Investigating population trends and winter ecology in Wood Thrushes

Photo Credit: Richard Hall

Wood thrushes are a species of songbird that breeds in the forests of the eastern US and migrates in the winter to the forests of Central America. Although they remain a relatively common bird, citizen science surveys throughout the US show that this species has been declining rapidly since at least the early 1970’s. Many possible causes of these declines have been put forward. In the breeding grounds, wood thrushes are affected by acid rain, disturbance, urbanization, and are known to be especially sensitive to forest fragmentation. A pair of wood thrushes nesting in a small forest patch is much more likely to lose their nest to predators or have their nests parasitized by brown-headed cowbirds (cowbirds do not build their own nest but lay eggs in the nests of other species) than a pair of wood thrushes nesting in a large contiguous forest. Another likely cause of declines is the loss and degradation of forest habitat used by wood thrushes during the winter – the amount of forest available in the winter range of wood thrushes is very small compared to the available habitat in the breeding range and these Central American tropical forests are disappearing fast.

A recent open access paper in Scientific Reports, available at http://rdcu.be/x8rH, estimates how breeding success changes with increasing bird density (the shape of density dependence) in several states in the northeastern US and finds that the shape of these curves at depends on the fragmentation level of forest. Surprisingly, the study shows that in ‘good quality’ states, such as Maine, with relatively intact forested areas, declines were more severe, density (number of breeding pairs of wood thrushes per hectare of forest; estimated from the citizen science surveys) was lower, and density dependence was weaker than in the ‘poorer quality’ states, such as Ohio, with relatively heavy forest fragmentation. This demonstrates that, while wood thrushes are sensitive to forest fragmentation in breeding habitat, breeding habitat loss or fragmentation is not the main cause of declines, rather the loss and/or degradation of habitat in the winter range in Central America is the culprit.

The same conclusion was reached by a demographic model of the population dynamics of wood thrushes throughout the entire ‘network’ of wintering and breeding regions, published last year in the journal Ecological Applications by Caz Taylor in collaboration with Dr Bridget Stutchbury (York University, Ontario), a leading expert on wood thrushes and other songbirds. The network model points the finger, particularly, at forest loss and degradation in the eastern part of the wood thrush winter range, the countries of Costa Rica, Nicaragua, and Honduras.

These high level modeling studies need to be followed up by on the ground research to understand what drives land use change in Central America and how changing landscapes in their winter range affects neotropical migratory birds. A new PhD student in our lab is planning to conduct such studies. Fabiola Rodriguez, an international student from Honduras, has participated in several projects focusing on neotropical birds in Central America including her MS research in Pennsylvania and seen first hand how research can serve as a foundation to inform conservation. Fabiola was inspired by the network model paper and by EEB’s strengths in Tropical Biology to apply to Tulane to do her doctoral research. She plans to work in Honduras conducting much needed field studies to better understand the winter ecology of wood thrushes. She will work with conservation partners so that their research will help to inform conservation actions and how to mitigate threats that currently affect migratory birds.

Taylor, C.M. 2017. The shape of density dependence in fragmented landscapes explains an inverse buffer effect in a migratory songbird. Scientific Reports 7: 14522 DOI:10.1038/s41598-017-15180-4 Open Access: http://rdcu.be/x8rH

Taylor, C.M. & B. J. M. Stutchbury. 2016. Effects of breeding versus winter habitat loss and fragmentation on the population dynamics of a migratory songbird. Ecological Applications 26: 424–437 doi:10.1890/14-1410.1