The I Love the Finger Lakes website has an article with tips for birding Horseheads Marsh.

Horseheads Marsh is a long, narrow, wet marsh bisected by railroad tracks. Observation is from the tracks, which are slightly elevated about the marsh. The length of this section of track is 1.4 miles. As you start along the tracks, the first part of the marsh is mostly cattails with a considerable amount of open water. The farther you go along the tracks, the more the habitat becomes populated with trees and shrubs. The types of birds you see change as you progress along the tracks, making this a relatively short but rewarding place to bird.