Efficacy of Trapping and Shooting in Removing Breeding Brown-Headed Cowbirds

Abstract

Trapping often is used to mitigate brood parasitism by brown-headed cowbirds (Molothrus ater). The efficacy of trapping to remove locally breeding cowbirds has not been compared to shooting, an alternative removal method. We used cluster analysis to group female cowbirds trapped and shot at Fort Hood, Texas, USA, during the 2003 and 2004 breeding seasons as potential local breeders (PLBs) or migrants based on their color, mass, ovarian development, and wing chord. The PLBs were paler and smaller, generally fitting the description of M. a. obscurus, the expected race of locally breeding cowbirds. We detected enlarged ovaries in 0% of migrants (n ¼ 1,634) and 18% of PLBs (n ¼ 959). We compared the efficiency (proportion of PLBs removed) and effectiveness (total no. of PLBs removed) of trapping and shooting. Monthly shooting efficiency was high (.90%) and always greater than that for trapping. Trapping efficiency was temporally variable but greatest in March (50%). Despite its inefficiency, trapping removed at least 8-fold more PLBs during March and April than shooting. Effectiveness of trapping and shooting was similar in May. Shooting removed 3-fold more PLBs in June than trapping. The cost of removing a single PLB by trapping in March and April was ,$4 but increased to $153 by June. Cost of removing a single PLB by shooting ranged from $14–19 during all months except March, when few breeding cowbirds were shot. Knowledge of the efficacy of cowbird removal techniques over the breeding season will allow managers to strategically apply control to enhance program benefits. (WILDLIFE SOCIETY BULLETIN 34(4):1107–1112; 2006)