The Thrush in Winter
The importance of fruit in the winter diet of the hermit thrush.
Insight into the habitat selection of birds in winter is growing, yet we still understand little about how changes to the habitat affect overwintering survival. Unfortunately, development of bottomland hardwoods and other forest types in the southeastern United States is encroaching upon the habitats available for overwintering birds.
A better understanding of the habitat features that are crucial to the survival of overwintering birds will help managers protect critical habitats.
The fruiting shrubs of southeastern forests provide a supplemental food source for overwintering migratory birds. We wanted to know how important the presence of winter fruiting shrubs was to these birds.
The Hermit Thrush
To examine this question, Nora Diggs, research scientist with the Migratory Bird Center, chose to study the hermit thrush, which breeds throughout the northeastern United States, most of Canada, and much of the western United States, and winters in southeastern United States, through Mexico to Guatemala.
Hermit thrushes are ideal for this study because they are common and territorial on their wintering grounds and forage on fruit. For example, hermit thrushes are known to be more common in areas with lots of wax myrtle (Myrica cerifera) fruit.
The Experiment
Nora is conducting research at Spring Island in Beaufort County, South Carolina, in a nature preserve administered by the Spring Island Trust. This 1,200 hectare (nearly 3,000 acres) preserve is divided into two tracts and is surrounded by wooded home lots.
She hopes to answer the following questions:
- How to hermit thrushes respond to changes in fruit abundance over the winter?
- Do hermit thrushes with more fruit in their territory fare better or worse than those with less fruit?
- What do the birds eat throughout the season?
- How important is fruit in their diet?
Nora captures the birds several times during the winter to monitor their weight and collect fecal samples. In addition, she removes a tail feather from each bird at the beginning of winter and records the replacement feather's growth. Also, blood samples are taken for carbon and nitrogen isotope analysis; this will aid in determining the winter diet of hermit thrushes.
SMBC Investigators:
Related Resources:
Photo Credits: hermit thrush pictures courtesy of Gerhard Hofmann
