Autumn Feathers: Lesser Goldfinch

Recently, while watching and photographing American Goldfinches and Pine Siskins in my neighborhood in the foothills of Boise, Idaho, I saw about a half-dozen slightly smaller, more yellow-bellied than the American Goldfinch this time of year, and sounding a bit different.  I figured I was looking at Lesser Goldfinches, so I snapped a few photos for later evaluation. (Compare sounds of Lesser and American)

I have never seen Lesser's in Idaho, much less my own neighborhood.  They are reported regularly in Idaho, I just haven't come across them until now.  Back in my Arizona days, Lesser's were the only type of Goldfinch I'd get at my feeders, but they sure looked different in full breeding plumage as compared to the those I saw this day.

So, how can you tell the difference between a Lesser and an American Goldfinch?

Good question!  My first clue on these little guys was simply my gut instinct about overall shape, size, and sound...and the fact that they were so different than the American Goldfinches all around them. 

Studying the photos, you can see a few more clues.  On the top photo, that little strong white patch on the male's wing (at the base of the primaries) is a good differentiating characteristic.

The olive green upper parts and nape help too.  The developing black forehead helps.  American's get black foreheads too, but none of them show any sign of this black forehead during this time of the year.

Looking at the Peterson guide, he shows the American Goldfinch with light colored bills and the Lesser's with dark bills.  I thought this was a sure key trait, but looking at my photos of American's in my area, they all have dark bills too.  What's up with that?! Non-breeders have dark bills.  Breeders have light bills.  (All About Birds and Sibley)

I need more field marks to clinch this identification.

Lesser Goldfinch have yellow on the underside of the tail, while American's have white.  Look back at the previous post of the American's and you can see a dramatic difference in the tail feathers.  In flight, the Lesser Goldfinch has white underwing patches and again the tail is different than the American Goldfinch.

What other clues can you suggest for telling the difference between Lesser and American Goldfinch?