Looking at the head and its distinct markings can really help distinguish one bird from another. Sometimes though a rump shot makes all the difference. Both have a bit of white directly under the tail but the rump is clearly different. American Goldfinch are white under the rump where as the Lesser Goldfinch are yellow all the way. Other shots here show the distinctly white and dark bands on the American Goldfinch’s wing and the darker greenish tinged head of the Lesser Goldfinch (in winter. It is black during breeding plumage).
Then add a White-crowned Sparrow (likely subspecies oriantha) and it gets easier (or more difficult depending on your perspective. More on that later.). This sparrow is much larger than the finches with a stouter bill. There are other distinct head, chest and wing patterns too but I will let you explore your own feeders and discover the details for your self. Once you do that you may begin to see the difference between the White-crowned Sparrow subspecies!
Some of the 40 or so American Goldfinch at my feeders these days are beginning their first molt of the year. Cool fact (thanks to All About Birds http://www.allaboutbirds.org/guide/American_Goldfinch/lifehistory) American Goldfinch molt twice a year. Once being in late winter, now! Check out the lovely hint of yellow popping out on one’s back and the black cap of another.
I am enjoying the new scope and this digiscoping experiment.