Brown-headed cowbirds are brood parasitic and do not build their own nests.
In late spring, the female cowbird looks for the nests of other birds in which to deposit her eggs. She typically removes
one or more of the existing host species eggs from the nest and lays her own eggs in their place. Some host species, including
bluebirds, will attempt to raise the young cowbird(s) as their own.
The problem is that cowbird eggs hatch at least
a day before host species eggs and cowbird nestlings are considerably larger than host species nestlings. The presence of
cowbird nestlings physically shield the host female adult bird from properly incubating her own eggs which may cause some
or all of them not to hatch. The larger cowbird nestling(s) gain a huge competitive advantage during feeding which deprives
the host species nestlings of critical nourishment. Often the host species nestlings die in the nest from lack of food.
Since
cowbird eggs hatch before host species eggs, the host species adult female curtails or suspends her incubation efforts and
concentrates on feeding the cowbird nestlings. This premature transition often dooms the unhatched host species eggs. If the
host species eggs do hatch, those nestlings will not get equal portions of food.
U. S. Fish and Wildlife Service Region
4 (Atlanta, GA) which covers North Carolina has interpreted Depredation Order (50 CFR 21.43) to mean you do not
need a federal permit to control cowbirds including removing cowbird eggs from host species nests provided you fully
comply with relevant state laws. The North Carolina Wildlife Resource Commission does not require
a permit to control cowbirds including removing cowbird eggs from host species nests. The key criteria for the application
of 50 CFR 21.43 is that cowbirds must be about to or have committed depredation on wildlife. Removing a host species
egg or eggs and depositing its own egg or eggs has been interpreted by USFWS to constitute evidence of depredation
on wildlife by a cowbird. The Depredation Order does not permit possessing or salvaging the
removed cowbird eggs.
Any questions regarding 50 CFR 21.43 and its application within Region 4 should be directed to Carmen Simonton (404-679-7049).
Any questions regarding North Carolina's requirements for cowbird control should be directed to Tammy Minchew (919-707-0391).
It is vitally important to monitor bluebird nests early and often throughout
each nesting cycle of the nesting season and take corrective action as needed.
