Brown-headed Cowbird (Molothrus ater) parasitism is recognized as a threat to the recovery of many federally listed songbirds. As a result, millions of dollars are spent each year on cowbird control, sometimes in areas that don’t need control. Improving the efficiency of cowbird control can free up scarce conservation dollars for other projects. One possible way to increase efficiency is developing the ability to predict parasitism rates of target species using the abundance of other avian species. With this in mind, we used a 13-year dataset to investigate the relationship between Southwestern Willow Flycatcher (Empidonax traillii extimus, SWFL) nest parasitism and local avian community composition. We hypothesized that parasitism would decrease with lower numbers of cowbird females, and with higher numbers of preferred hosts. In contrast, if cowbirds are attracted to areas with a higher abundance of host species, this could increase cowbird parasitism. Our top model showed that SWFL parasitism rates significantly decreased with decreasing cowbird female abundance and when preferred hosts were more abundant. In addition, SWFL parasitism rates also tended to decrease with increased numbers of other known host species. This approach could be used to decide whether an area should receive cowbird control in a particular year.

MARY J. WHITFIELD is the founder and Research Director at the Southern Sierra Research Station. She holds a B.S. degree in Wildlife and Fisheries Biology from U.C. Davis, and a M.S. degree from CSU, Chico. She has over 35 years of fieldwork experience in the U.S. as well as numerous Latin American countries including work on wintering Willow Flycatcher ecology. Her research interests include conservation biology, breeding ecology, behavioral ecology, full lifecycle research, and riparian systems. She has been involved with the Partners In Flight Western Working Group for over 15 years and is currently leading the PIF Western Motus Initiative.