This little bird is a Magnolia Warbler. We first saw one a few weeks ago at the Magic Hedge. I tried to get a photo but it kept hiding behind leaves. Over the next few weeks, more Magnolia Warblers started to show up and a few, like this one which was recently in Grant Park, were downright camera hogs!
I have a few more Magnolia Warbler pictures at my birdwatching blog, which is here.