Distribution, abundance, and vegetation associations of birds in Mississippi tidal marshes during the non-breeding season

Accurate estimation of populations of secretive marsh birds is difficult, especially during the non-breeding season when vocalizations are infrequent. Thus, dynamics of marsh bird populations remain relatively unknown, especially during the non-breeding season along the northern coast of the Gulf of Mexico. To address this lack of information about populations of marsh birds during the non-breeding season along the coast of Mississippi, we conducted 650 surveys along 130 line transects distributed across a spatially balanced sampling framework of tidal marshes from December to February 2018–2020. Using hierarchical distance-sampling models for unmarked populations, we estimated species-specific non-breeding population densities, abundances, and vegetation associations of eight species of marsh birds across a mosaic of tidal marsh communities, spanning oligohaline to polyhaline emergent and open marsh systems. We estimated that 127,000 Red-winged Blackbirds (Agelaius phoeniceus), 106,000 Marsh Wrens (Cistothorus palustris), 37,000 Seaside Sparrows (Ammospiza maritima), 17,000 Clapper Rails (Rallus crepitans), 15,000 Swamp Sparrows (Melospiza georgiana), 14,000 Nelson's Sparrows (Ammospiza nelsoni), 10,000 Sedge Wrens (Cistothorus platensis), and 2000 Common Yellowthroats (Geothlypis trichas) overwintered in the tidal marshes of Mississippi during our study. Further, we show that these species associate with different salinity ranges and vegetation communities within the broader estuarine marsh. Our population estimates and vegetation associations for vulnerable species provide important baselines from which assessments of future change can be compared.