Kingbirds (Tyrannus spp.) are aerial hawking insectivores of open spaces that occur in most physiographic regions of North America. Of the 8 species that breed north of Mexico, the Eastern Kingbird is the most widely distributed (Figure 1); along with the Gray Kingbird (T. dominicensis), it is the only species to breed in eastern North America. Despite its common name, the Eastern Kingbird breeds abundantly west of the Mississippi River, and its range extends to the Pacific Ocean. In the Great Plains and Far West it may breed sympatrically with 1 or 2 other species of kingbirds.
Kingbirds are well known for their aggressive nature. Indeed, Tyrannus means “tyrant, despot or king,” in reference to their aggressive defense of nests and mates, and their domination of other birds. Pairs are monogamous, maintain territories while breeding, and if both survive to the following breeding season, usually remate and reuse former territories. Nonetheless, extra-pair copulations and intraspecific brood parasitism may be common.
The Eastern Kingbird overwinters in Amazonia. While there, it maintains different social and feeding behaviors than are seen during the breeding season. Most individuals travel in flocks and forage on fruit, returning to North America to begin laying eggs by late May and early June. Clutch size varies geographically (mode of 3–4 eggs), but females raise only a single brood per season. This low productivity is likely related to the species' foraging habits and reliance on flying insects for food. Fly-catching is an exacting mode of foraging, and individuals appear to have a difficult time feeding large broods adequately, especially when cool, wet weather reduces the availability of flying insects. The extended period of post-fledging parental care (3–5 weeks) appears to limit parents to a single brood per year.
The reproductive biology of the Eastern Kingbird has been described in detail by several authors, including early work by Davis in central New York (Davis 1941c) and Montana (Davis 1955b), and later research by Murphy (Murphy 1983a, Murphy 1983b, Murphy 1983c, Murphy 1986c, Murphy 1986a) in western New York and eastern Kansas, and Blancher and Robertson (Blancher and Robertson 1985a, Blancher and Robertson 1985c) in eastern Ontario. Smith (Smith 1966d, Smith and Smith 1992b) studied the vocalizations of Eastern Kingbirds (and other kingbird species) extensively, and parental care behavior has been described by Morehouse and Brewer (Morehouse and Brewer 1968) in Michigan, and Murphy and his students in central New York (Rosa and Murphy 1994, Maigret and Murphy in press).