331. (American Ornithologists' Union check-list #) Marsh Harrier or Marsh Hawk: Circus hudsonius (Linn.) Adult male--Upper parts light bluish gray; rump white; tail barred black or brownish; breast light gray at upper part shading into white; belly white barred warm brown. Adult female--Upper parts dark brownish; wing coverts edged with light rusty brown; under parts dark buff streaked with brown. Immature birds resemble the adult female without the streaks on the belly. Male Length--19.00. Breeding Range--The entire United States. The nest is placed on the ground, and is "from three to seven inches high and a foot or more in diameter;" it is formed of grass, twigs, and sometimes moss. The eggs are white or bluish white, occasionally marked with pale brown spots. Size--1.80 x 1.45. The Marsh Harrier, or Blue Hawk as it is sometimes called, is easily recognisable by its white rump, which during flight is very conspicuous. They may be seen in almost any marshy or low scrub land, flying within a few feet of the ground or hovering in mid-air watching for their prey. During the mating season the male bird performs the most remarkable evolutions in mid-air, with the evident intention of winning the admiration of his would-be mate. They "frequently begin to incubate with the first egg and the young are hatched at intervals." The nesting season begins in Massachusetts early in May. Davie says that fresh eggs may be found from May 1st to June 15th or 20th, according to locality. The nests are placed in open marshes or low lands.