Millennium Park has beavers. Their dam is off the northeast corner of the property. There is a well-traveled but narrow path lined with bird houses that provides views of the beaver pond. (I saw a common yellowthroat (!) and a winter wren there the other day). Recently they've become more visible, cutting down many small trees that line the lower path. There are beavers in Needham now too, though perhaps not for much longer.
The problem of backyard beavers makes me think about passenger pigeons. After reading 19th century accounts of the depth of guano and stench associated with mass wild pigeon nesting sites, I've become convinced that as much as we mourn their extinction, there is no way they would have been tolerated in the modern world.
Bonus Charles River Peninsula bluebirds (who may just stick around the whole winter). Spotted a purple finch for the first time at the CRP last week (only blurry photos). Usually it's been a complete house finch zone.