The main colour and any markings are two vital things to look at if you're trying to identify a bird egg. It's not easy as the Royal Alberta Museum website notes bird eggs come in nearly every single "colour of the rainbow."
While many bird's eggs will have one solid colour, such as the blue of the American robin, others will have series of blotches, dots, speckles or other markings that are typical of a certain species. Blue jays, for example, have greenish eggs covered with brown spots, while the black-capped chickadee produces a speckled egg. The white eggs of the white-breasted nuthatch have abundant red-brown speckles, according to the "National Audubon Society Field Guide to Birds." The eggs of the ferruginous hawk have a series of blotches on them.
Sometimes the markings on a bird's egg have a much higher concentration on one particular end. An example would be the eggs of the mountain chickadee which, when they do have red-brown dots, have many more on the larger end of the egg.
Birds that nest in cavities in trees, such as kingfishers, owls, swallows and many types of woodpeckers, will usually lay eggs that are all or mostly white. The reason for this is to allow the parent birds to find the eggs in the dark hole more easily, notes the Royal Alberta Museum site..