EffEcts of ExpErimEntal cowbird rEmovals on brood parasitism and nEst prEdation in a Grassland sonGbird

1E-mail: bsanderc@ksu��edu 2Present address: Department of Biological Sciences, Virginia Polytechnic Institute and State University, 2125 Derring Hall, Blacksburg, Virginia 24061, USA�� 3Present address: Tetra Tech EC, 1750 SW Harbor Way, Portland, Oregon 97201, USA�� Abstract��—Brood parasitism and predation are t�o factors that limit seasonal fecundity in grassland songbirds�� We removed Bro�n-headed Co�birds (Molothrus ater) in a s�itchback experiment to examine the effects of brood parasitism and nest predation on the productivity of Dickcissels (Spiza americana)�� Nesting Dickcissels �ere monitored at four study plots in northeast Kansas in a t�o-year study�� Bro�n-headed Co�birds �ere captured �ith drop-in traps at t�o removal plots, t�o unmanipulated plots �ere reference plots, and treatments �ere reversed bet�een years�� To evaluate the effect of Bro�n-headed Co�bird removals, �e compared the percentage of nests parasitized, rates of multiple parasitism, clutch size, daily nest survival rates, and overall productivity per nest bet�een removal and reference plots�� Removals of Bro�n-headed Co�birds successfully reduced the probability of parasitism and rates of multiple parasitism, but only in one of t�o years�� Bro�n-headed Co�birds did not appear to contribute to nest losses, given that fe� nests �ere abandoned because of co�bird activity and that the probabilities of nest parasitism and nest survival declined simultaneously over the breeding season�� Overall, nest productivity sho�ed no difference bet�een treatments in either year, despite reduced rates of parasitism at removal plots in 2004�� High rates of nest predation minimized the potential benefits of Bro�n-headed Co�bird removals for increasing productivity of Dickcissels�� Our results demonstrate that removals can reduce parasitism levels but that the success of removal programs may vary annually, particularly in regions �here Bro�n-headed Co�birds and nest predators are abundant�� Management actions that minimize parasitism and predation by modifying habitat structure may provide better alternatives to programs based on removals�� Received 5 August 2006, accepted 25 February 2008.