Vermont Cooperative Fish and Wildlife Research Unit

The University of Vermont
Rubenstein School of Environment and  Natural Resources
Burlington, VT 05405

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Wood Thrush

Determinants of Wood Thrush nesting success

 

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Principal Investigator:  Therese Donovan

Cooperators:

Dick DeGraaf, U.S. Forest Service

Graduate Student:  Melanie Driscoll

Funding Sources:

U.S. Forest Service

Abstracts:

1)  We collected data on 212 wood thrush (Hylocichla mustelina) nests in central New York from 1998–2000 to determine the factors that most strongly influence nest success.  We used an information–theoretic approach to assess and rank 9 models that examined the relationship between nest success (i.e., the probability that a nest would successfully fledge at least 1 wood thrush offspring) and habitat conditions at different spatial scales.  We found that 4 variables were significant predictors of nesting success for wood thrushes:  (1) total core habitat within 5 km of a study site, (2) distance to forest–field edge, (3) total forest cover within 5 km of the study site, and (4) density and variation in diameter of trees and shrubs surrounding the nest. The coefficients of these predictors were all positive.  Of the 9 models evaluated, amount of core habitat in the 5-km landscape was the best-fit model, but the vegetation structure model (i.e., the density of trees and stems surrounding a nest) was also supported by the data.  Based on AIC weights, enhancement of core area is likely to be a more effective management option than any other habitat-management options explored in this study.  Bootstrap analysis generally confirmed these results; core and vegetation structure models were ranked 1, 2, or 3 in over 50% of 1,000 bootstrap trials.  However, bootstrap results did not point to a decisive model, which suggests that multiple habitat factors are influencing wood thrush nesting success.  Due to model uncertainty, we used a model averaging approach to predict the success or failure of each nest in our dataset.  This averaged model was able to correctly predict 61.1% of nest outcomes.  

2)  Despite two decades of research into habitat fragmentation and edge effects on nesting birds, critical information about how edges affect population viability of Neotropical migratory songbirds breeding in heterogeneous landscapes is still missing.  We studied nesting success and assessed population viability for Wood Thrushes (Hylocichla mustelina) breeding across a heterogeneous landscape in central New York from 1998-2000.  We monitored nests to estimate nest success in edge and interior habitats in both fragmented and contiguously forested landscapes.  In contiguous landscapes, daily survival rate did not differ between edge nests (0.963) and interior nests (0.968) (c2 = 0.19, p = 0.66).  In contrast, in fragmented landscapes, daily survival estimates were higher in interior (0.971) than edge (0.953) nests (c2 = 3.1, p = 0.08).   Although these results support the hypothesis that edge effects are more severe in fragmented landscapes, the effect on population viability is of utmost concern.  To this end, we used a Monte Carlo approach to model the population viability of Wood Thrushes for each landscape-habitat type combination, as well as for the entire system.  We let birth rates, juvenile survival rates, and adult survival rates vary stochastically for each habitat-landscape combination, and computed the finite rate of increase (l) for the Wood Thrush population.  Assuming all birds that successfully fledged a brood would attempt a second brood, and a maximum of three nesting attempts per season, results of 250 trials show that l averaged below 1 for fragmented edge habitat, approximately 1 in contiguous edge, and above 1 for fragmented interior and contiguous interior habitats.  Additionally, l averaged 0.98 for the regional population, suggesting edge effects have an influence on population viability.  However, the 95% Monte Carlo confidence intervals ranged from 0.85 and 1.12.  High uncertainty in birth rates led to significant variation in birth rates in Monte Carlo trials, such that even changes in the probability of double brooding had limited impact on system-level viability.  Our model suggests uncertainty in Wood Thrush population stability, although population growth likely would be reduced if the distribution of breeding females is altered, if birth rate or adult or juvenile survival decreases, or if human activities create more edge habitat.

Publications:

Driscoll, M., T. Donovan, R. Mickey, A. Howard, and K. Fleming.  2005.  Determinants of Wood Thrush nesting success: a multi-scale, model selection approach. Journal of Wildlife Management 69:699-709.
Driscoll, M., and T. Donovan.  Landscape context moderates edge effects: Nesting success of Wood Thrushes in central New York.  Conservation Biology 18:1330-1338.

Selected Links:

Wood Thrush species account
Wikipedia
Vermont Department of Fish and Wildlife 

 

 

 

Send mail to tdonovan@uvm.edu with questions or comments about this web site.
Last modified: May 09, 2007