690 BULLETIN 5 0, UNITED STATES NATIONAL MUSEUM 



Aesalon fusco-caerulesc.ens Heine and Reichenow, Nom. Mus. Heine. Orn., 1890, 



261, part (Brazil; Chile). 

 Rhynchofalco Jusco-caerulescens fusco-caerulescens Swaxn, Syoopt. Accip., pt. 4, 



1922, 210 (Argentina, Patagonia, and Tierra del Fuego). 

 Hypoiriorchis fusco-caeruleufi Gurnev, Ibis, 1SS2, 158, in text. 

 Falco thoradcus "Illiger," Lichtenstein, Verz. Doubl. Berlin Mus., 1823, 62 



(Paraguay). 



Subgenus Aesalon Kaup 



Small falcons (wing about 186-231 mm.) with length of cere on top 

 equal to less than one-third the length (curve) of culmen; depth of bill 

 at base less than one-third the distance from nostril to tip of maxilla; 

 distance from bend of wing to tip of longest primary covert little, if 

 any, less than distance from latter point to tip of longest primary; 

 basal phalanx of middle toe without transverse scuteila, sexes totall}^ 

 unlike in coloration, and young similar to adult female. 



Bill moderate in size, its depth at base of rhamphotheca slightly 

 greater than its width at same point and very slightly less than distance 

 from anterior margin of cere (in front of nostril) to tip of maxilla; 

 culmen regularly decurved from base, distinctly but not sharply 

 ridged; gonys rather strongly convex, ascending terminally, very 

 indistinctly (if at ah) ridged; maxillary tomium with "tooth" very 

 prominent, the maxillary unguis rather slender, compressed. Nostril 

 moderate in size, nearly circular. Wing long and pointed, the longest 

 primary exceeding distal secondary by about half the length of wing 

 or slightly less; second, second and third, or third primary (from 

 outside) longest, the first (outermost) not longer (usually shorter) than 

 fourth; inner webs of first and second primary emarginated, the 

 emargination on first nearly opposite tip of sixth primiary, that on 

 second between tips of thhd and fourth or fourth and fifth primaries; 

 outer webs of second and third primaries slightly sinuated. Tail 

 about three-fifths as long as wing, slightly but distinctly rounded, 

 the rectrices (12) moderately rigid, not tapering terminally. Tarsus 

 longer than middle toe without claw (about equal in length to middle 

 toe with claw), the upper portion feathered in front for about one-third 

 its length or slightly more, elsewhere covered with small hexagonal 

 scales, these larger on acrotarsium, especially along inner side, where 

 much larger, decidedly transverse, and arranged in a single longitu- 

 dinal series, the extreme low^er portion of acrotarsium with several 

 frontal transverse scuteila; outer toe, without claw, extending about 

 to penultimate articulation of middle toe, the inner toe (without claw) 

 falling short of same point. 



Plumage and coloration. — Plumage compact though rather soft, the 

 primaries and rectrices not very rigid; orbital region quite bare. 

 Sexes conspicuously unlike in adult stage. Adult males bluish gray 

 above with black shaft streaks, interrupted by a band of whitish, 



