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 is 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.