Truly "urban birding"! The common and garden offer a good birding break for folks downtown on other business. It is very rare that there are birds worth coming to look for outside of migration. That being said - I have seen Barred Owl on several occasions and it is the one of the best spots in Boston for Yellow-bellied Sapsucker. Warblers can concentrate in the Garden during migration, and it is a good spot to observe the variations in Hybrid Black Ducks and Mallards : )
Although Saltmarsh Sharp-tailed Sparrow is not exactly a target bird - it does represent the amazing diversity of birds you might find here during migration. This particular bird was discovered feeding on Monument Hill with several Swamp Sparrows, White-crowned Sparrows, White-throated Sparrows, Song Sparrows, Savannah Sparrows, and a Lincoln's Sparrow! In past years monumnet hill has seen Clay Colored Sparrows during migration as well.
The Garden and its abundance of flowering trees is a bigger draw for Warblers - Any warbler is possible here during migration, and this past year the Garden was also very productive for thrushes in one day I had Swainson's, Veery, Hermit, and Wood Thrush.
Outside of Migration the possibility of a good bird or two is ther but not likelly!
This map of the Common and Garden in 1890 is still fairly accurate. A ball field here and gazebo there is really the only changes. Monument hill previously mentioned is South of Frog Pond.
Common and Garden courtesy of 
