The study is Bird Mortality at a Glassed-In Walkway in Washington State, by R.E. Johnson and G. E. Hudson. It was published in 1976 in Western Birds, 7:99-107. elibrary.unm.edu/sora/wb/v07n03/p0099-p0107.pdf
The most common bird in the study between May 1968 to March 1970 (28 birds out of 155) was the pine siskin, http://identify.whatbird.com/obj/79/_/Pine_Siskin.aspx, a small finch with a brown-streaked body. American Robin (turdus migratorius - http://en.wikipedia.org/wiki/American_Robin) and yellow warbler (Dendroica petechia - http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Yellow_Warbler.html) were next most common, with 9 each out of 155 specimens.
Migration seasons, especially fall, were most lethal for birds. The location in this study was a glass walkway that does not reflect but seems completely transparent. An invisible barrier.