The materials that birds use to build their nests vary widely between species, from twigs and grasses to mud and even their own saliva. Hawks typically use coarse, dry branches and twigs as primary construction materials, and many line their nests with bark or other finer materials. For most bird species, hawks included, once construction is finished and their eggs are laid, home-building is complete. But University of Vermont Professor Bernd Heinrich found that broad-winged hawks continue lining their nests with green sprigs of vegetation throughout the nestling stage, and those green fronds appear to be carefully selected.
Heinrich examined the contents of a broad-winged hawk nest in western Maine after the eggs hatched to learn the species and purpose of the vegetation used. His paper in Northeastern Naturalist (2013) reported that the birds delivered two large, green fronds to the nest per day, on average, for the first 18 days, and one frond per day in the succeeding 17 days, until the young birds fledged. Twenty of the 55 sprigs came from northern white cedars, 14 consisted of five species of ferns, and another 11 were from sugar maples. Much smaller numbers of balsam fir, red spruce, white ash, and red maple were also included.
“Of my nine examinations of the nest, in all but the last one the nest mold was lined with either ferns or northern white cedar,” Heinrich wrote. “On the other hand, during the course of my observations, the most readily available green material was maple leaves.” In fact, the closest white cedar trees were half a kilometer away, and the ferns came from the ground, even though the hawks are seldom observed on the ground unless capturing prey. Heinrich believes the cedar and ferns were chosen on purpose.
Previous research on European starlings has suggested that sprigs of fern and cedar, which contain medicinal and antiseptic compounds, serve to rid nests of pathogens by suppressing bacteria and boosting the young birds’ immune function. They also lie flat, as do sugar maples, and Heinrich speculates that their purpose is to provide a clean surface for food.
“Hawk parents deposit the bodies of their prey directly into the nest mold,” he wrote. “Providing a ‘clean plate’ should help store fresh meat and would be beneficial, especially at high temperatures...If the parents procured a surplus of meat that was partially torn apart, then the fresh substrate would serve as a relatively clean surface where dismembered prey would be subjected to a reduced bacterial load that would retard spoilage.”