Passive acoustic monitoring provides predictable and reliable underestimates of population size and longevity in wild Savannah Sparrows

Abstract

Many breeding birds produce conspicuous sounds, providing tremendous opportunities to study free-living birds through acoustic recordings. Traditional methods for studying population size and demographic features depend on labor-intensive field research. Passive acoustic monitoring provides an alternative method for quantifying population size and demographic parameters, but this approach requires careful validation. To determine the accuracy of passive acoustic monitoring for estimating population size and demographic parameters, we used autonomous recorders to sample an island-living population of Savannah Sparrows (Passerculus sandwichensis) over a 6-year period. Using the individually distinctive songs of males, we estimated male population size as the number of unique songs detected in the recordings. We analyzed songs across 6 years to estimate birth year, death year, and longevity. We then compared the estimates to field data in a blind analysis. Estimates of male population size through passive acoustic monitoring were, on average, 72% of the true male population size, with higher accuracy in lower-density years. Estimates of demographic rates were lower than true values by 29% for birth year, 23% for death year, and 29% for longevity. This is the first investigation to estimate longevity with passive acoustic monitoring, and adds to a growing number of studies that have used passive acoustic monitoring to estimate population size. Although passive acoustic monitoring under-estimated true population parameters, likely due to the high similarity among many male songs, our findings suggest that autonomous recorders can provide reliable estimates of population size and longevity in a wild songbird.