Tuesday, March 31, 2015

Why are Robins’ Eggs Blue?

It’s finally spring in Massachusetts, even if it doesn’t feel like it just yet, and many people associate the beginning of spring with robins returning back to the northeast from their annual migration. Once the robins have had their fill of worms in the beginning of spring (up to 13 worms in one day!), they will begin nesting in late April through May and eventually lay their notorious bright blue eggs. But why are there eggs so blue? It seems somewhat counterproductive to the species, as one would imagine that bright blue eggs would stand out against the brown nest and surrounding earthy tones, and that predators could spot the eggs in an instant.

Surprisingly though, robins have evolved this attribute as a survival strategy due to intense sexual selection. A female robin lays blue eggs as a health indicator, meaning that a female with very vivid and brightly colored eggs will be a strong and healthy female. The male robin can be sure of this because in order to protect these conspicuous colored eggs, the female must be strong and healthy. Of course, this adaptation is not only based on the female’s health status. A male robin will instinctively be more drawn to the brightest and the bluest eggs, encouraging him to take better care of them.  If the eggs are not bright blue, the male will subconsciously not put enough effort into being attentive to the nest. This could be because the energy that he would put in to taking care of a nest with an “unfit” mother may be too great, and the chance of success for the young to fledge (and eventually reproduce and pass on the father’s genes) may be too low. Likewise, if the male does not have the gene that causes him to care more efficiently for bright blue eggs, the mother will have to pick up the slack, and the chance of success for the nest plummets.

Generally, when the eggs are hatched, the male will take care of the chicks while the female builds another nest and lays more eggs, those of which are hopefully just as vibrant as the first. Since taking care of multiple nests with eggs that are so attractive to predators is a two-bird job, the vibrant colored eggs encourage cooperation between the pair, and encourages the parents to stick together all season and continue to reproduce. The benefits of having blue eggs out way the risks, and therefor leads to an overall accomplishment for the ultimate goal, which is successfully passing on genes.

Erika Nevins (Group B)


  1. Great post! This is such a cool evolutionary way for both parents to care about their offspring. I think that its really interesting that it makes both the male and the female work together in order to have the most young survive. I also think that it is really interesting that the birds can see the differences between different shades of blue, its not something I realized they could do.

    -Madison Boone

  2. Evolution is so cool! This is a really great example of how evolution factors into parental care. I wonder if any other egg-laying species rely on similar health-indicators? Nice post, it was a very interesting read and thought to consider!

    -Michael Salhany

  3. This is a very interesting post! Exactly as you mentioned above, when I glanced at the picture of the blue eggs in the nest, I totally thought that the parents might lose those eggs very easily if they don't pay enough attention to them. However, your explanation is very clear and makes a lot of sense. Since it's explained above that natural selection has acted on the blue color of the eggs to keep the parents together and induce reproduction, do you think that fecundity selection is favored over viability selection in this species?

    -Phi Duong