Optimizing surveys for marsh songbirds: does timing matter?
Analysis of data from point counts, a common method for monitoring bird population trends, has evolved to produce estimates of various population parameters (e.g., density, abundance, occupancy) while simultaneously estimating detection probability. An important consideration when designing point counts is to maximize detection probability while minimizing variation in detection probability both within and between counts. Our objective was to estimate detection probability for three marsh passerines (Marsh Wren [Cistothorus palustris], Swamp Sparrow [Melospiza georgiana], and Yellow-headed Blackbird [Xanthocephalus xanthocephalus]) as a function of weather covariates and evaluate the temporal variability in detection probability of these three species. We conducted paired, unlimited radius, 10-min point counts for our three study species at 56 wetlands in Iowa from 20 April – 10 July 2010. We conducted surveys in the early-morning (30 m before sunrise to 3 hr after sunrise) and late-evening (3 hr before sunset to 30 m after sunset) hours during consecutive survey periods to minimize any daily variation in detection probability. We recorded all birds detected during point counts and also recorded both weather variables and time to sunrise or sunset before each survey. We estimated detection probability for each species as a function of weather and temporal covariates using the N-mixture model in the “unmarked” package in Program R. Mean detection probability ranged from 0.272 (SE = 0.042) for Marsh Wren to 0.365 (SE = 0.052) for Swamp Sparrow. Time of season was positively correlated with detection probability for Swamp Sparrow but was negatively correlated with detection probability for Yellow-headed Blackbird, suggesting that detection probability increases throughout the breeding season for Swamp Sparrow and is highest early in the breeding season for Yellow-headed Blackbird. Detection probability of Marsh Wren did not vary temporally nor did we find any effect of weather covariates on detection probability for any species. Understanding how detection probability for marsh passerines varies throughout the breeding season allows for targeted survey efforts to maximize detection probability for these species. Furthermore, consistent detection probability of marsh passerines between morning and evening survey periods increases the opportunity to conduct surveys for these birds, which allows for greater flexibility to increase spatial and temporal replication of surveys could lead to more precise estimates for desired population parameters.
This is the peer reviewed version of the following article: Harms, Tyler M., and Stephen J. Dinsmore. "Optimizing surveys for marsh songbirds: does timing matter?." Journal of Field Ornithology 89, no. 3 (2018): 276-286, which has been published in final form at doi: 10.1111/jofo.12265 . This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.