Behavioral differences among sex and age classes of the brown-headed cowbird and their relation to the efficacy of a control program

Published source details Rothstein S.I., Verner J., Stevens E. & Ritter L.V. (1987) Behavioral differences among sex and age classes of the brown-headed cowbird and their relation to the efficacy of a control program. The Wilson Bulletin, 99, 322-337.
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Summary

Ranging studies of brown-headed cowbirds Molothrus ater (an obligate brood parasite) in the Sierra Nevada, California, USA, suggest that they commute up to 7 km to specific feeding sites, where they congregate in large numbers. This study investigated the effectiveness of removing cowbirds from an area of the Sierra National Forest by trapping at two such sites, Wishon Lakes Pack Station and the nearby Camp El-O-Win horse corrals.

Between 12 June and 7 July 1981, cowbirds were captured at Wishon Lakes Pack Station with Potter traps and, to a lesser extent, a large (c. 1.5 m²) decoy trap baited with wild bird seed. Traps were kept open and baited on 19 days (on average, for over 5 hours/day): 12–17 June, 19–26 June, 29 June–2 July and 7 July. Trapping was also carried out at Camp El-O-Win on 26 June (after horses were brought to the site), 1 July and 7 July (for less than 10 hours in total).

From 29 May to 9 July, numbers of cowbirds visiting Wishon Lakes were monitored during 5-minute spot counts. In addition, surveys were carried out (by two observers) in 15 meadows within 7 km of the pack station during 2–11 June (i.e. immediately before the start of cowbird removal) and 29 June–8 July.

Prior to trapping, numbers of cowbirds visiting Wishon Lakes reached daily peaks of 47–60 individuals during 2–11 June. From 12 June to 7 July, 125 cowbirds (96 males, 29 females) were captured at the site, and peak numbers visiting fell to 16 individuals by 6–9 July. An additional 10 individuals (5 males, 5 females) were trapped at Camp El-O-Win between 26 June and 7 July.

Between 2 and 11 June, two observers recorded 36 (33 male, 3 female) and 47 (38 male, 9 female) cowbirds during surveys in the meadows surrounding Wishon Lakes. Between 29 June and 8 July (i.e. during cowbird control), the same observers recorded 13 (9 male, 4 female) and 23 (22 male, 1 female) individuals respectively, suggesting that cowbird numbers (particularly males) declined between the two periods.

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