About us: We own a wild bird feeding supply nature shop in East Lansing, Michigan,
a store that provides a wide variety of supplies to help you enjoy the birdwatching hobby.

This blog was created to answer frequently asked questions & to share nature stories and photographs.
To contribute, email me at bloubird@gmail.com.

Sunday, July 28, 2019

Odd specked cream colored egg in bluebird nest

I live in Northern Minnesota and have bluebird nesting boxes. Last year, a bluebird pair were very interested in the box. All of a sudden they seemed upset and then were gone and never nested. When I opened the box, there was an odd egg in the box that apparently didn’t belong to the bluebirds. It was specked cream colored and appeared larger than a bluebird egg. Thanks for any answer you can provide. Kathy

Cowbird eggs are white to grayish-white with brown or gray spots or streaks. The Brown-Headed Cowbird is a common native bird throughout North America. The male brown-headed cowbird is glossy black with a contrasting dark brown head, while the females are dull grayish brown. A female cowbird can have several mates but instead of building their own nest, she will lay her eggs in the nests of other bird species which foster the babies. She chooses several nests of other bird species to deposit one egg.

Studies have shown that the cowbirds don’t just drop an egg and move on. The females occasionally check in on the little bundles they’ve left behind. An experienced bluebird may have recognized the egg and abandoned the nest. If the bluebird tries to remove the foreign egg, the female cowbird may return and destroy the whole nest. This tactic forces the birds to start another batch of eggs and the cowbird can then try to slip another egg into the nest. The cowbird wants the advantage of her egg hatching first so her baby will have the best chances of survival.

Another sign of a cowbirds presence is an intact egg on the ground under active nests. Female cowbirds often evict one or more of the host eggs before they lay their own. Or she may eat the egg instead or damage it and leave it in the nest.

Cowbird nestlings do not oust host eggs or young from the nest, or kill the host's chicks. However, since they tend to hatch earlier, develop faster, and crowd out or reduce the food intake of the hosts' nestlings, often only the cowbird survives to fledging. In one study with one Cowbird and two host nestlings, the Cowbird got 50% of the food. Bluebirds abandoning a nest with a cowbird egg may have been the smartest solution. Cowbirds are native, so they are protected under the Migratory Bird Treaty Act so there was nothing you could do about the egg without a permit.

For additional information about Brown-headed Cowbirds, visit this species’ profile on All About Birds.

Sources: 
1.  http://www.sciencenow
2. https://nestwatch.org/
3. http://www.sialis.org/cowbirds.htm 

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