Yesterday evening I had the pleasure of narrating a cruise down the lower CT River to view one of the most impressive North American avian spectacles - the autumn staging of Tree Swallows. For those who have not heard about this, up to a half million (!) Tree Swallows congregate each evening during late summer and early fall to roost in a marsh along the river. A boat is needed to reach this location, and Connecticut Audubon Society EcoTravel helps to arrange access to this special event. Several evening river cruises are run each year at this time. If you haven't seen this yet, you must. This is the sort of thing by which even non-birders can be amazed.
My 400mm lens is just too much to capture the display these birds put on, but this small fraction of the flock gives an idea of the density involved.