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May 2020 Natural Mystery Answered

posted Jun 9, 2020, 7:46 PM by Jonathan Poppele   [ updated Jun 9, 2020, 7:48 PM ]
The all new Peterson Field Guide to North American Bird Nests by Casey McFarland, Matthew Monjello, and David Moskowitz is in press. But its release date is still nearly a year away. That didn't stop Cheri Stockinger, Kirsten Welge, Liz Jaeger, Matthew Johnson and Mike Holtz from correctly identifying the species that laid these eggs and sharing some insights into the color and why these particular eggs are so dark. An honorable mention also goes out to Maren Miller who made a compelling case for another species. Congratulations to all of you!

As Matthew Johnson notes for us, these are the “robin's egg blue” eggs of an American Robin, Turdus migratorius.

Liz Jaeger summarizes the features here that are characteristic of American Robin.

“The nest has some mud on the exterior and a light lining of grass on the interior. Eggs are smooth, glossy, and oval, and have a relatively consistent color (i.e. not mottled or speckled).”

American Robins are one of a handful of songbirds nesting in Minnesota that can lay smooth, glossy, unmarked, blue-green eggs. Others include the Eastern Bluebird, European Starling, Gray Catbird, and Blue Jay.

Mike Holtz notes that the “nest structure doesn't fit bluebirds or starlings.” As Matthew Johnson noted about Eastern Bluebirds, and Kirsten Welge pointed out about European Starlings, both are cavity nesters and neither use mud in nest construction. Neither do Blue Jays.

That eliminates all the candidates except for the Gray Catbird. And Maren Miller made a compelling case for this being a Catbird nest:

"Gray Catbirds are known to lay dark green-blue eggs, approximately 1-5ish per clutch, that measure approximately 1 inch in length. Obviously, these eggs are dark green-blue (not light blue, like a Bluebird's or an Egret's; and not overtly speckled, like a House Finch's); and the clutch size of 3 eggs is certainly reasonable. Although we don't know how long each egg is, we can consider them in relation to the height of the nest. Gray Catbird nests are generally about 2 inches tall, meaning that we could stack two 1 inch Grey Catbird eggs on top of one another lengthwise to hit the top of the nest. When I "eyeball" the eggs in relation to the nest, it looks as though two eggs stacked on top of one another lengthwise would approximately equal the height of the nest—not an exact science, but again, the Grey Catbird egg hypothesis seems feasible.

Nest: If this nest doesn't belong to a Gray Catbird, a Gray Catbird should probably consider moving in here, because it's just about ideal for this bird—from the materials used to build the nest, to the nest's structure, to its immediate location and surrounding habitat. Gray Catbirds build nests with an eclectic array of materials, including twigs, straw, mud, and reeds, and the interior of their nests are often tightly woven—all of which we see in this photo. Gray Catbirds also tend to build their nest in "shrubby" locations that are close to the ground (unlike Robins, who prefer leafy trees), and the photo seems to document a brush-y habitat. Finally, Gray Catbirds are a pretty common species of bird on/around Picnic Island, so it would certainly make sense to see a GC nest with eggs in this location."

Mike Holtz
says he thinks “catbird eggs would be even darker, but I'm not sure that's enough to discredit this analysis.

As it happens, this nest was not in dense shrub, but on the horizontal limb of maple with a large wild grape vine growing along side—though that is not evident from the photo. I have been researching the nests of these two species, and haven't found anything in my study to eliminate Gray Catbird as a candidate based only on this photo. I'd love to hear from you if you have insights on this.  What I can offer is verification that this is a robin's nest—as I watched the mother return shortly after I took the photo of the eggs.

Maren also had some good insights as for why these eggs are such a dark color, explaining:

"I once read that certain birds' eggs appear green-blue because of a green-blue bile substance called biliverdin that is present in the female. A darker green-blue eggshell suggests that there is more biliverdin in the female; and biliverdin is also an antioxidant, so darker green-blue eggs indicate that the female and her brood are healthy. Male Catbirds seem to know that darker eggs are healthier eggs, which works in the female's favor: males are more inclined to stick around and help the female raise their chicks if they sense those chicks and the female bird are healthy."

Mike Holtz added to this “The eggs could be a darker blue in a first brood, when levels of biliverdin are higher in the female.

All spot on. The blue-green color in the eggs of these birds comes from biliverdin which, as Matthew Johnson pointed out, is derived from hemoglobin. Levels of biliverdin are higher in healthy females, and tend to fall off with each successive brood over the summer.

Congratulations again to Cheri, Kirsten, Liz, Matthew and Mike Holtz for correctly identifying these eggs, and another shout-out to Maren for all the great insights.

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