A House Sparrow building a neat round nest in a tree is an odd and seldom-seen sight. Recently, in western North Dakota I watched a male House Sparrow busily constructing a round grass nest in the crotch of a coniferous tree, a location a robin might choose. This was at a I-94 rest stop in the midst of vast, treeless prairie. The building offered no nooks or crannies, the kind these birds often use. There were no signs to build behind, and no nest boxes or other cavities. Usual building sites were non-existent. I've always thought House Sparrows, an introduced species, were obiligate cavity nesters. I did find one Internet site, from the University of Michigan, that mentioned use of both deciduous and coniferous trees for non-cavity nest construction, but dozens of sites did not mention it. The nest, built entirely of long strands of grass, was about twice the size of a softball. The nest entrance was an obvious hole on one side of the structure.