This morning, I birded the Birch Street Community Gardens in Concord and saw the bird that Dylan Jackson found yesterday and identified as a possible Bell's Vireo, but was reluctant to say for certain. I saw the bird and believe that he is correct. I got a brief but excellent view and then a couple of other short views as I tried for a photo (one shot of the rear end but it shows the wing bar)). It has a vireo bill, plain face with no spectacles or eye line, single wing bar, greenish back and yellow flanks. Combined with Dylan's description, I cannot come up with anything else that it could be.
Dylan had it in the middle section of the garden yesterday, in the dead bush covered with morning glory vines by the feeders. This morning I had it in the northwest corner of the garden, feeding in the smartweed (the low weeds with the reddish leaves and pink seeds) by the tall flagpole. It was feeding with a number of other birds, and flushed when they did to the bushes on the northwest corner edge and then into the tangle of vines in the tall trees where I lost it.
The bird was skittish and did not respond to pishing. If the House Sparrow and Finches are feeding in the smartweed, I'd suggest approaching slowly and quietly and not too closely. If you can avoid flushing the other birds, the vireo may be more likely to stay feeding.