A Method for Distinguishing Flight Calls of Several Western Birds

Abstract

Billions of birds migrate under the cover of darkness, making them difficult to detect except by calls given in flight. Recording and identifying these calls can document the species of birds passing overhead and provide an index to their numbers. However, flight calls of some species are quite similar and difficult to tell apart. We investigated a method of identifying calls of several species whose calls are difficult to distinguish: the Solitary (Tringa solitaria) versus Spotted (Actitis macularius) Sandpipers and the White-crowned (Zonotrichia leucophrys) versus Brewer’s (Spizella breweri) and Clay-colored (S. pallida) Sparrows. We generated audiospectrograms of diurnal flight calls of known identity and inspected these for qualitative criteria by which the calls could be distinguished without the need for measurements or statistical software. We then tested the efficacy of these criteria on a new set of previously identified flight calls. Consideration of multiple criteria allowed identification of ~50% of one of the two types of sandpiper calls analyzed and ~60% of the sparrow calls, so a significant fraction remained unidentifiable by this method. Nevertheless, we hope researchers and sound recordists will apply this guide to improve our understanding of migration throughout western North America. We also encourage recordists to contribute additional visually verified recordings to allow us or others to perform similar tests on other species and species groups.