I just saw something on Nova Now that is so interesting I had to tell someone.
You are familiar with Cowbirds, I’m sure. They lay their eggs in other birds’ nests and the other birds raise their babies. This is getting to be a problem because the baby Cowbird is a big little guy and if the foster parents are small birds, they spend all their time and energy satisfying the large dominant bird and their own babies starve. Some species of small song birds are dangerously diminished in number because of Cowbirds.
A researcher noticed that a female Cowbird was frequently near a nest where she had laid an egg, and seemed to be watching the nest, but thought that was preposterous. Later he decided to remove the cowbird eggs from the nests of the birds he was studying so their own offspring would have a better chance. Then a strange thing happened. The nests that he had removed the Cowbird eggs from were trashed. The eggs were broken and the nest was torn up. Impossible as it might seem, it looked like vengeance for the missing cowbird eggs.
So he set up an experiment. He made many nest boxes with holes big enough to allow a Cowbird in, making them inaccessible to snakes, squirrels, and other possible culprits. When Cowbirds laid eggs in the nests, he removed them. Then, on half the nests, he slid in an insert that made the hole too small for a Cowbird.
All the nests that had Cowbird eggs removed that were still accessible were trashed. The ones that were not accessible were not. He mounted cameras and actually filmed mother Cowbirds tearing up the nests and breaking the eggs.
This gives a whole new dimension to why the foster families accept the baby cowbirds. They have learned that if they do not raise the intruder, there will be disastrous consequences. Cowbirds are not lazy bums, they are little winged mobsters intimidating other birds into raising their young.
I hope researchers and others who build nest boxes to attract certain kinds of birds start making the entry holes as small as they can be to allow the species they want using the nest.