The eastern bluebird lays the smallest blue egg – approximately 4/5 of an inch – of all the songbirds that lay blue eggs. The eggs of western and mountain bluebirds are slightly larger than the eastern bluebirds, but smaller than the eggs of robins, blue jays, starlings, thrushes and catbirds.
It's rare to find bluebird eggs in an open nest, as bluebirds typically build their nests in a cavity. Blue eggs are also commonly laid by sea-dwelling birds. Larger, blue-green eggs are laid by great blue herons, little blue herons and snowy egrets and great egrets. Cormorants, skimmers and common murres lay eggs that range in color from blue to ivory.