of these birdsongs may have an adaptive function as well.

The researchers recorded songs from common nightingales and thrush nightingales that live in allopatric populations in the Czech Republic and Poland. They took these song recordings and taxidermic dummies of each species to a sympatric population in Poland. They walked around the breeding areas and figured out where the male territories were for each species. When they found a male in his territory, they set up their speakers and a taxidermic dummy of either a common nightingale or a thrush nightingale; then they played 5 minutes of song that was recorded from the same species as the dummy. They watched the territorial male's behavior and wrote down everything he did for that 5 minutes. They returned the next day and repeated the process with the other species' dummy and song recordings, so each male was exposed to both species' songs. The researchers also repeated the process with the song and a dummy of a third species, the willow warbler, a songbird that lives in these sympatric areas but does not compete for the same resources.

Even though the nightingales never showed any interest in the willow warbler dummies, they all approached the nightingale dummies. Not only did they approach the nightingale dummies, but they often attacked them as well. The common nightingales were much more aggressive towards their own species than the thrush nightingales were towards their own species. However, when played the other species' song, the common nightingale was less aggressive whereas the thrush nightingale was more aggressive. What does this mean?

Territorial males of most species respond much more to intruders of their own species. In this way, the common nightingale is more "typical" in their territorial behavior. However, remember that the thrush nightingale in these sympatric populations learns a song that has common nightingale elements in it. It makes sense that they would respond to the songs of both species, because in their mind, either song could be produced by their own species.

This brings up the question of why these sympatric population thrush nightingales sing a hybrid song in the first place? One possibility is that female thrush nightingales may prefer males that sing these more complex songs. Another is that a mixed song is more effective at defending your territory from both species. More research is needed to see if either or both of these hypotheses are true.

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Further reading:

Reif, J., Jiran, M., Reifová, R., Vokurková, J., Dolata, P., Petrusek, A., and Petrusková, T. Interspecific territoriality in two songbird species: potential role of song convergence in male aggressive interactions, Animal Behaviour, 104, 131-136 (2015). DOI: 10.1016/j.anbehav.2015.03.016.

Image Credits:

Common nightingale (left) by Frebeck at Wikimedia Commons

Thrush nightingale by (right) Locaguapa at Wikimedia Commons