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Literature Review

Summer tanager

Piranga rubra cooperi

USFWS: No status
CDFG: Species of Special Concern

Background

Distribution, Abundance and Trends. The summer tanager breeds across the southern United States from California, as far north as the Kern River valley, to Florida, and in the eastern United States as far north as 40° N. Two subspecies are currently recognized. One, P. r. cooperi, breeds in the southwest from California to west Texas and northern Mexico. The other, P. r. rubra, occupies the remainder of the range to the east. The western subspecies inhabits riparian woodlands and, at higher elevations, woodlands dominated by mesquite and salt cedar. The summer tanager winters from central Mexico south through Central America to Bolivia and Brazil. It occurs in small numbers in winter in southern California, southern Arizona and in southern Florida.

The North American breeding population has remained generally stable since the mid-1970's, although some populations in the eastern United States and along the Colorado River have declined. It was formerly considered common in the lower Colorado River valley by Grinnell (1914), but only 216 individuals were estimated to be present by 1976 (Rosenberg et al. 1991). Habitat destruction is the likely cause of the decrease.

Little is known of the breeding biology of the species. Summer tanagers nest in mature riparian groves dominated by cottonwoods and willows. Early arrivals from wintering grounds may appear in late March, but the main migration is April through early May. Nesting is primarily May through June. The nest is built on a horizontal limb of large trees including cottonwoods, usually 10 to 35 feet above the ground, and often over an opening such as a creek bed. The nest is a loosely built, shallow cup of weed stems, leaves, bark, and grasses, lined with fine grasses. From 3 to 5, but usually 4 eggs are laid. Incubation is approximately 12 days. Tanagers eat insects, including bees and wasps, and small wild fruits.

This species is known or suspected to nest in the Plan area in Mission Creek, the Whitewater River Canyon, and Palm Canyon, and also migrates through the area on its way to more coastal and northern habitats. There are also records from the Whitewater River delta and the Coachella Valley Preserve, but whether it nests in these areas or only uses them in migration is not known.

Threats and Limiting Factors. The major threat is loss of habitat due to human activity, including flood control. Cowbird parasitism may be a contributing factor, although parasitism of summer tanager nests appears to vary significantly by geographic area. In southern Illinois, 11 of 13 nests observed were parasitized, while in the South Fork Kern River Valley only 1 of 16 nests was subject to parasitism. The extent of cowbird parasitism in the Plan area is not known. Collisions with wind turbines and other towers during migration could also be a source of mortality. In Leon County, Florida, 146 summer tanagers were killed at a television tower during spring migration. The actuality or potential of mortality from wind turbines, communication towers, and transmission towers in the Plan area is not known. Overall in California, the population of summer tanagers has declined severely in response to elimination of riparian willow and cottonwood forest. The species is designated a Species of Special Concern by the state.