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Buteo swainsoni
Listed as "Threatened" by the State of California
Life History
The Swainson's hawk is a large buteo with a body length of around 21 inches.
Although this bird is about the same size as a red-tailed hawk it can be distinguished by its
relatively long, narrow pointed wings. Additionally, the breast and belly may be light with a
characteristic dark, wide "bib" from the lower throat down to the upper breast and light
colored wing linings. The tail is gray ventrally with a subterminal dusky band, and narrow,
less conspicuous barring proximally. The sexes are similar in appearance except that the
females are slightly larger and heavier than the males.
A great majority of the Swainson's hawks are migrators, nesting in northwestern
Canada, the western U.S., and Mexico, then wintering in South America, a round trip which
can exceed 14,000 miles. Swainson's hawks are summer breeders in California with
approximately 80 percent of the pairs nesting in the southern Sacramento and northern San
Joaquin Valleys. The migrating birds return to California between late February and early
April.
Swainson's hawks are monogamous. The average clutch is three or four eggs and
incubation lasts 34 or 35 days with both parents participating in the brooding of eggs and
young. The young fledge in 42 to 44 days but remain with the adults until they depart in the
fall. Large groups of 100 or more individuals may congregate in holding areas in the fall.
The purpose of these holding areas is thought to be related to increasing energy reserves for
migration; aggregation into larger migratory groups, which includes assisting the young; and
providing pairing and courtship opportunity for unattached adults.
Swainson's hawks nesting in the Central Valley are generally found in scattered trees
or along riparian systems adjacent to agricultural fields or pastures. These open fields are the
primary foraging areas. Swainson's hawks generally search for prey by soaring and several
hawks may be seen foraging together following tractors or other farm equipment capturing
prey escaping from farming operations.
The Swainson's hawk was historically regarded as one of the most numerous raptors in
the state. The dramatic decline in the population of the Swainson's hawk has been attributed
to the loss of native nesting and foraging habitat, and more recently to the loss of suitable
nesting trees. This loss of nesting habitat within riparian areas has been
accelerated by flood control practices and bank stabilization programs.
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