Structure of breeding calls in three closely related bird species (Calidris Merrem, 1804; Scolopacidae)

We studied calls of three sandpiper species 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 great knot, Calidris tenuirostris (Horsfeld, 1821); surfbird, Calidris virgata (Gmelin, 1789); and red knot, Calidris canutus (Linnaeus, 1758). Calls diverged unevenly across species--RRCs and Song diverged the most and ATs the 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 suppressed in some call types). 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.