STERCORARIIDyE: JAEGERS, OR SKUAS. 979 



Jaeger: Length 17.00-20.00 ; wing 12.00-13.00; tar.sns 1.75-1.87 ; bill 1.35-1.50 ; tail 5.00- 

 6.00, the long feathers up to 9.00. Nearly mature : Pileum, lateronuchal region, and whole 

 upper parts, as in adult. Under parts white (as in adult), but clouded everywhere with dusky 

 patches, most marked across breast, on sides, flanks, and under tail-coverts, leaving middle of 

 belly and throat nearly pure. Varying degrees of this dusky nubilation approach in some 

 specimens nearly to the uniform dusky plumage below characterized ; in others fade almost 

 into the pure white of the normal adult, connecting the two states perfectly. The tarsi of the 

 darkest specimens have small yellow blotches ; others not. Melanotic state (adult) : Wholly 

 deep dusky ; darker and more plumbeous above ; lighter and with a fuliginous tinge below ; 

 pileum black; lateronuchal region yellow; remiges and rectrices black; feet black. Imma- 

 ture (size and general proportions nearly of adult ; bill and cere perfectly formed) : Feet mostly 

 black, but with some yellow blotches. Upper parts unadulterated with any rufous bars ; the 

 deep brownish-black pileum has appeared, and sides of neck have obtained their yellow shade, 

 which contrasts conspicuously with the fuliginous background. Evidences of immaturity, 

 however, are found on under parts, where the dark color is mi.Ked with illy-defined transverse 

 bars of ochraceous ; rufous is also found at bend of wing and on under wiug- and tail-coverts. 

 Primaries still wliitish outside, as are also the rectrices. The middle tail-feathers project 2.50, 

 and have the tapering form of those of adults. Younger (small size, delicate bill and feet, 

 little projection of the central rectrices, general moUipilose condition of plumage, etc.) : The 

 rufous of the very young bird, instead of giving way everywhere to dusky, yields to this color 

 only on upper parts and crown ; on sides of head, neck, and whole under parts, whitish pre- 

 vailing, only interrupted by indistinct dusky bars. The yellow of sides of neck has not yet 

 appeared. There are the same white spaces on bases of wings and tail that exist in very young 

 birds. The middle tail-feathers project about 2.50. Young-of-the-year in August (size con- 

 siderably less than that of adult ; form every way more delicate ; wings more than 1.00 

 shorter ; bill and feet much slenderer and weaker) : Bill in some specimens light bluish-horn ; 

 in others greenish-olive, the terminal portion brownish-black. Tarsi and greater part of toes 

 yellow. Plumage everywhere rayed and barred witli ruf(jus and brownish-black. On head 

 and neck the light ochraceous rufous predominates, tlie dark color only appearing as a narrow 

 line along shaft of each feather. Proceeding down neck to back, the longitudinal lines gradu- 

 ally spread until between shoulders they occupy the whole of each feather, except a narrow 

 l:)order of rufous, which latter is of a deeper tint than on head. Passing down throat to breast, 

 the rufous becomes decidedly lighter — almost whitish ; while the dark bnnvn, which on the 

 throat exists only in longitudinal lines, changes to transverse bars of about equal width witli 

 the light rufous bars with which it alternates. This pattern prevails over the whole under 

 parts, the transverse bars being broadest on flanks and under tail- and wing-coverts, narrowest 

 in middle of belly. Primaries brownish-black, narrowly tipped with rufous, their shafts yellow- 

 ish, tlioir iuner webs fading basally into white. Tail like wings; middle feathers projecting 

 about 0.75. Such are the plumages of the normally colored birds, from tlie first featlicriug to 

 tlic fully adult dress, irrespective of the melanotic state; but some individuals show tlie dark 

 phase at all ages, and when adult pair with birds of the normal plumage, giving rise to inter- 

 mediate specimens in every degree between the extremes. Eggs 2-3, resembling those of 

 Numcnius borealis, and finite as variable in ground-color and markings but not so pear-shaped ; 

 sizf 2.00-2.40 X 1.50-1.70, averaging about 2.30 X 1.65; the nest is placed on the ground 

 in open places, in the gra.ss, moss, or heather. This Jaeger is found at times in most parts of 

 tlie world, but breeds normally in arctic and sul)arctic regions of l)oth hemispheres, ranging in 

 winter to South Africa, Australia, New Zealand, and portions of South America. It breeds 

 abundantly in some localities in British America and Ala.ska, and is of common occurrence in 

 tlio U. S. in winter, on both coasts and in the interior (Illinois, Colorado, etc.). It has many 

 synonyms, of wliich crepidatiis is the most prominent, and used by those who apply parasiticus 

 to tlic next species; richardsoni was based on the melanotic state. 



