The bird on the left is causing me to think, which is a good thing. There are a couple of things odd about it, I saw it today May 4, 2012 here in Northern California and it is pretty late for a Golden-crowned Sparrow overall. Although, when I do see late Goldens, they tend to be up in higher elevation sites, such as where this one was. But the real weird part is that it looks like a Golden-crowned except with a pale supercilium and dark eye line, features of the White-crowned Sparrow! This made me wonder if it is a Golden x White-crowned Sparrow hybrid, a rare but known hybrid. BTW – you can click on the photos to get a bigger version of the shot!
In fact, that hybrid combination I have had twice in my backyard over the years. This is the only photo I have easily available of one of these birds. I was able to trap it and photograph this bird in the hand, the photo appears in Beadle and Rising’s photo guide to sparrows from a few years back. The hybrid is similar to today’s bird, but the striping pattern is much more well defined on the presumed hybrid. Also the bill is pinkish, like a White-crowned, and definitely unlike a Golden-crowned Sparrow.
Back to today’s bird. the bill is dark with a “horn” colored base to the lower mandible, pretty typical of Golden-crowned Sparrow. A close look at the primaries shows that they are pretty worn and frayed, suggesting that this is a younger bird (hatched in 2011). The weird bird was with a typical Golden-crowned and I could not see any size, structure or back color differences. It just had this funny head pattern. My impression is that this is a Golden-crowned, not a hybrid, but it is a particularly “retarded” bird in its head molt, or perhaps a bird that will not fully gain an adult type head pattern (likely a low hormone bird). The pale supercilium look is seen on winter Golden-crowns, although not as well marked as on this bird, and it may be a retained pattern common to Zonotrichia which few if any Golden-crowns ever get to show. It may be that you need a first spring immature, and one with low hormone levels during the head molt to get this? Well, that is my theory, and I have no idea if it is a good one or not. If the bill had been pink, or the back greyish-brown I would have gone for the hybrid theory.
Reminder – pelagic schedule is up for this summer – fall.