I like loons, but only was familiar with summer common loons.
As I got more into birding in the last few years, I became familiar with winter common loons. I didn't realize they changed coloration for some reason, until I saw an injured one near my park. I called on it and they captured it for rehab. There's not a lot that stick around our area--most just pass through on their way to northern Michigan.
Because of the color change in another group of birds, I signed up for the course on warblers so I can identify them better when they pass through in spring and fall.