I've been fortunate enough to see them twice during migration, but only once did I photograph them. They swooped through our yard and settled into a field across the street, diving and foraging among the weeds. They dove and hopped and rustled through the grass obviously after something specific, but I couldn't determine just what. They were only there about 5 minutes before lifting en mass and moving eastward in a blue-green swirling dervish. They flew all around me, not bothered by my presence. Cornell says their ability to use plant foods help them survive periods of bad weather. Maybe they were picking up some yummy seeds. Normally, they catch insects in flight and nibble berries.
Cornell also states that outside of the breeding season the Tree Swallow congregates into enormous flocks and night roosts, sometimes numbering in the hundreds of thousands. They gather about an hour before sunset at a roost site, forming a dense cloud. They swirl around like a living tornado and as darkness approaches they then wheel low over the cattail marsh or grove of small trees. Large numbers drop down into the roost with each pass of the flock until the flock disappears.
The Tree Swallow uses many feathers from other birds, usually waterfowl, in its nest. The feathers help keep the nestlings warm so they can grow faster. They help keep levels of ectoparasites, like mites, low too. They normally nest in tree cavities or nest boxes. Cornell Lab of Ornithology has a NestCam which occasionally highlights Tree Swallows but today, the NestCam is showing Barn Owls. You can link to Cornell Lab of Ornithology's blog, Round Robin, to the right under Nature & Photography Blogs.