At about 8:00am this morning I was passing by the 'Towhee Pond' at Sweetwater Wetlands and heard a vireo that seemed out of place. Much to my delight it was a singing RED-EYED VIREO. It was in the largest willow (there are 3) at the northwest corner of the 'West Settling Basin'.
a map of Sweetwater can be seen here at Tucson Audubon Society's website:
I stayed on the bird after the initial sighting for about 45 minutes until Jerry Boch, Mark Stevenson, and Thomas Staudt arrived and heard the bird singing immediately. It took a good 30 minutes or so before looks were had by all. This location is where we left it at about 10:00. It was singing from the south side of the island in the middle of the 'Overlook Pond'. It was seen while standing at the hand-railed section of the overlook area.
A discussion for the possibility of Yellow-green Vireo:
I've posted 4 pictures. There never was a good photo opportunity, for this bird never really left the tops of the trees until a brief look at the Overlook Pond. The pictures could be better (always) but enough features are seen to talk about.
The flight shot shows the nice yellow-wash on the breast sides well. This is in contrast with the underwing coverts which appear paler. They were actually very white in person. Yellow-green should have lemony yellow underwings. (The Sibley Guide to Birds). This pic. also shows a nice long white supercilium. The color on the back reflecting the sunlight is the mostly olive color of the back contrasting with the greyish head, good for Red-eyed. There was little yellow in the back color.
The shots from underneath show how there's very little contrast between flanks, chest-sides, undertail coverts, and belly. On Yellow-green Vireo I've noticed some brighter colors extending into the auricular region as well as across the chest. It's quite noticable at times. This is described well in Peter Pyle's Identification Guide to North American Birds. Peter also describes a Yellow-green as having a dusky eye line. A couple of the pictures show a pretty obvious blackish eye-line very noticable in the field as well.
This is all great, however the bird was singing it's head off for nearly 2 hours straight which helped a lot in keeping track of it for others who might be on their way. At no point did it's song strike me as odd for anything but Red-eyed. However I have a bias growing up in Pennsylvania and hearing a ridiculous amount of these birds. This I felt was the most diagnostic part.
Birds are birds and these 2 species have been confused in the past. I'm going with Red-eyed. Hopefully someone will be able to record it and/or get some better pictures so this can be investigated further if need be...