If the nest failures on Fifth Avenue over the last few years have you down, hop on the M79 Bus at 79th and Fifth and take the bus crosstown to the last stop at 79th and Riverside. From there it is a short walk to the nest. The pair should have eyasses (babies) around May 1st plus or minus 5 days.
If you get off the bus at 79th and Riverside, cross Riverside and walk along the dirt path, being careful as you cross the on ramp and off ramp of the highway. Then walk under the overpass, and take the stairs down to the Boat Basin Café. If you keep making right turns you'll pass the bar, take a small set of stairs and then a long set of stairs. Once you get to the bottom keep going straight (north) about two blocks and you'll be at the nest. For better views, climb up to the exit ramp and look back. The river cools things down, so dress as though it's 10-15 degrees cooler than the city.
Update 4-30-09: The Boat Basin Café is very busy on the weekends and can be frustrating to navigate through. As an alternative, you can enter the park at 83rd Street, and wind your way west to the 84th Street underpass, cross under the highway and walk south to the nest.