Vocalizations of Unicolored Jays (Aphelocoma unicolor) at Montebello, Chiapas, Mexico

Abstract

Unicolored Jays (Aphelocoma unicolor) are little-known social corvids of Mexico and Middle America. Like other birds in the family Corvidae they possess a vocabulary composed of sounds usually referred to as calls rather than songs. Here we provide the first description of this species’ rich vocabulary. We studied vocalizations of color-banded Unicolored Jays in Chiapas, Mexico from January through May of 1987. There they live in cooperatively breeding groups of four to nine birds. We distinguish 697 long-range call variants from our recordings, which probably did not exhaustively sample the range of jay sounds on our study site. The sonograms of the sounds can divided into broad categories with finely graded spectrum-like variation within them, and generally no sharp boundaries between them. Nearly any sonogram in the vocabulary can be connected structurally to any other through a range of intermediates.

The jays tended to introduce many new related variations on a call theme in a single calling session, and many of these variants were not recorded again. There is some indication that the jays favored using a few call types in particular behavior contexts, but they used the great majority of them in a range of widely varying settings. The jay groups shared only a minority of their call collections with one another, but many of the unshared calls were rarely recorded and may not have represented typical vocal differences between them. A few commonly used calls did seem to be peculiar to one or a few jay groups.

Vocalizations recorded on our study site in 1987 differ notably from those recorded there in 2006, strongly suggesting cultural turnover in the local assortment of calls. Recordings from other A. unicolor populations in southern Mexico show marked geographic variation on a regional scale. We suggest that the tendency of Unicolored Jays to produce series of similar and apparently new calls in spates, geographical variation in calls on scales small and large, and turnover in calls at a single locality may all reflect call learning, call improvisation, and the process of random vocal change known as culture drift. We also describe a variety of the jays’ soft short-range calls, some of them strikingly similar to those in other species of Aphelocoma, and others, including some common ones, not previously known in this genus. Like other Aphelocoma, A. unicolor on our study site had rattle calls (one fast and one slow), and whisper song. The precise similarities between certain calls of A. unicolor and those of its relatives show either a startling degree of fidelity in vocal copying over thousands of generations, or a strong innate tendency to reproduce certain details of their vocabularies.