Wednesday, August 18, 2010

Eastern Bluebird Egg Color

Earlier this season a neighbor up the road near Durand called to tell me about the 3 white eggs in one of her bluebird nest boxes.

Bluebird eggs are usually blue.  Tree Swallow eggs are white.

I asked:  Could they be Tree Swallow eggs?

Tree Swallow Nest

No, she said.  It's a bluebird nest.  There are no feathers lining the nest.  I've seen bluebirds going in and out of the box.

White bluebird eggs are unusual, but not unheard of.   Curious, I went over to take a look.

White Eastern Bluebird Eggs

No doubt about it.  She had white bluebird eggs.

As it turned out, this would be the first of 4 nests with white eggs this season.

That got me thinking.

Why are bluebird eggs blue?  Don't most cavity nesting birds - songbirds, Belted Kingfishers and Wood Ducks - lay white eggs?  How do bird color their eggs?

Beyond genetics, some the explanations for egg color seem obvious.

Speckled eggs makes sense for Killdeer, who nest on the ground, or Yellow Warblers who lay their eggs in open nests.  It’s more difficult for predators to spot eggs that blend in with their surroundings.   

But why would cavity nesting House Wrens, who also lay speckled eggs, need to camouflage them?

Scientists at Oxford University recently suggested a completely different explanation for speckled eggs.   They hypothesized that the speckles might increase shell strength, compensating for thinner egg shells caused by calcium deficiencies in their diet. 

The scientists looked at eggs of the Great Tit (Parus major) and found that eggs with the thinnest shells had the most speckling.  And the thinner the shell, the darker the spots.  They concluded that chemicals in the speckle pigment may work like glue, supporting weak areas of shell, protecting them from breaking.  

But what about those plain white eggs?   

There seem to be obvious advantages:  in the low light of tree cavities and tunnels, white eggs should be easier to spot; and it takes less energy to lay a plain white egg.   

The “less energy” hypothesis has to do with anatomy and biochemistry of the hen’s oviduct.   It’s complicated, but here’s the simpliflied version.  As eggs are laid, glands along the hen’s oviduct deposit pigments on top of the basic white shell, comprised primarily of calcium carbonate - a mineral found in chalk, limestone and marble. 

The amount of pigment and the timing of pigment release determine the base color of the egg and its distinctive markings (the speckle pattern).   As the egg enters the oviduct, the first pigment released creates the base color of the egg.   Blue and green are created by pigments known as the biliverdin and the zinc chelates.
According to the North American Bluebird Society, in 5-9% of reported nest observations, bluebirds laid clutches in which the eggs were a uniform, lighter color – white, pale pink or pale blue.  

It was once believed that laying white eggs was a genetic trait – like blue eyes and brown eyes in humans.  Bluebird hens that lay white eggs, will always lay white eggs.

But now there's a new theory about egg color in bluebirds:  it  may be an indication of the health of the female that triggers a higher level of paternal care from the males.  Older females and females in better body condition lay more pigmented blue-green eggs.

Perhaps the stress brought about by cold and wet spring weather had something to do with the increased incidence of white bluebird eggs in our nest boxes this season.

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