干旱区生态环境知识资源中心

The incidence of Brown-headed Cowbird (Molothrus ater) parasitism of Southwestern Willow Flycatchers (Empidonax traillii extimus) is highly variable, ranging from less than 10% at some sites to over 50% at others. Parasitism usually results in complete loss of flycatcher reproductive output because most parasitized nests are deserted or fledge only a cowbird, although birds that desert often renest. Despite the reduced reproductive output from individual flycatcher nests, it is not clear that cowbird parasitism affects Southwestern Willow Flycatcher population sizes. Cowbird control reduces parasitism rates and increases the reproductive output of Southwestern Willow Flycatchers, but there is no firm evidence yet that it has resulted in any increases of flycatcher populations or forestalled declines, suggesting that populations may be limited by other factors, such as habitat. Cowbird control may nevertheless be an appropriate management option because some populations may benefit and there may be benefits that have not been detected. However, cowbird control efforts should: (a) be applied cautiously and when baseline data indicate serious impacts, because control is expensive and has a number of potentially negative aspects; (b) be geared towards critical assessments of the efficacy of the control, with increases in flycatcher population sizes being the ultimate measure of efficacy; and (c) be regarded as a short term measure, not a permanent management activity.