Birds have been known to build nests out of various and sundry materials, ranging from leaves and twigs to saliva and feces, depending on the species, range and material availability. Hawks appear to prefer coarse, dry branches and twigs to build the foundation of their nest, and similar to most other birds nest construction is considered complete once eggs have been laid. However, Professor Bernd Heinrich of the University of Vermont has found that broad-winged hawks (Buteo platypterus) will continue refreshing nest lining with green vegetation throughout the nestling stage.
Heinrich inspected the nests of broad-winged hawks in Maine after the eggs had hatched to discover the composition and purpose of vegetation within. His research found that the birds deliver 2 fresh, green, large fronds to the nest each day for the first 18 days post hatching, then for the following 17 days one frond is installed, until fledging. Species composition yielded 55 northern white cedar sprigs, 14 fern fronds from 5 different species and 11 sugar maple twigs. Statistically less significant amounts of fresh vegetation used by the hawks came from balsam fir, red spruce, white ash and red maple.
Over the course of nine nest inspections, the first eight found nests lined with fresh cedar or ferns, but the most interesting point is that for the duration of the hatchlings rearing maple was the most readily available material. The closest white cedar trees, Heinrich noted in his paper published in Northeast Naturalist in 2013, were half a kilometer away and ferns grown on the forest floor, a place rarely frequented by hawks unless retrieving prey. The conclusion reached becomes cedar and fern species were chosen on purpose, for a reason beyond availability and structure.
Research on building materials used by other species revealed that fern and cedar fronds have antiseptic properties, which could aid in preventing bacterial and pathogenic intrusion. This would be especially helpful when bring back partially disemboweled prey during summer months. Heinrich posits that the fronds could provide a veritable “clean plate” to serve hatchlings from, reducing bacterial loads and probable spoilage.
Heinrich, Bernd “Why Does a Hawk Build with Green Nesting Material?” Northeastern Naturalist 20(2):209-218. 2013