The Brown-headed Cowbird is a small migratory bird species that parasitizes the nests of other bird species by laying its eggs in their nests. This can be problematic for the host species as the cowbird chicks often outcompete the host’s chicks for food and resources. Many people want to know how to keep Brown-headed Cowbirds away from their backyard bird feeders and nesting boxes in order to protect other bird species. There are a few methods that can be effective for deterring cowbirds.
Why are Brown-headed Cowbirds a problem?
Brown-headed Cowbirds are considered “brood parasites”, meaning they lay their eggs in the nests of other bird species, leaving those host species to raise the cowbird chicks. Cowbird chicks often hatch earlier and grow faster than the host chicks, monopolizing food resources and causing starvation or premature fledging of the host chicks. Their parasitism has contributed to population declines in many songbird species.
Which bird species are impacted?
Brown-headed Cowbirds primarily target smaller passerines that build open, cup-shaped nests such as warblers, vireos, flycatchers, and sparrows. Some vulnerable host species include the Kirtland’s Warbler, Black-capped Vireo, Southwestern Willow Flycatcher, Least Bell’s Vireo, and Dickcissels. Any small songbird is potential target for cowbird parasitism. Larger cavity nesting species like woodpeckers may also be parasitized. Over 220 North American bird species have been documented as hosts.
Methods to Deter Brown-headed Cowbirds
There are a few different tactics that can be used to try and deter Brown-headed Cowbirds from utilizing backyard bird feeders and nest boxes:
Use Cowbird-resistant Nest Boxes
One of the most effective ways to prevent cowbird parasitism at nest boxes is to use specially designed “cowbird-resistant” nest boxes. These boxes have a smaller entrance hole (1 1/8 inches) that allows smaller songbirds access but restricts larger cowbirds from entering and parasitizing the nest. Restricting access prevents cowbirds from laying eggs.
Use Exclusion Devices
Another deterrent option is to place a metal cowbird exclusion device over or around the entrance hole of nest boxes. These devices have slots that songbirds can slip through but cowbirds cannot fit their larger body size through. This blocks cowbirds from entering the box.
Relocate Bird Feeders
Brown-headed cowbirds often congregate near bird feeders where they can find potential host bird species vulnerable to parasitism. Simply relocating bird feeders away from known nesting sites can help reduce cowbird activity and nest parasitism rates in the area. Move feeders at least 300 feet away from nest boxes.
Use Life-sized Predator Decoys
Cowbirds seem to avoid areas where predators are perceived to be active. Placing life-sized plastic decoys of predatory hawks or owls near nesting areas may frighten cowbirds away and prevent them from laying eggs. The decoys are most effective when moved around occasionally so they don’t become ignored.
Remove Cowbird Eggs
For nest boxes or birdhouses already parasitized, removing cowbird eggs promptly after they are laid can allow the host birds to successfully raise their own chicks. However, you must check nests daily for cowbird eggs which often requires a special permit.
Limit Availability of Preferred Foods
Cowbirds have a strong preference for white millet. Eliminating this seed from bird feeders may discourage cowbird visitation. Offer alternative seeds like nyjer, safflower, sunflower chips, peanuts, and suet instead. Discourage cowbirds from sticking around.
When are Cowbirds Most Problematic?
The risk and rates of cowbird parasitism are highest at specific times of year. Understanding seasonal patterns can help guide deterrent efforts:
Spring Migration (March – May)
Cowbird activity and parasitism increases during spring as migrants return and breeding begins. Efforts should focus on deterring scouting female cowbirds from laying eggs early in the season.
Summer Breeding Season (May – July)
Parasitism rates peak in summer when nesting and egg-laying is at its height. Deterrents are critical during this period to prevent cowbird access to active nests.
Fall Migration (August – October)
Parasitism declines but continues in the fall. Late nesting species are still at risk as cowbirds join flocks preparing for southern migration.
Signs of Cowbird Parasitism
Being able to identify the signs of cowbird activity can help reveal parasitism risks and issues:
Cowbirds Present
Simply observing cowbirds actively using feeders and scouting in nesting areas indicates an elevated threat level that should be addressed.
Multiple Eggs in Nest
The presence of more than 4-5 eggs in songbird nests often signals a cowbird has laid an egg. Cowbird eggs may be slightly different in size or color.
Fast Nestling Development
Cowbird chicks develop more rapidly than host species young. A large, loudly vocalizing chick in a nest likely indicates a cowbird chick monopolizing resources.
Dead Host Young
Cowbird parasitism frequently leads to starvation or ejection of smaller host young. Dead chicks around a nest are a red flag.
Preventing Cowbird Parasitism in Specific Bird Species
Extra efforts may be needed to protect particularly vulnerable bird species from cowbird parasitism using the following methods:
Kirtland’s Warbler
– Use jack pine forest habitat management to limit suitability for cowbirds
– Trap and remove cowbirds from warbler breeding grounds
– Monitor nests closely and remove cowbird eggs promptly
Black-capped Vireo
– Create shrubland habitat with dense vegetation that impedes cowbird movement
– Install vireo-sized nest box entrances and exclusion devices
– Remove cowbird eggs from nests
– Use decoy traps to capture female cowbirds
Southwestern Willow Flycatcher
– Conserve dense willow and cottonwood riparian nesting habitat
– Monitor nests weekly and remove cowbird eggs
– Remove grazing from riparian areas to discourage cowbirds
Golden-winged Warbler
– Manage breeding habitat for shrubland and young forest beneficial to warblers over cowbirds
– Install small entrance nest boxes cowbirds can’t access
– Eliminate cowbird feeding and perching sites near warbler nesting habitat
Dickcissel
– Convert overgrown fields to shorter, sparse vegetation where cowbirds are less likely to forage
– Mow or graze nesting fields after dickcissel breeding season to detract cowbirds
– Use audio playback of dickcissel vocalizations to distract cowbirds from locating nests
Wood Thrush
– Maintain shady, dense understory vegetation that impedes cowbird movement
– Add a cowbird exclusion device to nest boxes
– Remove cowbird eggs promptly after parasitism
– Use plastic predator decoy in nesting site to deter cowbirds
Habitat Modifications to Deter Cowbirds
Altering habitat structure and vegetation patterns in yards and natural spaces can make an area less suitable and attractive for nest parasitizing cowbirds:
Remove Perching Sites
Cowbirds often perch on elevated dead snags, poles, fences, etc. to survey for potential host nests. Eliminating these perches can deter cowbirds.
Increase Understory Density
Thick, dense shrub and vegetation layers make it harder for cowbirds to move through habitat and find host nests to parasitize.
Reduce Edge Habitat
Cowbirds thrive along habitat ecotones like forest edges and riparian corridors. Minimize these transitional edge areas in the landscape.
Eliminate Open Ground
Bare, open ground provides ideal cowbird feeding habitat. Reduce open areas by planting ground cover vegetation, leaf litter, etc.
Convert to Native Plants
Non-native plants tend to create simpler, more open habitat structure beneficial to cowbirds. Restoring native plant diversity creates complexity.
Landscape Management Strategies to Deter Cowbirds
Several natural landscape management practices can help create conditions less favorable to nest parasitizing Brown-headed Cowbirds:
Tree Thinning
Selective cutting and thinning of overstocked forest reduces suitability for cowbirds which prefer forest edge habitat.
Prescribed Burning
Use of controlled burns can alter vegetation in ways that discourage cowbirds by removing groundcover, perches, and woody shrubs.
Mowing & Brush Cutting
Regular mowing or mechanical cutting of dense shrub and grassland eliminates protective nesting cover used by species vulnerable to cowbird parasitism.
Livestock Grazing
Targeted rotational grazing can be used to remove vegetation structure and create more open ground less favorable to nesting cowbird hosts.
Pesticide Restrictions
Certain pesticides reduce insect availability which may limit cowbird food resources and ability to feed young. Usage restrictions can be beneficial.
Coyote Control
Reducing coyote control efforts allows this cowbird predator to help naturally limit local cowbird populations and parasitism pressure.
Chemical Cowbird Repellents
No pesticides or chemical products are currently registered specifically for repelling or controlling Brown-headed Cowbirds. Some general bird repellents may have minor effects:
Methyl Anthranilate
Formulated as a liquid or gel and applied to surfaces, methyl anthranilate smells unpleasant to deter birds. Impact on cowbirds is limited.
Avitrol
Causes distress and flock alarm in birds that ingest it. Mixed responses in cowbirds, often just redistributes problem.
Fogging Agents
Propane cannons or other devices used to create dense fog may temporarily deter cowbirds but effectiveness is highly variable.
Ultrasonic Devices
Speakers broadcasting ultrasonic noise have not demonstrated reliable repellency for cowbirds or most problem bird species.
Summary
In summary, Brown-headed Cowbird parasitism can be a significant threat to populations of many songbird species. There are number of tactics that can be implemented to help deter cowbirds from utilizing backyard bird feeders and nest boxes:
– Install cowbird-resistant nest boxes with small entrances or exclusion devices
– Relocate feeders away from nesting areas
– Use frightening predator decoys near nest sites
– Remove cowbird eggs promptly after parasitism events
– Limit availability of preferred cowbird foods
– Monitor for signs of cowbird activity and parasitism
– Modify habitat structure to make it less suitable for cowbirds
– Manage landscapes using grazing, burning, mowing, etc.
– Target control efforts during spring and summer when parasitism risk peaks
– Focus extra protection on particularly vulnerable host species
Implementing one or more of these exclusion, harassment, and habitat modification strategies can help successfully protect songbird nests from problematic Brown-headed Cowbird parasitism.
References
| Source 1 | All About Birds – Brown-headed Cowbird https://www.allaboutbirds.org/guide/Brown-headed_Cowbird |
| Source 2 | The Cornell Lab – Managing Cowbirds https://www.birds.cornell.edu/home/managing-cowbirds/ |
| Source 3 | The Old Farmer’s Almanac – Deterring Cowbirds https://www.almanac.com/deterring-cowbirds-brood-parasites |
