Not all birds make good mothers.

In what is called brood parasitism, some females will lay their eggs in another bird’s nest, relinquishing all parental care of their offspring to the host female. Rearing a parasitic nestling, however, reduces the chances of survival for the host bird’s offspring. The best thing for a bird to do is remove the foreign egg before it hatches.

But how do birds identify an egg that’s not their own?

My research partner, Claudia Ki, and I were curious as to what visual cues signaled a foreign egg to female birds.

To find the answer, we placed plastic eggs resembling those of a house sparrow in the nests of 14 eastern bluebirds at our local field site. Eastern bluebirds lay eggs that are light blue-green in color. House sparrows lay eggs with brown speckling on a whitish-blue background. We isolated the colors and pattern of those eggs and created four models: whitish blue, brown, an equal ratio of both colors, and a replica of the house sparrow egg.

Each day we’d check the nest boxes at our field site. What we found is that some females rejected one or more of the model eggs, while others accepted all variations and left them in their nest. Females who rejected at least one fake egg always picked the speckled one. This led us to conclude that pattern rather than color is how birds identify parasitic eggs.

One possibility as to why a female was more likely to reject the patterned egg over the other models is due to the contrast between the speckles and the color of the egg. In the low lighting of the nest box, the contrast between the light and dark colors in a speckling pattern stand out. The other eggs might have blended with the female’s own clutch or the brown color of the nest material.

As for birds that didn’t reject the intruding eggs, they might not have noticed a difference between their own or the effort to remove them was too high.

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