Cooperative Fish and Wildlife Research Units Program: Vermont
Education, Research and Technical Assistance for Managing Our Natural Resources

We investigated how habitat patterns across three spatial extents influenced habitat selection decisions and demographic patterns for black-throated blue warblers (Dendroica caerulescens) at 20 study sites in west-central Vermont, USA from 2002-2005. These sites represented gradients of habitat patterns at different spatial extents, including: 1) territory-level understory shrub density, 2) patch-level understory shrub density occurring within 25 ha of territories, and 3) landscape-level habitat patterns occurring within 5 km radius extents of territories. We considered multiple vital population parameters including abundance, age ratios, pairing success, and annual fecundity. We found that territory-level shrub cover was most important for determining which individuals occupied which habitats but that landscape-level habitat structure strongly influenced reproductive output. Consistent with long-term studies of this species in New Hampshire, sites with higher territory-level shrub density had higher abundance, were more likely to be occupied by older, more experienced individuals, and males that were paired compared to sites with lower shrub density. However, annual fecundity was higher on sites located in contiguously forested landscape where shrub cover was low. In addition, we found evidence that the effect of habitat pattern at one spatial level depended on habitat conditions at different levels. The interaction between territory-level and landscape-level habitat structure influenced both abundance and annual fecundity. Abundance was highest at sites located in more fragmented landscapes with the highest shrub densities, but females occupying these same sites fledged fewer offspring per year. Our results suggest that the proximate cue of territory-level shrub density used for breeding territory selection by this species may be decoupled from realized fitness, where individuals are unable to recognize and occupy habitats best suited for reproduction. We considered ecological factors associated with this disconnect including predation, parasitism, and food limitation, and suggest that cowbird parasitism is a leading factor contributing to disconnect between selection cues and fitness in this system.