Great Tit grammar

Winter is tough for small birds. The bare trees and the need to spend more time finding food to compensate for higher energy demands in cold weather means that they might be more exposed to a range of predators. They often join other birds in mixed flocks which might afford them more protection, as more eyes keep an eye for Sparrowhawks or cats. Birds have another card up their wings: alarm calls. Alarm calls can be very diverse and research has shown that they can be specific to each predator: responding appropriately is crucial, therefore alarm calls have meaning: flying away might be good if a cat is about, but staying still or hiding in a bush might be a better response to an aerial predator. Tits are highly vocal and have a broad repertoire of calls, which they repeat in different combinations. Some calls are used to beckon other tits when food is found, others to mob predators, yet others as contact calls. They are also very responsive and inquisitive and will approach to you if you imitate them.
 A research team from Japan, Germany and Sweden, led by Toshitaka Suzuki, has been investigating communication in Japanese Great Tits, Parus minor, a close relative of the European species that used to be considered a subspecies (the photo above is of the European Great Tit). Using playback experiments of natural and artificially manipulated calls they show that Great Tits responded very differently to different calls. One call, which they named D call (also called the 'jar' call), is used by birds that are alone, and their partner is more likely to join them after hearing the call, a 'come and join me' call. When researchers played this call on a loudspeaker, great tits approached the loudspeaker. Another call, ABC calls, also called 'chicka calls' are used in an anti-predator context, and playback calls elicit scanning behaviour like the tits are looking for potential danger, 'watch out'. Often tits combine different calls are combined in different sequences, as we combine words in a sentence, for example they add their D call to the ABC call, e.g. ABC-D: upon hearing this, the birds watch left and right as it searching for a predator in their surroundings, but they also approach the loudspeaker, so that both calls combine the separate meanings into a 'sentence', which is used in a mobbing context.
This video shows the sounds separate and in combination, and illustrate how similar the Japanese Great tit calls are to European great tits.
 The researchers wondered if the order of the call matters, that is, there is syntax in the vocalisations. If order is important, then the birds should respond to the ABC-D but not to an artificially generated D-ABC call. They played both calls to 34 wild Japanese Great Tits individually in the winter and counted the number of scans and if the birds approached the loudspeakers. These were their results:

Responses of Japanese great tits to playbacks of ABC–D and D–ABC calls (a) Number of horizontal scans made by tits in 90 s (b) Percentage of trials in which tits approached within 2 m of the loudspeaker. From Suzuki et al 2016. This Creative Commons Attribution 4.0 International License.
The birds scanned and approached the loudspeaker much more when the natural ABC-D call was used, strongly suggesting that the meaning of the vocalisation is lost when the natural order is altered, and that, indeed, birds use grammar. The combination of calls in the context of grammar enriches the possibilities of meaning of a limited number of calls, making bird language much more fascinating and complex that we could have anticipated.

More information
Toshitaka, S. N., Wheatcroft, D. & Griesser, M. Experimental evidence for compositional syntax in bird calls. Nature Communications (2016). doi:10.1038/ncomms10986