The Ruby-throated Hummingbird, the only breeding hummingbird species in eastern North America, is a familiar summer inhabitant of woodlands, parks, and gardens from central Canada to the Gulf Coast that occupies the largest breeding range of any North American hummingbird. In the United States, the western boundary of its breeding range ends along a remarkably straight north–south line, just east of the 100th meridian. The breeding range, which extends in west-central Canada to British Columbia, conforms closely to the range of eastern deciduous and mixed boreal forests (1).
Adult female Ruby-throated Hummingbirds have an average mass of only 3.5 grams, adult males just 3.0 grams. Despite their tiny size, many of these birds fly nonstop across the Gulf of Mexico during fall and spring migration, a round-trip of more than 1,600 km. To accomplish this, individuals often double their body mass by fattening on nectar and insects prior to departure. The proportion of this species' population that uses trans-Gulf versus circum-Gulf routes is largely unknown, as is regional connectivity in migration routes and overwintering areas.
Although the Ruby-throated Hummingbird is common and widely distributed, many fundamental aspects of its migration, breeding ecology, and molt remain poorly understood. At the time of this writing, for example, there has been only one study of nesting success in this species.