The tree in the photo above, and others like it on the front terrace of the New York Public Library, has been permanently marked by Yellow-bellied Sapsuckers, a small, popular species of woodpecker.
Taken at the end of October 2010, the photo below shows two of the birds in action, tap-tap-tapping their way into the bark of the trees. As the name suggests, Yellow-bellied Sapsuckers drill shallow holes with their beaks to literally suck sap out of trees. They get a two-for-one deal using this technique, as they can also eat insects that are drawn to the sap.
We should see more of the Sapsuckers when the birds begin to migrate north in April or May.