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So, Why is the Egg Blue?

By Jake Francis, Director of Environmental Education — There have been a number of bird species nesting in and around the Schrader Center for the last month (including red-shouldered hawks, red-bellied woodpeckers, mallards, and many more).  Recently we found hatched robin’s eggs on the ground along the Hardwood Ridge trail.

There are two interesting things going on in this photo. First, we see that the egg is bright blue (in fact this could be any thrush egg, but we know it is a robin because of the robin’s nest situated above the shell). The blue coloration of the egg is created by the deposition of the mother’s blood pigments on the egg, notice that the outer portion of the egg is blue while the inner side of the shell is not. There have been many scientists who have hypothesized about the adaptive significance of the blue coloration in thrush eggs. Some claimed that color blind mammals would be less likely to see a blue egg in a brown nest than a white egg, others believed that the blue coloration helped protect embryos from sunlight damage. Recent research suggests that perhaps the blue coloration of thrush eggs serves as a signal of their mothers’ fitness to male thrushes, causing them to invest more energy in feeding the young after they hatch. Some data supports that egg coloration correlates with the mother’s fitness in other families (e.g., Pied Flycatcher).  And, male Robins seem to feed supposed hatchlings from brighter eggs.

In this photo we can also see that an invertebrate was likely eating the remainder of the amniote of this egg (notice the trail of slug or snail excretion at the opening of the egg). This is the first instance that either I or Greg Park, Senior Naturalist, have heard of gastropods eating the left-over amniote of bird eggs. Perhaps some of our readers can lay out egg shells and try to catch a snail or slug red-handed!

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