I've been thinking about.

Birding.

How hard could it be? There are 900 or so North American birds. To be a good birder, all you have to do is be able to recognize each one.

Well, okay, there is a little more to it than that. There are a few multipliers. Sometimes the females look different from the males. The juveniles are always a little different from the adults. So it’s 900 birds, times a few variations for each. If you have to use the sounds they make to help in identification, there are songs, calls, flight notes, and chips; a few more multipliers.
And then there are gulls. Gulls are special. Each kind of gull looks different each year until they reach breeding maturity. That can take up to four years. Not only do feather colors and patterns change seasonally and by year, but bills and feet can change colors too.

Spring breeding plumage tends to be brighter than fall plumage. Non-breeding plumage can be altogether different. Different birds go from breeding to non-breeding plumage at different times. Even two birds of the same species will change at different times!
Here is an example of a simple identification challenge from last May. Two birds. Same beach. Same day. Same time.

One.


The other.


By plumage, these two look completely different, but physically, they are exactly the same. Breeding plumage. Non-breeding plumage. On the same day. In the same place. It’s the same kind of bird.

Black-bellied Plover.