The Invasive Brown-Headed Cowbird

​​​Purpose of Controlling Cowbirds

 

The combination of habitat loss and brood-parasitism in southern California has prevented willow flycatcher populations from increasing, and without management their populations may continue to decline. In southern California, cowbird parasitism has been recorded to be the most influential cause of nest failure, and because of the willow flycatchers' short breeding season they are significantly impacted by brood-parasitism. In addition, much of the willow flycatcher and Least Bell's Vireo habitat has been converted into agricultural areas.  In these areas, when livestock overgraze it results in damage of the understory vegetation which the willow flycatcher relies on. It also results in less moisture retention in the area destroying the suitable habitat. Both the Least Bell's Vireo and willow flycatcher rely on riparian habitat which is now often surrounded by agricultural areas. Since the presence of livestock and grazing ungulates provides the brown-headed cowbird with favorable habitat, our management plan focuses on reducing the continuous presence of large groups of ungulates near riparian areas in combination with cowbird trapping, which has shown to have positive effects on local levels.

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Goals

1. Significantly reduce cowbird population through cowbird trapping

2. Protect Least Bell's Vireo and willow flycatcher by reducing presence and abundance of cowbirds in riparian habitat

3. Manage cowbird population and endangered species population with a successful long-term plan

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Problems with Historic Methods

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Trapping and killing cowbirds is a temporary solution, and does not significantly prevent parasitism in large-scale areas and does not protect California's endangered birds long-term. Our plan seeks to combine a short-term management plan of cowbird-trapping, with a long-term management plan in which habitat suitability is maintained for the willow flycatcher and Least Bell's Vireo, and cowbird abundance in riparian habitats is reduced.

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Management Methods
 

​​​​​​Habitat and Livestock Management:

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Much of the land in the western United States is public land owned by the federal government. Approximately 175 million hectares is managed by the Bureau of Land Management (BLM) and the U.S. Forest Service (FS). Of this land, approximately two-thirds is grazed, and the BLM is responsible for managing the grazing. Therefore, our livestock management plan addresses public land rather than private land. Our goal is to cooperate with the BLM to coordinate rotational grazing and the Natural Resources Conservation Service to establish and manage buffer zones around the riparian habitat. 

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Brown-headed cowbirds have been found to be most abundant along edges of riparian habitat and in feeding areas. There has also been evidence that shows cowbirds may select habitats based more on favorable feeding areas than host abundance. Therefore a thick buffer zone of about 1 km should be maintained around the natural and undisturbed habitat of the willow flycatcher and Least Bell’s Vireo on both sides of the stream. The managed buffer zone should consist of a canopy produced by tall and medium trees, and layers of shrubs and vegetation to reduce the ease of accessibility to host nests and reduce cowbird parasitism.

 

In addition, livestock and horses should be managed using a rotational grazing system. Rotational grazing of pastures will allow recovery of vegetation, increased pasture productivity, healthier livestock, improved water quality, and overall community improvement, as well as economic benefits. Rotational grazing will prompt the cowbirds to move along with the livestock rather than remaining in an area near the riparian habitat. The combination of a buffer zone and rotational grazing will keep livestock a sufficient distance away from host species to significantly reduce cowbird parasitism.

Livestock and horses should also be prevented from grazing in certain areas during cowbird breeding seasons (April to July), and should remain in other areas of the pasture for the duration of that time. A distance of 2-3 km away from edges of the buffer zone barrier should be sufficient to reduce cowbird parasitism. Despite evidence that shows commuting distances up to 7 km by some cowbirds, majority of breeding-feeding flights are often less than 3 km. Length and time of breeding season may vary depending on location and should be adjusted accordingly. Following the breeding season grazing ungulates may be permitted to re-enter the area while maintaining a rotational grazing system.

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The livestock and habitat management plan is expected to significantly reduce cowbird parasitism of host nests because they are less likely to commute the further distance to the nests, and will also have less success finding the nests through the dense canopy buffer zone. Additionally the cowbirds will be located in pastures furthest from the riparian habitat specifically during breeding season, resulting in increased productivity for the Least Bell's Vireo and southwestern willow flycatcher.

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Cowbird Trapping:

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In combination with livestock management methods and a vegetation buffer zone, cowbird trapping will be utilized to reduce cowbird abundance. Traps will contain approximately 13 decoy brown-headed cowbirds and will include a small opening in an upper corner to remove cowbirds for killing via cervical dislocation or euthanasia via carbon monoxide off-site. Any species other than the brown-headed cowbird that are accidentally caught will be released immediately. Traps will be set up in open pastures near the livestock and also near the edges of the buffer zones in low grass areas. Since cowbirds are drawn to the calls and presence of the decoy birds, as well as the ungulates, the traps are expected to significantly reduce the number of brown-headed cowbirds and thus parasitism of host species' nests. 

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