Ground-Nesting Warblers on the Eastern Shore of Maryland: Declining Population Trends and the Effects of Forest Composition and Structure

Abstract

Changes in forest size and composition can affect Neotropical migratory songbirds breeding in Eastern North America, leading to population fluctuations and declines. In 2012–2014, we conducted point counts at locations previously surveyed in 1981–1983 and analysed Breeding Bird Atlas data from 1980–2013 to estimate long-term population trends of two declining ground-nesting warblers on the Eastern Shore of Maryland within the Atlantic Coastal Plain: the Ovenbird Seiurus aurocapilla and the Worm-eating Warbler Helmitheros vermivorum. We detected significant declines for Ovenbirds in both datasets, but we found conflicting data for Worm-eating Warblers. During 2012–2015, we investigated the effects of forest characteristics, such as vegetative composition and ground cover, on occupancy and nest success. Ovenbirds were found more where there were more and larger trees but were negatively affected by the proportion of pines, whereas Worm-eating Warblers were more abundant when there were higher numbers of trees and more pines. Nest success was not affected by vegetation other than the proportion of pine trees or leaf litter depth, both of which increased nest success. Worm-eating Warblers with more pine needles in their nests also had increased nest success. This study presents a nuanced view of how forest composition (proportion of pine trees) of the Atlantic Coastal Plain in Maryland influenced ground-nesting warblers differently, and suggests that effectively managing ground-nesting warblers in this area may differ from other regions of Eastern North America due to its unique vegetative composition, structure, and history.