Could territorial birdsong be directed at other species?

Question

Birds sing to attract mates and deter conspecific rivals. But often, birds of different species may compete for the same resources. Are there studies of territorial song being used to ward off rivals of different species?

In other words, what if birds can understand the meaning (or intent) of many species' vocalisations, even if they can only produce those of their own species?

Answer 1

There are some very cool examples of inter-species communication. This first one could even be considered a step beyond your question:

• Flower et al (2014) Deception by Flexible Alarm Mimicry in an African Bird, Science. "Fork-tailed drongos (Dicrurus adsimilis) use false alarm calls to scare other species away from food that they then steal. We show that drongos mimic the alarms of targeted species. Further, target species reduce their response to false alarm calls when they are repeated. However, the fear response is maintained when the call is varied. Drongos exploit this propensity by changing their alarm-call type when making repeated theft attempts on a particular species."
• Spottiswode et al (2016) Reciprocal signaling in honeyguide-human mutualism, Science. "African honey-guide birds are known to regularly lead human honey-hunters to bee colonies, and the humans, on opening up the nest, leave enough mess for the birds to feast on. Spottiswoode et al. show that when the honey-hunters make a specific call, honey-guides are both more likely to come to their aid and more likely to find them a bee's nest. This interaction suggests that the birds are able to attach a specific meaning of cooperation to the human's call—a rare case of mutualism between humans and a wild animal."

However, I suspect these examples are more the exceptions that prove the rule. I think there is generally only limited understanding among avian species. A dominant individual might advertise its territorial claim to a resource, but this is against all-comers. If an individual from a dominant species sings, other subordinate species might well recognise the song as coming from a dominant rival. But I don't think the subordinate bird thinks to itself, "oh, there's a big blue bird claiming her territory, I don't want to intrude". I think it is a more basic, "I'm avoiding that dominant bird."

I'm basing this from (many years) watching inter-species avian interactions. There's clearly a lot of dominance behaviours around a common resource, but these are usually based on direct interactions where a larger or more aggressive bird drives off other individuals from a food source etc.

I'm sure it's not impossible that territorial song might be used to discourage other species as well as conspecifics. Particularly among the "smarter" families such as parrots or corvids. I'd be very interested to hear of other examples.

Answer 2

I feel like most of the examples of heterospecific vocal communication in birds as to do more so with predator alarm calls, though are some examples for mobbing or distress calls as well -

Wheatcroft, D., & Price, T. D. (2013). Learning and signal copying facilitate communication among bird species. Proceedings of the Royal Society B: Biological Sciences, 280(1757), 20123070.

Ridley, A. R., Child, M. F., & Bell, M. B. (2007). Interspecific audience effects on the alarm-calling behaviour of a kleptoparasitic bird. Biology Letters, 3(6), 589-591.

Magrath, R. D., Pitcher, B. J., & Gardner, J. L. (2007). A mutual understanding? Interspecific responses by birds to each other's aerial alarm calls. Behavioral Ecology, 18(5), 944-951.

Hurd, C. R. (1996). Interspecific attraction to the mobbing calls of black-capped chickadees (Parus atricapillus). Behavioral Ecology and Sociobiology, 38(4), 287-292.

Fallow, P. M., & Magrath, R. D. (2010). Eavesdropping on other species: mutual interspecific understanding of urgency information in avian alarm calls. Animal Behaviour, 79(2), 411-417.

Aubin, T. (1991). Why do distress calls evoke interspecific responses? An experimental study applied to some species of birds. Behavioural Processes, 23(2), 103-111.

Fallow, P. M., Pitcher, B. J., & Magrath, R. D. (2013). Alarming features: birds use specific acoustic properties to identify heterospecific alarm calls. Proceedings of the Royal Society B: Biological Sciences, 280(1754), 20122539.

There are also a lot of great references in the online Lit Cited of this book - Interspecific Communication: Gaining Information from Heterospecific Alarm Calls (https://link.springer.com/chapter/10.1007/978-3-030-39200-0_12). I did a search for 'heterospecific' and saw 20+ papers describing this phenomenon.