The Northern Oriole (Icterus galbula) is rarely reported to be parasitized by the Brown-headed Cowbird (Molothrus ater) (Friedmann 1963, Friedmann et al. 1977). Rothstein (1977) demonstrated that the Northern Oriole rejects cowbird eggs virtually 100% of the time—and often within minutes of laying.
While examining nests in a small riparian woodlot west of Fort Collins, Colorado, I found two parasitized nests of Northern (Bullock's) Orioles. One contained three oriole eggs and one cowbird egg; the second contained one egg of each species. I found the first nest at approximately 0830 and the second at 0930 on the same day, 23 June 1977. Two other oriole nests were situated in the woodlot, but one was inaccessible and the other contained four young orioles.