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    Although the "dwarf" race of the Brown-headed Cowbird (Molothrus ater obscurus) is sympatric with the Crissal Thrasher (Toxostoma dorsale) in the lower Colorado River valley, I observed no parasitism in 15 thrasher nests. To determine whether or not the absence of cowbird eggs was caused by egg rejection, I experimentally parasitized nine thrasher nests. Cowbird eggs were ejected by thrashers at all experimental nests. To test the prediction that ejection or absence of cowbird eggs reduces nesting mortality, I compared thrasher nesting success to that of a sympatric accepter, the Abert's Towhee (Pipilo aberti).

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    Finch, Deborah M. 1982. Rejection of cowbird eggs by crissal thrashers. The Auk. 99: 719-724.


    Brown-headed Cowbird, Molothrus ater obscurus, Crissal Thrasher, Toxostoma dorsale, Abert's Towhee, Pipilo aberti, egg rejection

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