What will winter bring?

With fall and winter on the horizon, it's time to start thinking about birds that might turn up in the backyard.  Even with good long looks, a few possibilities pose ID challenges.  Life would be great if all of the birds that turn up at our feeders were well-marked adult males.  The odds argue against that, so there's plenty to keep us on our toes.  Consider the following:

The birds above are so drab that if you notice them at all, they might make you wonder if they're worth a second look.  Are they?    

The yellow bird above isn't well-marked, but once winter arrives, some "drab" birds will actually look pretty bright if they pop up in the yard.  That alone would warrant a second look at this bird, if the fact that it's hanging out on a hummingbird feeder doesn't.  What's the most important thing to look at on this bird?

Green and white female-type hummingbirds are always fun.  If you're lucky enough to get a good look, there are a few things to size up when narrowing down an ID.  Sometimes it takes a good study of a photo to get the ID right.  Bill size, the shape and pattern of the tail, and the shape and size of wing feathers can all provide a good clue.  Is a green and white hummer a big deal, or just a Ruby-throat?