Shifts in call type and acoustic structure across the breeding season of a polyandrous shorebird, the Spotted Sandpiper (Actitis macularius)

Breeding birds vocalize to find mates, defend territories, warn of predators, and coordinate parental care, among other social functions. Birds are known to modify their vocal behavior with respect to social contexts, but the extent and generality of this phenomenon in shorebirds is not well understood because shorebirds are understudied models in avian bioacoustics. We employed passive acoustic monitoring to record vocalizations of free-living Spotted Sandpipers (Actitis macularius), a polyandrous shorebird, to determine how their vocal behavior changes across the breeding season. To do so, we collected recordings from nests in the morning and early evening across breeding stages and extracted time spent calling, call rate, and several acoustic structure variables for three main call types: “weet call”, “trill call”, and “pipe”. Focusing on incubation and post-hatch breeding stages, we hypothesized that Spotted Sandpipers prioritize different call types in different stages, and we explored whether call usage or acoustic structure varied by site, time of day, or between individual nests. Spotted Sandpipers spent more time vocalizing in the morning and displayed significant differences in vocal behavior by nest and site. While total time spent calling did not change significantly from incubation to post-hatch, different call types were prioritized in each stage: usage of weet and trill calls was higher during incubation, suggesting these calls function to coordinate parental activities, while pipe usage was higher in the post-hatch stage, suggesting pipes are directed to chicks and may serve anti-predator functions. Changes in acoustic structure (call duration, number of notes, and note rate) of weet and trill calls were consistent with the observed changes in time spent calling for those call types. Prioritizing different call types in different contexts, and adjusting the acoustic structure of specific calls, likely helps these shorebirds accomplish the suite of social tasks necessary for successful reproduction.