Living on the Edge

Using nest cameras and on-the-ground observations of nesting behavior, the research team analyzed differences in parental behavior between three separate landscape types. The goal of this approach was to assess whether forest fragmentation has an impact on nesting success, given that broad-winged hawks show a preference for unbroken, continuous tracts of forest.

Forest loss and fragmentation result from human activities such as urban expansion, timber harvest, clearcutting, and other forms of development. Between 1973 and 2000, these activities played a significant role in the available habitat for forest-dependent birds throughout the eastern United States. Fragmentation, or the dividing of forest, has both direct and indirect impacts on forest-dwelling raptors. One side effect is the creation of more forest edges. This forces broad-winged hawks to nest nearer to edges where their predators, such as the Great Horned Owl, prowl. More edge habitats could also mean increased competition between species seeking nesting territories, potentially further displacing broadwings. Forest cover alterations can also influence abundance and composition of small mammals, birds, and amphibians that make up most of the broad-winged hawk diet. For example, staple prey species like the eastern chipmunk struggle to survive when forest cover is reduced.

This study compared nesting behavior amongst three provinces within the state, all experiencing varying degrees of forest fragmentation. These three focal areas were Ridge and Valley, the Piedmont, and the Appalachian Plateau. The Ridge and Valley is 59% forested with contiguous forested areas, linear ridges, and very developed valleys and agricultural land. Even so, forest cover in this region has been relatively stable from the 1990s to 2005. The Piedmont is 32% forested with highly fragmented forests interspersed with suburban developments. This region experienced a 6% decrease in forest cover during the same 15-year time frame. The Appalachian Plateau is 71% forested, contains many areas of uninterrupted forest, and has undergone an increase in forest cover by about 2.4% during those 15 years.

Interestingly, there was no clear difference detected in parental behavior between the three landscapes. Over the course of 1,357 observation hours on 13 different nests, the research team found that the composition, delivery rate, diversity of prey and nest materials, as well as attendance of parents to the nest, all remained relatively similar. This implies that broad-winged hawks can thrive in a variety of forest types and adjust to available resources as necessary, at least during the nesting season.

Researchers also noticed a strong representation of eastern chipmunks in prey deliveries to the nest at above 16% of total prey deliveries. The presence of parents at all nests decreased significantly from week 1 of the nestling period to week 2, and continued to decrease as the nestlings developed, reflecting a similar trend found in broadwing nests in Puerto Rico. As nestlings develop, they become capable of thermoregulating for themselves and require less immediate attention from their parents. This means dense forest cover may be an important factor in nesting success, given that these parents spend a large portion of their time away from the nest, foraging. With less cover, the nestlings are more exposed. The team observed a variety of nesting materials being brought to the nests, including fresh oak and hemlock sprigs, dead sticks, and bark. Interestingly, fresh sprigs were often delivered to the nest after the young were fed, supporting a proposed hypothesis that this plant material may contain volatile compounds that help deter insects.