Structure of breeding calls in three closely related bird species (Calidris; Scolopacidae)

We analyzed calls of three related sandpipers to document species’ similarities and differences. We hypothesized that functionally different calls would differ in degree of divergence. We studied two nuptial calls (complex “Song” and rhythmically repeated aerial call, RRC) of males, and a general-purpose call (“alarm” trill, AT) given by parents of both sexes in the presence of humans and other potential predators, in a small clade: great knot, Calidris tenuirostris (Horsfeld, 1821); surfbird, C. virgata (Gmelin, 1789); and red knot, C. canutus (Linnaeus 1758). Calls diverged unevenly across species, but RRCs and Song diverged most and ATs least. Vocalizations of great knot and surfbird were most similar to one another, in agreement with a recently proposed phylogeny. Despite species differences in single acoustic traits, calls were evolutionarily conservative at higher structural levels, such as rhythmic temporal delivery of RRCs and harmonic structure (e.g., the fundamental frequency was commonly suppressed). Some acoustic qualities that differed across species were similar across call types within species (e.g., tonality in red knot calls). Trait similarity across different calls suggests that a species’ calls cannot evolve independently of one another: common mechanisms of vocal production across different calls may impede differentiation within a species’ repertoire.