Pennsylvania is a state with beautiful rolling hills, grasslands, old forests, majestic rivers, and scenic mountain views – perfect backdrops for birdwatching. The change of seasons causes a dramatic fluctuation in bird populations. We generally understand that the movement of birds is a response to food availability, rather than directly to cold temperatures. Birds that feed on insects or fruit can make a living in Pennsylvania only during the warmer months, when these food sources abound. Frozen water restricts access to abundant aquatic life for water birds. The resulting movement of birds has an effect on Pennsylvania birdwatching.
Below are excerpts from an excellent series of articles by Daniel Brauning of the Pennsylvania Game Commission on the Seasonality of Pennsylvania Birds. The complete series of articles are available on the Game Commission website under Bird Watching.