Dorothy is a Master Gardener, former newspaper reporter, and the author of several books. Michael is a landscape/nature photographer in NM.
Is This the Devil of Birds?
They Put Their Energy Into Producing Eggs
When photographs of cowbirds are posted on the web, people respond by saying things like: "I hate those birds!" or "I have tried to run them off from my backyard." They post such comments because of the female cowbird's unique way of raising her young, which is to not raise them at all.
Rather than spending any time building a nest for her eggs, the female cowbird puts all of her energy into producing eggs and does so at the rate of up to three dozen in a single summer. When she is ready to lay her eggs, she begins to look for nests that other birds have constructed (or have already constructed). When the host bird is not at the nest, the female cowbird slips in and pushes one of the eggs out of the nest and replaces it with her own, referred to as "nest parasitism". The baby cowbird is often raised by the unsuspecting host bird, who becomes a foster parent through no fault of her own.
Should the host bird destroy the cowbird's egg, the cowbird will often retaliate by destroying the entire nest in what many people have referred to as "mafia behavior."
"Chester" the One-Legged Cowbird
How to Deter Cowbirds
To deter cowbirds, only use feeders made for smaller birds, such as tube feeders with short perches and smaller ports (with no catch basin at the bottom of the feeder). Stay away from platform trays and NEVER spread food on the ground.
Put out the food that cowbirds don't prefer, such as suet, whole peanuts or safflower seeds. Steer clear of the things they do prefer, like sunflower seeds, cracked corn, and millet. Always clean up any feed that spills over to the ground below your feeders.
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Cowbirds are native to the United States and as such are protected under the Migratory Bird Treaty Act. It is unlawful to use lethal control without a permit (in most instances), which includes the removal of their eggs from a nest. There are occasions, however, when unpermitted control of cowbirds can be permissible (under special circumstances that are outlined in the act). In Michigan and Texas, you can obtain permits to trap the cowbirds if they are threatening the survival of endangered species like Kirtland’s warbler, golden-cheeked warbler, and black-capped vireo.
Host Species Baby Birds Suffer
When the unsuspecting foster parent bird raises the young cowbird, it is usually done at the expense of her own babies. The cowbird eggs require a shorter period of incubation than most other songbirds, so they usually hatch first and grow large very quickly, allowing them to get the most food from the parent birds. The nesting success of the host species is significantly lowered because of the actions of the cowbird hatchling.
Because the cowbird doesn't depend on a single host species exclusively, the impact of the actions is spread across many populations. Over 200 different species of North American birds are known to have been affected when their own eggs are tossed out of the nest, only to be replaced by the egg of a cowbird.
There are bird species that recognize that an egg is not their own, such as the yellow warbler. Those birds will either remove the egg from the nest or simply build another nest right on top of the cowbird's egg. Some people think that the birds that do accept the eggs and raise the hatchling as their own are simply doing it for self-preservation because of the cowbird's reputation for the destruction of any nest that rejects their egg.
There Are Other Egg Dumpers!
Apart from the New World cowbirds, avian brood parasites include Old World cuckoos, some African finches, African and Asian honeyguides, and the South American black-headed duck. Opportunistic egg-dumping occurs among swallows, waterfowl, and others, but these guys (cowbirds) are pros.
— Joe Eaton, in an article written in 2007 in The Berkeley Daily Planet newspaper
They Began Acting Out of Necessity? Maybe
One theory (the one that is most accepted) is that cowbirds may not have malicious intent on their minds when they expect other species of birds to raise their young. They may have begun acting out of a natural necessity to ensure the survival of their own species. At one time, these birds followed the massive herds of bison across North America. They were able to feed on the insects that were kicked up by the huge bison's hooves, and also on the seeds that were displaced by the hooves.
The bison herds were constantly on the move, so the cowbirds had to follow. If they had stayed in one place long enough to raise their young, they would have perished so they altered their strategy of breeding. When the bison herds disappeared, the cowbirds adapted by following domestic cattle that had inherited the range from the previous herds that were no more.
Another theory is that they were only able to follow the bison herds because their breeding strategy, which already existed, allowed them the freedom to do so.
No one can say with any certainty if either of these theories holds water.
- http://www.heraldcourier.com/community/birds-have-many-different-ways-of-raising-their-young (Retrieved from website on 5/8/2018)