Advertising by western grebes: bout-length variability and signal confusion in a multiple-use call

Studies of song variability in birds have contributed greatly to our understanding of how animals use vocal signals to communicate information concerning their individual identity, species, sex, and territorial or pairing status to other members of the population. Most of these studies have focused on the songs of male passerine songbirds, where learning plays an important role in the development of song variability. Here we examine variability within a multi-functional call of a nonpasserine species, the western grebe (Aechmophorus occidentalis). In this observational study, we focused on variability in the lengths of consecutive call-bouts given by individuals within different behavioral contexts. Early in the breeding season, Advertising appeared to serve as an initial contact between two/one courting birds, which then approached and eventually engaged one another in visual displays. After pairing, however, both sexes continued to use Advertising calls in a variety of contexts that involved mate recognition or parent-young communication. Cases of signal confusion or misidentification of pairing status were rare and generally brief, even within the contexts of a large colony. During the pre-nesting period, unpaired birds usually gave bouts of 3-5 calls, while paired birds gave 1- or 2-call bouts. During incubation, birds returning to their nests often Advertised repeatedly to their mate using 1- or 2-call bouts in nearly equal proportions. Replies by their mate from the nest were more likely to be 2- or 3-call bouts (74%) and appeared to guide the returning bird to the nest. Finally, post-nesting parents calling to their young or mates primarily used 1- or 2-call bouts. We suggest that bout-length flexibility likely plays an important role in providing redundancy in a noisy environment and in limiting signal confusion in this multi-functional call.