Decoding the chaffinch "rain" call: a female-directed alarm call?

Acoustic communication is vital for many animal taxa. Many songbirds have elaborate communication systems and large vocal repertoires consisting of learned, complex songs, and calls that are usually simpler in structure. While the functions of songs have been well researched, the functions of bird calls are often difficult to deduce from the context. A well-known example is the "rain" call of the common chaffinch (Fringilla coelebs): although chaffinches are very common and "rain" calls are conspicuous and frequent, the function of this call is still a mystery. It has been proposed to serve such diverse functions such as song substitute in territorial contests, predator alarm call, or within-pair coordination. Here, we systematically tested these hypothesized three functions, using a combination of two playback experiments and field observations. We found that chaffinches did not react to "rain" call playbacks with the same aggressive behavior as to song playbacks. Predator vocalizations, however, consistently elicited "rain" calls. In addition, when a female was visible, male chaffinches used "rain" calls more often both during predator simulation and in the actual presence of predators. Since the "rain" call is only uttered during the breeding season and it is associated with the presence of a female and predators, we propose that the "rain" call is a specific alarm call used in the context of defense against nest predators.