Demographic responses of shrub-successional birds to patch area and edge
Sarah E. Lehnen, PhD
Advisor: Amanda Rodewald
Dissertation: An evaluation of survivorship and habitat use of early-successional birds during the breeding season: Implications for conservation
Due to steady population declines since the 1960s, conservation of shrubland birds has become an important issue facing land managers across eastern North America. Because most shrubland habitats are ephemeral and patchily distributed, effective conservation requires knowledge of shrubland bird responses to patch characteristics and configuration. In this dissertation, we (1) tested the extent to which shrubland birds may be area sensitive, (2) evaluated evidence for 5 alternative hypotheses to explain the previously documented pattern of edge avoidance by shrubland birds, and (3) evaluated the importance of landscape configuration to shrubland birds.
To test area-sensitivity, we mist-netted and banded shrubland birds in 13 regenerating clearcuts between 2002-2006 in southeastern Ohio. After quantitatively adjusting for increased availability of birds for capture in larger patches, only four of the six species showed positive correlations between capture rate and patch size, with capture rates averaging 22% higher in the largest patch compared to the smallest patch; this relationship was only statistically significant for the Yellow-breasted Chat (Icteria virens).
Despite weak evidence of lower densities in smaller patches, we had strong evidence of lower densities in edge habitat during the early morning. We evaluated evidence for five hypotheses that could potentially underlie this pattern: 1) lower usage of edge habitat over all time periods, 2) lower usage of edge habitat during the morning because of social interactions, 3) temperature regulation leading to preferred use of interior habitat during the morning, 4) lower densities in edge habitat due to passive displacement, and 5) larger territories near edge habitat. To test these mechanisms, we examined nest success, settlement, and territory size and placement of three shrubland species and radio-marked 37 males of one shrubland specialist, the Yellow-breasted Chat, during 2005 and 2006. We found little support for lower nest success near edge habitat or edge avoidance in nest or territory placement. Settlement patterns were consistent with predictions of the passive displacement hypothesis. Models strongly supported the hypothesis of larger territories near edges for only one species, the Prairie Warbler (Dendroica discolor). Radio-marked Yellow-breasted Chats did not avoid edge habitat based on telemetry data although there was some support for seasonal variation in edge habitat use. Collectively, results supported the passive displacement hypothesis and, to a lesser extent, the social behavior and territory expansion hypotheses.
To evaluate the importance of landscape composition to shrubland birds, we used variables at the plot, patch, and landscape level to model capture rates. Models including plot variables and the amount of shrubland habitat in the landscape received the most support. Support for amount of shrubland habitat in the landscape rather than patch size suggested higher densities of birds in clustered patches and/or that birds may have used multiple patches within the season. Banding and telemetry data also indicated that inter-patch movements were not uncommon.
Overall, we detected weak evidence of lower densities in smaller patches and no evidence of active edge avoidance by shrubland birds. Due to strong evidence of passive displacement and the importance of landscape composition, we suggest that managers would support the highest densities of shrubland birds through the creation of larger, clustered patches with minimum edge-to-area ratios