Responses to Nest Predation 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.
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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.
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Author Jeff Hoover dwarfed
by cypress in Cache River Wetlands. Photo courtesy Anthony
Corso. |
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 and Plant Ecology
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