Cassin's Kingbird is a large, noisy flycatcher conspicuous on its breeding grounds in the United States and Mexico. Its scientific name describes its aggressive, domineering behavior toward potential predators and rivals of its own species, and its loud, frequent vocalizations.
The species was first described by William Swainson in 1826 from a specimen taken in Mexico. A later description in 1850 by George Lawrence provided its common name honoring John Cassin (1813–1869), an eminent Philadelphia ornithologist who has been memorialized in the common names of 5 bird species (Mearns and Mearns 1992a).
Cassin's Kingbird is a partial migrant, with individuals breeding in the United States and northern Mexico moving south in fall. A resident population, which has been much less studied, occurs in central and southern Mexico. Cassin's Kingbird breeds at higher elevations than other North American kingbirds. The species of large tree chosen for its nest site varies geographically; sycamore, cottonwood, willow, oak, and pine are commonly used in riparian areas in arid parts of its range.
The ranges of Cassin's and Western (T. verticalis) kingbirds overlap geographically and partially in elevation; the ecological separation of these 2 species remains an intriguing question. Competition between the 2 species appears minimal in nesting and foraging habitats with ample insect prey. The 2 species occasionally nest in the same tree. Close nesting by pairs of Cassin's Kingbirds is rare. Both species join to mob hawks and other potential predators. In suboptimal or marginal habitats, Cassin's Kingbird successfully defends individual foraging areas and forces Western Kingbirds to nest farther from foraging areas.
In its migratory population, Cassin's Kingbird raises 1 brood per year. The female gathers nesting material, builds the nest, incubates the eggs, and broods the nestlings, while the male perches nearby. Both mates feed nestlings, but the female spends more than twice as much time feeding the young as the male does.
Relationships among the members of the genus Tyrannus have intrigued many ornithologists, and studies by Hespenheide (1963), Smith (Smith 1966d), Ohlendorf (Ohlendorf 1971, Ohlendorf 1974), Goldberg (Goldberg 1979), and Blancher and Robertson (Blancher 1982, Blancher and Robertson 1984, Blancher and Robertson 1985b) supply much of the existing knowledge of Cassin's Kingbird.