Does egg rejection account for the low incidence of cowbird parasitism in chestnut-collared Longspurs (Calcarius ornatus)?

Abstract We experimentally parasitized Chestnut-collared Longspur (Calcarius ornatus) nests with mimetic and nonmimetic Brown-headed Cowbird (Molothrus ater) eggs to determine whether the low frequency of parasitism reported for that grassland host is due to egg rejection behavior. Longspurs accepted all 23 mimetic eggs (real and wooden) and rejected 9 of 22 blue nonmimetic eggs. Our results suggest longspurs accept cowbird eggs even though they are physically capable of ejecting them from their nest. Thus, the low parasitism frequency reported for that species is not a result of egg rejection behavior, but may be due to antiparasite strategies that reduce their chances of being parasitized (e.g. nest defense behavior).