546 PROCEEDINGS OF THE ACADEMY OP 



shorter and weaker than that of the adult, and wanting its very acute tip, and 

 sharply-defined r.dges and angles ; brownish-black, fading into dull flesh-color 

 at the base of the under mandible. Front white, but the crown and nape show 

 onsiderable traces of the black that is to appear, which is now mixed with a 

 good deal of light-brown. The pearl-blue of the back and wing coverts is 

 everywhere interrupted by irregular patches of light grayish-brown, showing a 

 tendency to become transverse bars ; this grayish-brown on the tertials deepens 

 into brownish-black, and occupies nearly the whole extent of each feather. 

 The primaries differ from those of the adult in having less of the silvery gloss, 

 and the inner white spaces are more marked, being in fact much like those of 

 the adult hirundo. The rump and under parts are pure white. The tail inten- 

 sifies, so to speak, its adult characters as regards color; and, independently of 

 any other feature, will always serve to identify the species. It is deeply emar- 

 ginate, but the lateral feather is not greatly produced, surpassing the second 

 by scarcely more than the latter surpasses the third. Its inner web for an inch 

 or so from the tip, and both webs of the other feathers, are quite decidedly 

 grayish-black; the intensity of this color, and also its extent, decreasing suc- 

 cessively on each feather from without inwards, so that the central pair scarcely 

 deepen their color at the tips. The outer web of the lateral feather generally 

 stays pretty uninterruptedly white, but sometimes is just at the tip invaded by 

 the darker color of its inner web. 



The preceding descriptions embrace all the well characterized stages of 

 plumage of this species which are known to me, though there are, of course,- 

 intermediates in great variety between those given. It is indeed a little 

 remarkable, the number of specimens iu immature or winter plumage which 

 find their way into collections. Of the numerous examples before me, just one- 

 half are in this state, all showing white fronts, and the usual deep black hand 

 through the eye. There would seem to be something peculiar in the habitat 

 of this species, to cause it to differ so remarkably from its allies hirundo and 

 macroura in this respect. I have purposely gone considerably into detail re- 

 garding these immature stages, because of the great similarity which exists be- 

 tween the species, and the same ages of li S. Havelli" if, indeed, the latter be 

 really distinct from it. The question of the relationship of the two has been fully 

 discussed under the head of " 5. Havelli." 



Sterna Forsteri affords a good illustration of a species, bearing so intimate a 

 general resemblance to another, as to be confounded with it at first glance, and 

 yet when carefully examined proving to be totally distinct. It is perfectly easy 

 to separate it from the hirundo by its characters of bill, wings, tail or feet, 

 either of which taken alone would identify it. The following table will exhi- 

 bit at a glance the distinctive features of our three most intimately allied spe- 

 cies, between which, it will be observed, there is a complete and gradual 

 transition in almost every respect. 



Differential Diagnoses of S. Forsteri, hirundo and macroura. 

 S. Forsteri. Bill (average) 1-60 along culmen ; depth at base -40; robust. 

 Bill orange-yellow, nearly its terminal half black. White space between eye 

 and cutting edge of upper mandible broad. Under parts white. Outer web 

 of first primary silvery; the inner webs also of the others strongly hoary, with- 

 out well-defined white spaces. Tail bluish-pearl, like the back, its lateral 

 feather greatly produced (average nearly 7 inches in length) ; it3 outer web 

 white, inner the color of the rest of the tail. Legs long and stout; length of 

 tarsus (average) rather over -90 of an inch ; orange-yellow, tinged with ver- 

 milion! Length of tarsus, middle toe and claw 2 inches. 



S. hirundo. Bill (average) 1-45 along culmen ; depth at base -33 ; moderate. 



Bill vermilion-red ; its terminal third black. White space between eye and 



utting edge of upper mandible narrower than in Forsteri. Under parts lightly 



washed with plumbeous, fading into white on the throat and abdomen. Outer 



[Bee. 



