The Phainopepla inhabits arid wood lands and deserts of the southwestern United States and Mexico. This long-tailed, crested bird is common and conspicuous in the Sonoran Desert, where it perches atop mesquite or palo verde trees, calling with a questioning " Wurp? " while flicking its tail and turning from side to side. The female in gray plumage, and the male in glossy black, flutter and zigzag, especially in pursuit of flying insects. In flight, the male flashes white wing patches, a striking contrast with the shimmering black plumage which inspired the Greek name Phainopepla, or "shining robe."
Phainopeplas breed in two distinct habitats at different times of the year, an unusual pattern among North American passerines. Between February and April the species breeds in the Sonoran Desert in Arizona and in the portion of the Sonoran Desert extending into California, called the Colorado Desert. As summer heat intensifies and berry supplies dwindle, Phainopeplas vacate the desert. In May, they arrive in the oak and sycamore canyons of Arizona and California, where they breed through July. The two distinct breeding seasons have prompted conjecture that the same individuals breed in both habitats each year. However, specific migratory routes are unknown, and it remains uncertain whether the same birds breed in both desert and woodlands.
The differences in Phainopeplas' social behavior between desert and woodland breeding grounds are as striking as the physical contrasts between the habitats. In the desert, mated pairs vigorously defend feeding territories against intruders. In woodlands, Phainopeplas often nest in loose colonies of from 3-15 pairs, have overlapping home ranges, and mob nest predators such as the Western Scrub-Jay (Aphelocoma californica) in groups. Several members from the colony may feed at the same fruiting bushes such as holly-leaf redberry (Rhamnus ilicifolia) and blue elderberry (Sambucus mexicana). These social differences between habitats may reflect differences in the availability and distribution of fruit. In woodlands, fruiting at any given bush is relatively ephemeral, and the bushes thrive in open areas away from shaded nest sites. In the desert, Phainopeplas depend on fruiting desert mistletoe (Phoradendron californicum), which parasitizes the same trees used for nesting, and produces a stable, long-lasting supply of berries.
Early accounts of the Phainopepla described nesting behavior in southern California woodlands ( Merriam 1896a , Myers Myers 1907 , Myers 1908 , Myers 1909b ) and in Arizona's Sonoran Desert ( Rand and Rand 1943 ). More recently, Walsberg ( Walsberg 1977 ) compared social behavior and daily energy expenditure of desert and woodland populations. In the desert, Phainopeplas are inextricably linked to desert mistletoe, upon which they subsist almost entirely in winter. Noted as early as 1914 ( Grinnell 1914b ), this reliance on desert mistletoe has spurred studies on the morphological specialization of the Phainopepla digestive system ( Walsberg 1975 ), the impact of the fruit-eating specialist on desert mistletoe seed dispersal (Larson Larson 1991 , Larson 1996 ), and fluctuations in mistletoe berry crops as a determinant of Phainopepla breeding success ( Walsberg 1977 , Anderson and Ohmart 1978 ). Ongoing investigations include microgeographic study of population genetics (R. Fleischer and W. Boarman pers. comm.), comparison of mating strategies of territorial and loosely colonial Phainopeplas (MC), and description of the diverse vocal repertoire of the species, including imitations of other birds that both sexes use in distress calls (MC).
Ever since Gilman ( Gilman 1903b ) noted the disjunct breeding seasons and habitats of the Phainopepla, various authors have remarked on the question of whether the same birds breed in both seasons ( Dawson 1923 , Crouch 1943 , Phillips et al. 1964a , Walsberg 1977 ). If so, Phainopeplas would be behaviorally flexible-in migration, breeding, and social structure-to a degree not documented in any other North American bird. All major studies of Phainopeplas have been conducted in the United States, and comparable data on breeding habitats and migratory movement in Mexico are lacking, including such basic information as the extent of any north-south migration over the border. The question of whether the remarkable behavioral flexibility of the Phainopepla is exhibited by individuals or by separate populations of birds remains an unresolved and pressing issue.