Mechanisms underlying population dynamics of a migratory songbird in a fragmented forest
Trine, Cheryl Lynnette
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Permalink
https://hdl.handle.net/2142/23210
Description
Title
Mechanisms underlying population dynamics of a migratory songbird in a fragmented forest
Author(s)
Trine, Cheryl Lynnette
Issue Date
1996
Doctoral Committee Chair(s)
Robinson, Scott K.
Department of Study
Biology
Discipline
Biology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Ecology
Biology, Zoology
Language
eng
Abstract
Fragmentation has been implicated as a cause of long-term changes in migratory songbird populations. Some problems associated with fragmentation are generally considered edge effects (nest predation and cowbird brood parasitism), whereas others are attributed to fragment size and isolation (low pairing success, area sensitivity, unstable population sizes). I studied the impact of Brown-headed Cowbird (Molothrus ater) brood parasitism and of nest predation on the reproductive success of Wood Thrush (Hylocichla mustelina) hosts in three large ($>$1000 ha) forest tracts in southern Illinois. In these forests, nests were typically multiply-parasitized (1.5-4.4 cowbird eggs/nest). Host fledging success was reduced by 18-20% per cowbird egg. Multiple parasitism also reduced cowbird fledging success. The study revealed that all three study sites were likely population sinks (produced too few offspring to compensate for adult mortality) because of the combined effects of high levels of parasitism and nest predation. An examination of parasitism levels and nest predation rates near and far from edges revealed no strong gradients near edges. This suggests that the landscape composition (e.g., relative amount of forest and agriculture) was a major factor determining nest predation and cowbird parasitism levels. In my study sites, local population dynamics appeared to be governed by decision rules used by adult birds when deciding whether to return to their previous year's breeding sites. Adults were more likely to return to areas where they had previously nested successfully. These decision rules may help them avoid sink habitat. Adults, however, did not avoid areas of high parasitism, which suggests that they lack adaptive responses to sink habitat in which parasitism is the chief cause of reproductive failure.
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