A challenge in learning to identify birds is the variability in plumage between sexes and between adults and juveniles. Here are two examples of age differences in plumage. This is an adult Little Blue Heron. It is mostly slate blue with a little purple on the neck.
This is a juvenile Little Blue Heron. Where’s the blue??? You have to look closely sometimes or you may mistake the all-white juvenile Little Blue Heron for a Snowy Egret at a distance. You need to consider the bill and leg coloration to be sure.
Another example is the well-named White Ibis. It is completely white except for the black wing tips that are visible in flight.
But the juveniles are a mix of brown, gray and white. This is a youngster with a Glossy Ibis in the background.
Add seasonal breeding plumages to gender and age variations and you may need to learn 6 or more different wildly different variations for one species of bird!
Birds in My Viewfinder 
My favorite examples of this are the bald eagle and gulls, where the juveniles have changing plumage for up to five years before settling into adult coloration. (Like gulls aren’t hard enough already!)
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