Tree Swallows are somewhat territorial when nesting and breeding, chasing other Tree Swallows away from their chosen nest cavities and defending a small area around them. You see them a handful at a time from the late spring through the summer, but by the end of summer, once they've reared their young, they start to congregate together in large flocks, particularly in the evening.

In one spot in Connecticut, this phenomenon has grown to one of the great natural events of the year: on Goose Island, a small grassy island in Old Lyme, something like 300,000 Tree Swallows gather to roost together each night for a few weeks before it's time to migrate. Savvy naturalists and boat operators have taken notice and now offer Swallow tours of the Connecticut River. This September, Andy and I got a chance to see it all for ourselves.