Tree Swallows nest in tree cavities abandoned by other species. Bluebird enthusiasts rue the presence of these swallows in bluebird boxes. The swallow’s dependence on nesting cavities, often real-estate in short supply, has some interesting consequences. Tree Swallows are early spring migrants and breeders—the early bird gets the nesting cavity. Tree Swallows fiercely defend their breeding territories against other Tree Swallows. As a result, many tree swallows are unable to breed. These “floaters” are ever vigilant nest holes abandoned, for whatever reason, by the territory holders (Winkler et al. 2011). I took this photo on Wednesday of a Tree Swallow at a local nursery, while Erika bought supplies for her garden.