Conservation Partners Defragment Bottomland Forests and increase Songbird Nesting Success

Neotropical migratory birds face continued threats resulting from the loss, fragmentation and degradation of natural habitats. Research during the past 30 years has demonstrated the negative effects of habitat fragmentation (increased brood parasitism by cowbirds and decreased nest survival) on migratory songbirds breeding in temperate bottomland forests, and land acquisition/conservation efforts that “defragment” forests are possibly the best solution to restore or improve bottomland forest ecosystems for these birds. The Cache River Joint Venture Partnership (JVP; ILDNR, USFWS, and TNC) formed in 1991 in an effort to conserve and restore some 60,000 acres of bottomland forest habitat in the Cache River watershed of southern Illinois. Through the early 2010s, the JVP had successfully acquired and re-forested over 20,000 acres of non-forested (primarily agricultural) land. With the backing of the JVP, we collected data during 1993-1995, and again during 2010-2012, documenting nesting success of various species of bird prior to and after most of this land-use conversion, respectively. We predicted that, as a result of the reduction of forest fragmentation, rates of cowbird parasitism would decrease and nest survival increase between the two time periods. We also predicted that some measure of landscape composition (e.g., percent landcover in agriculture within a particular radius around nests) would be correlated with nesting success. We analyzed data collected from 1554 nests (22,238 exposure days) representing 7 species. As predicted, nest survival increased and cowbird parasitism decreased between the early and late time periods. Nest survival was most affected by percent agricultural landcover within 1 km, whereas for cowbird parasitism it was percent agriculture within 5 km. These results represent the merging of scientific research with conservation in action in an effort to evaluate the success of conservation efforts which, in the end provided tangible benefits to birds breeding in bottomland forests.