The USDA Blog has a good short article on bird watchers and what the can do to prevent spreading invasive species around.
You can find the article at: http://blogs.usda.gov/2011/08/15/what-can-bird-watchers-do-to-fight-invasive-species/
This topic is definitely a relevant one. When I (Chris Evans is writing this post) worked in Georgia, we found the only known location of garlic mustard in the state, which was located at a very popular location for viewing neotropical migrants. In all likelihoood, a birdwatcher unintentionally transported seeds in from another state when visiting the site. The recommendations discussed in this article are also relevant for hunters, anglers, hikers, nature photographers, and anyone else enjoying the outdoors.