According to Sialis, Eastern bluebirds and robins both commonly have blue eggs. Robin eggs are larger, often quarter-sized and paler blue, while Eastern bluebird eggs are deeper blue and smaller in size, closer to the diameter of a dime. However, many birds have variety in egg coloration and occasionally lay blue eggs.
Mountain bluebirds, for example, commonly have light blue or blue-white eggs, as do Western bluebirds, though these can occasionally lay white or off-white eggs. House finches sometimes lay blue-white eggs, often with black or olive speckles. European starlings, on the other hand, lay glossy blue or green eggs.
All About Birds describes cowbirds as having blue eggs on occasion, though they can also be white or green with speckles, and it also describes house sparrows as sometimes having blue eggs, often spotted to some degree. Because color varies even within the same species, it may take multiple nest factors to positively identify a species. Although both Eastern bluebirds and robins most commonly lay blue eggs, their eggs are different textures, sizes and shades, with different nest types and materials. All of this information together makes it easier to recognize a species by its nest or its egg fragments.