Egg rejection by Cowbird hosts in grasslands

We tested Field Sparrows (Spizella pusiila), Vesper Sparrows (Pooecetes gra mineus), Lark Sparrows (Chondestes grammacus), Grasshopper Sparrows (Ammodr amus savannarum), Dickcissels (Spiza americana), Eastern Meadowlarks (Sturn ella magna), and Western Meadowlarks (S. neglecta) to determine whether the low level (<10%) of observed parasitism by Brown-headed Cowbirds (Molothru s ater) on these grassland hosts is a result of egg rejection. Western Mead owlarks rejected 78% of artificial and real cowbird eggs, Eastern Meadowlar ks rejected 36% of artificial cowbird eggs, and Dickcissels rejected 11% of artificial cowbird eggs. None of the other hosts regularly rejected cowbir d eggs. Thus, egg rejection may account for some, but not all, of the low l evel of observed parasitism on grassland hosts in the Midwest. Meadowlarks were also tested with nonmimetic eggs, and the remaining hosts were tested with undersized mimetic and nonmimetic eggs when possible. All hosts, with the exception of the Field Sparrow, demonstrated some level of rejection of the nonmimetic eggs. These results suggest that some grassland hosts, whic h apparently have been in contact with cowbirds the longest, have evolved s ome form of rejection behavior that might have selected for mimetic eggs in cowbirds. The intermediate levels of rejection by both species of meadowla rks also may indicate that rejection is increasing in these populations.