Most fair-sized cities host one or more birding clubs. They organize field trips, invite guest lecturers, show films of good birding destinations, provide educational services to local schools and, if they're good ones, they are involved in local environmental projects. Save the swamp, save the prairie patch, save the forest...
Part of the fun in joining such groups is being with people who get excited when you bounce into the meeting room some evening bellowing, "This morning I saw a Lawrence's Warbler!" and they get excited, too. If you see a Lawrence's Warbler, you can bet that the next morning you'll be leading a field trip so others can see it as well.
Wikipedia has both an international List of ornithological societies and an international List of environmental organizations. Also there are these special-focus sites:
For many birders in the US, the main event of the year is the Christmas Bird Count sponsored by the Audubon Society and the U.S. Fish and Wildlife Service. Each year, between December 20 and January 1 or 2, thousands of birders all across North America fan out over the landscape actually counting each individual bird.
This count is extremely valuable because it helps keep track of how well the various species are coping. Quite a few of the more specialized species grow rarer all the time, while some others seem to be adapting, and those living in disturbed habitats even thriving. A review of all past and recent Christmas Counts is available on Audubon's Annual Summaries of the Christmas Bird Count, 1901-Present page.