Most hawks construct the frame of their nest from coarse dry branches and line it with finer materials before the eggs are laid. However, Buteo platypterus (Broadwinged Hawk) and some other hawk species resume to build on the nest lining in the nestling stage, by adding fresh sprigs of green vegetation. Various hypotheses for the latter behavior have been suggested. I examined the contents of a Broad-winged Hawk nest in western Maine repeatedly to gain insights into the function of the green sprigs. The birds added on average two large fresh green fronds (each 15–42 cm in length) per day to the nest during the first 18 days after their young hatched, and they continued to add about 1 frond/twig with fresh leaves per day during the last 17 days the chick was in the nest. These fresh greens consisted of ten species of plants, including five species of ferns and two species of conifer. Of my nine examinations of the nest, in all but the last one the nest mold was lined with either ferns or Thuja occidentalis (Northern White Cedar). I compared the percentages of these greens with their local availability near the nest, and conclude that the birds selected for a flat but feathery leaf structure. The literature suggests various possible functions of greenery in nests. I distinguish between greens added onto the nest, and greens added later on post-hatching into the nest lining as a layer, and conclude that nest hygiene is the most plausible explanation for green vegetation in hawk nest linings.