The easiest way to find out who is nesting in your nestbox is to ID an adult bird incubating, but incubation doesn't start the last or nest to last egg is laid (depending on the species.) The location and nest construction are also great tips. For detailed descriptions of nest construction and egg identification, see Nest and Egg ID (also with links to photos of nests, eggs and young) and the nest contents ID matrix. U= uncommon, but it happens (e.g., about 3-5% of bluebirds eggs are white. Y=Yes. MB=may be. A "wreath" is a concentration of pigment on one end of the egg. More ID links below. For non-cavity nesting bird eggs, or eggs found outside the U.S. see the searchable database at the Royal Alberta Museum Virtual Egg Exhibit.
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