Perch proximity correlates with higher rates of Cowbird parasitism of ground nesting Song Sparrows

The reproductive success of avian brood parasites depends, to a great exten t, on their ability to locate host nests that are at the appropriate stages of the host laying cycle. Consequently, brood parasites are expected to po ssess elaborate mechanisms and search modes to locate potential host nests. Through observing a population of song Sparrows (Melospiza melodia) parasi tized by the Brown-headed Cowbird (Molothrus ater) we examined two specific factors that may influence cowbird parasitism of a ground nesting host. Pr oximity to potential perches was a significant predictor of cowbird parasit ism, but overhead nest visibility,either classified dichotomously as visibl e or not, or measured as the absolute area of a nest visible to an observer , was not correlated with the likelihood of parasitism. Comparisons with pr evious studies suggest that female cowbirds use similar nest searching mech anisms in open habitats, irrespective of the height of host nests.