A Philadelphia Vireo. It has the vireo slightly thick bill that curves downward at the tip. The yellow at the throat and under the tail distinguishes it from a Warbling Vireo. Warbling can have yellow on the flanks and breast but the Philadelphia usually has more yellow at the throat and undertail. The Philadelphia also has a black line through the eye that connects to the base of the bill. While Red-eyed Vireos have a line through their eyes they have gray heads and white bellies. The Orange-crowned Warbler can look similar to the Philadelphia in the spring but they have thin pointed bills and only small white arcs above and below the eyes whereas the vireo is quite noticeable. Likewise female and immature Tennesee Warblers can appear similar but they would also have a thin pointed bill and white at the bottom of the belly and under the tail. Hope this helps.
