36 A CATAXOGtJE OF THE BIBDS 



as freely as their parents, and their reproductire organs exhibit 

 no deterioration. Is this a case, then, where a physiological 

 difficulty to Darwinism is surmounted ? or is it a mere case of 

 the iatermingiing of races or varieties, the progeny of which 

 mixture reverts to the one form or the other of the parents? 

 The black iadividual mentioned by Mr. Hepburn, in the brood 

 of two gTey birds or Hooded Crows, would appear to point to 

 such being the fact. 



The only character distinguishing the Hooded from the Car- 

 rion Crow is the grey colour of the back and belly of the former ; 

 in form, size, and proportions they perfectly agree ; the habits 

 are very similar, and the nest and eggs are undistinguishable. 

 In fact, it would seem that the Hooded Crow is a northern, the 

 Carrion Crow a southern race of one and the same species. And 

 that at the boundary line, where the two races meet, they pair 

 freely together and breed without the least reluctance. The 

 northern form migrates in winter southwards, and considerably 

 overlaps the boundary line, and flits again in spring, a few indi- 

 viduals occasionally remaining and pairing with their southern 

 kindred, as, for instance, the Scarborough example mentioned by 

 Yarrell. A solitary occurrence of the same kind in N'orthumber- 

 land came to my knowledge some years ago. There is nothing 

 extraordinary in this ; for many species of birds of partially mi- 

 gratory habits do the same ; that is, while some individuals re- 

 main in the country to breed, others leave to do so in their 

 northern haunts. The Woodcock is a notable example of this 

 fact ; the Crossbill, Short-eared Owl, Siskin, Lark, Golden-crested 

 "Wren, Ruff, and many others do so likewise. 



16. Rook. C. pktjgiiegijs, Linneus. 



Corvus frugilegus, Bewick, Hist. Brit. Birds, Ed. 1847, I., 78. 

 „ Yarrell, Hist. Brit. Bii'ds, Ed. 2, II., 91. 



There is scarcely anywhere a well wooded domain in the two 

 counties without its rookery ; indeed, it almost seems that this 

 resident species is increasing since its natural check, the larger 

 birds of prey, have been removed. 



