Abstract
We investigated susceptibility of Song Sparrow (Melospiza melodia) nests to discovery by nest-searching Brown-headed Cowbirds (Molothrus ater) at Delta Marsh, Manitoba. We assessed the importance of nest habitat and microhabitat as well as host activity in influencing the likelihood of parasitism by measuring vegetation and behaviour of hosts at nests that were (1) not parasitized, (2) parasitized once and (3) parasitized more than once, by the same female or by more than one female (i.e. multiple parasitism). None of the variables measured was significantly related to parasitism, but non-significant trends consistent in both years of the study suggested that concealment and host conspicuousness play important roles, but quantifying them is difficult and complex. A significant relationship between nest mass and the probability of parasitism further supports this suggestion, because nest mass should reflect nest-building activity as a whole.
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