Using autonomous recording units to detect individual marsh birds in the Northern Gulf of Mexico.

Abstract

Marsh birds are highly elusive and select wetland habitats that are difficult to navigate, as well as easily damaged by human observers. Autonomous recording units (ARUs) have been used to determine presence or absence of marsh bird species, but little is known about distance effects on detection probability. Therefore, our objectives were to 1.) evaluate if ARUs can be used to accurately count three marsh bird species (e.g., Clapper Rail, Least Bittern, Seaside Sparrow), 2.) estimate detection probability curves based on ARU recordings, and 3.) determine the area surveyed by the ARU for the three species. We arranged ARUs to record calls from our marsh bird species broadcasted from Bluetooth speakers at fixed distances. We replicated natural calling scenarios by playing calls in different number combinations of individuals, ranging from 1 to 10 birds. To reduce interference from real bird vocalizations, we conducted our experiment in a recently burned pine savanna habitat that had similar herbaceous vegetation structure to the coastal emergent wetland habitats preferred by these species in southern Mississippi. We used Raven Pro bioacoustics software to produce sonogram images of the broadcasted calls to count individuals in each recording. Detection probability of each species by distance was calculated in program R. Results showed that ARUs may be useful for counting individuals at close distances for some species (< 100m), but most counts biased low. In Clapper Rails and Least Bitterns, count accuracy decreased between 100 and 125 meters from the ARU but closer (50-100 meters) for Seaside Sparrows. There was also a significant decline in count accuracy with increasing chorus size. With further study and advancing technology, ARUs may be able to supplement marsh bird surveys and limit logistical issues.