FALCONID.E DIURNAL RAPTORES. 467 



white ; tibiae and lower tail-coverts immaculate ; across the abdomen and flanks (immedi- 

 ately in front of the tibiaeHs a broad interrupted belt of longitudinal black blotches.those on 

 the abdomen tear- shaped, on the flanks larger and more irregular, throwing off bars toward 

 the edge of the feathers; whole pectoral area variegated only with a few shaft-streaks of 

 black (these growing broader laterally), and sometimes washed with rusty. Lining'of 

 the wing ochraceous white, with sparse diamond- shaped spots of pale rufous, and shaft- 

 streaks of darker; under surface of primaries white anterior to their emargination, 

 beyond which they gradually deepen into black; the innermost ones are finely mottled 

 with slaty, and with imperfect transverse bars of the same. 



Male. Wing, 13.50-16.50; tail, 8.50-10.00; culmen. .95-1.08; tarsus, 1.40-3.20; middle toe. 

 1 . 60-1 . 70. Weight, 2^-3 Ibs. 



Female. Wing, 15.25-17.75; tail, 9.50-10.50; culmen, 1.00-1.15; tarsus, 3.15-3.40; middle 

 toe. 1.70-1.80. Weight, 3-4 Ibs. 



Young (No. 28,154, Philadelphia, 'J. Krider). Above similar to the adult, but lacking 

 entirely any rufous tinge, the scapulars and wing-coverts more variegated with whitish. 

 Tail light grayish brown (very much lighter than the rump), tinged, especially basally, 

 with rufous, narrowly tipped with white, and crossed with nine or ten narrow, curved 

 bands of black ; upper tail- coverts white, with broad bars of black. Head as in the adult, 

 but the rufous wanting, leaving the streaks black and white ; forehead more broadly 

 white ; chin and throat wholly white, the latter with a collar of dusky streaks across the 

 lower part; whole pectoral region entirely immaculate, pure white; abdominal band as 

 in the adult; tibiae somewhat tinged with ochraceous, unvariegated. 



In its typical form, B. borealis may usually be distinguished from 

 the lighter-colored examples of B. borealis calurus by having the 

 posterior lower parts (tibiae and lower tail-coverts) free from trans- 

 verse bars or other markings, and in the absence of transverse 

 bars on the tail anterior to the usual subterminal one. These 

 differences are by no means absolutely constant, however, since, 

 while eastern specimens occasionally occur which have these parts 

 more or less distinctly barred, western specimens with the same 

 parts immaculate are sometimes found. Such, however, are in 

 either case exceptional. 



Young birds from the eastern side of the continent occasionally 

 have the tibiae more or less barred or spotted, but usually such 

 markings, if present, are either small or faint in color compared 

 with western specimens of corresponding age. 



b. calnrus. 



SCBSP. CHAB. Adult. Similar to borealis, but darker, or with more rufous and 

 blackish in the plumage ; tibiae usually distinctly barred with rufous, or else nearly 

 uniform rufous or dusky ; throat usually with dusky prevailing, sometimes wholly of 

 this color. Tail frequently with more or less distinct indications of numerous blackish 

 bars. In melanistic individuals, which are not uncommon, the whole plumage nearly 

 uniform sooty black or dark brown, the breast, however, usually paler, sometimes rufous 

 or ferruginous. (Between this plumage and the lightest-colored individuals every pos- 

 sible intermediate coloration occurs.) Young. Much darker than the corresponding 

 stage of borealis. In melanistic specimens, whole plumage blackish brown, varied only 

 with fulvous edgings to the feathers and concealed white spotting; the tail grayish 

 brown, with about nine broad black bars. In the lighter-colored individuals, the tibiaa 

 with very distinct usually heavy transverse spots of dusky. 



