Here are the answers for the birding pop quiz:
1) Willow Flycatcher. The coloration on the beak is a bit misleading here (due to lighting), but the call (supplied in the hint) is diagnostic. Mia and Jen, nice work!
2) There's not too much to go on with this photo. The olive green back and white belly help narrow it down, but the slight, blurry hint of red on the eye, along with the broken eye-ring, point to Red-Eyed Vireo--Nice work Jen!
3) I cringe at this sign every time I visit the Gilbert Water Ranch...oh the shame it brings upon Arizona birding. As you all mentioned, it's an Abert's Towhee, and the White-throated Sparrow was pictured in place of the proper White-crowned Sparrow. Every state has a few skeletons in the closet I guess...
4) There are only two wood Thrushes (so, excluding Robins and Bluebirds) one can expect to see in north central Arizona in the summer. This bird is unusual in that is seems to lack the eye ring found both on Swainson's and Hermit Thrushes. The lack of any rusty coloration on the tail, and the overall gray coloration, point towards Swainson's, but to be honest I'm not 100% on this bird.
Thanks to all for playing!