Responses to Nest Preadation and Brood Parasitism in a Migratory Songbird

Birds have evolved life history traits that tend to maximize lifetime reproductive success, and these traits include behavioral responses to factors limiting reproductive success. Behavioral responses may be especially important for birds breeding in highly fragmented landscapes where increased nest predation and increased brood parasitism by Brown-headed Cowbirds can greatly reduce reproductive success.

I studied color-marked populations of Prothonotary Warblers (Protonotaria citrea) in the fragmented bottomland forest of the Cache River watershed during 1993-2000 to determine whether or not these birds responded to nest predation and brood parasitism in ways that reduced the negative effects of each. Experimental and non-experimental data demonstrated that individual Prothonotary Warblers returned to sites between years in response to their reproductive success (as limited by nest predation). Between-year site fidelity increased with an increase in the number of broods produced with approximately 80% of double-brooded males and females returning. Individuals returned at rates of approximately 30% and 50% when they produced zero or one brood, respectively. Brood parasitism by cowbirds reduced the reproductive success of Prothonotary Warblers as a result of decreased hatching success of warbler eggs and decreased survival of warbler nestlings. The warblers accepted brood parasitism and did not choose nest sites inaccessible to cowbirds, defend nests during the egg-laying period, desert parasitized nests, or avoid returning to sites where they had been parasitized. The results of this research indicate that these birds may be able to avoid chronically high rates of nest predation by not returning to areas where nest predation eliminates nesting success. Prothonotary Warblers, however, may be especially vulnerable to ecological traps where rates of nest predation are low, levels of brood parasitism are high, and they are producing mainly cowbird young.

The Cache River Wetlands (CRW) bottomland forest restoration project in southern Illinois, including the Cypress Creek National Wildlife Refuge and Cache River State Natural Area, continues to provide a unique opportunity to incorporate the results of songbird research within the project area into management recommendations and the restoration plan. These recommendations will be validated in the long term with continued research as restoration proceeds. In the CRW area, our previous research on the bird community has established the importance of connecting and enlarging existing tracts of forest, of restoring and managing a wide variety of floodplain habitats, and of the importance of bottomland forests for birds during the winter. In addition, we now know that the rate and amount of water-level fluctuations during the breeding season influence the rate of nest predation, in turn affecting season-long reproductive success and ultimately influencing the patterns of site and territory fidelity of birds breeding in bottomland and swamp forests.

Continued research on the bird community in the Cache River Wetlands will expand our knowledge of how the restoration of hydrology in off-channel wetlands affects the diversity, abundance, and nesting success of birds within the bottomland forest ecosystem. The Prothonotary Warbler will continue to be the focal species for determining the success of hydrologic restoration and also for determining the effect of bottomland forest restoration on brood parasitism by cowbirds. This research will increase our ability to effectively and efficiently restore hydrologic processes and manage bottomland forests for those avian species that are dependent on functioning bottomland forest systems. The results of this research will have broad application in the Mississippi ecoregions and will assist with other bottomland forest restoration efforts in Illinois (e.g., Emiquon and the Illinois River project and the Kankakee River restoration project).

Jeff Hoover, Center for Wildlife Ecology

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