Abnormal Bluebird Eggs

When I took this photo a few weeks ago, I didn't notice anything out of the ordinary.  I was happy to see 5 Eastern Bluebird eggs in a nest box that had not been productive this season. 

I didn't take them out of the nest box for a close inspection.

Next time I looked, 3 had hatched.   If the other 2 eggs were going to hatch, it would happen soon.

Two days later,  I went back to check on them.  The chicks looked good, but the eggs were cold.   That's when I took them out for a closer look.

"normal" bluebird egg and 2 "runt" eggs

While I'd never seen one before, I knew right away one of the eggs was a "runt."  It was round and about half the size of a normal bluebird egg.  I didn't realize the other egg was abnormally small (by 25%) until I compared it with a "normal" bluebird egg.  Both had tiny yellow yolks - but no sign of an embryo.

What causes dwarfed eggs?

Scientists don't know for sure.

But "runt" eggs are not as unusual as I thought.  The Western Foundation of Vertebrate Zoology has more than 1,000 in their collection.  And "runt" eggs have been found in most all species - not just bluebirds.

While they may not "know" much about them, scientists have correlated the occurrence of "runts" with the age of the female (younger birds are more likely to lay them) and a temporary impairment of the reproductive tract.

They don't believe it's an inherited trait, despite the 1983 report by my old friend Larry Zeleny (founder of the North American Bluebird Society).  Larry observed a banded hen in that laid 4 runt eggs in 3 separate clutches in one season.

Next time I find an abnormal egg, I'll contact the NABS to find out who might want to study it.