BROOD PARASITISM BY BROWN-HEADED COWBIRDS ON BROWN THRASHERS - FREQUENCY AND RATES OF REJECTION

Rates of brood parasitism by Brown-headed Cowbirds Molothrus ater on n ests of Brown Thrashers Toxostoma rufum in south-central North Dakota ranged from 3% to 18% from 1984-1986. The average observed rate of par asitism, 12%, was higher than typically reported for Brown Thrashers. We conducted an experiment to determine if these high observed rates o f parasitism were a result of unusually high rates of parasitism by co wbirds or unusually low rates of rejection by thrashers. Brown Thrashe rs removed 58% of experimentally-placed cowbird eggs. This rate of rej ection is significantly lower than that previously reported for Brown Thrashers and for other rejector species. We conclude that thrashers i n this area are parasitized at a high rate, but that low rates of reje ction are at least partially responsible for the high observed rate of parasitism. These results suggest that the classification of Brown Th rashers as a rejector species should be reevaluated and that further s tudy of geographic Variation in cowbird host behavior and the role of learning in egg recognition is warranted.