In the lingo of Ornithology, a summer resident is a bird that breeds in your home region but leaves for warmer latitudes or lower elevations during the colder months of the year. Generally speaking, the summer resident arrives in spring, nests and raises its young and then departs in late summer or autumn for their winter range.
Here along the Colorado Front Range (and, indeed, in much of the country) tree swallows are among the first summer residents to arrive, often by mid March. Yesterday, while birding at South Platte Park, I encountered 13 of these agile birds, snaring insects above Redtail Lake. Since they nest in cavities, these small but aggressive insectivores compete with bluebirds, Prothonotary warblers and other species for those nest sites and, since they often out-compete bluebirds for man-made nest boxes, they are not always welcomed by some members of the birding community.
Then again, all swallow species consume a prodigious number of nuisance insects and anyone who lives near ponds or lakes is sure to appreciate their presence. After nesting and raising their young, tree swallows depart for the south before an autumn cold snap wipes out their prey; unlike most swallows, which winter in Central or South America, tree swallows are satisfied to stay along the Gulf Coast and thus get a head start for the spring migration.