CALCUTTA SPARROW-HAWK. 193 



Mr. Gurney says that there is a constant sexual 

 difference of size between the two birds; that is, that 

 the difference between the male and the female of M. 

 brachydactylus is always less than between those of 31. 

 hadius. This certainly was the case with those he 

 kindly sent me, as will be seen by referring to the 

 table in the previous notice. But then, if my surmise 

 is true, that they are varieties of the same species, 

 how are the skins originally selected by the collector, 

 and se]3arated into the two divisions? Why surely by 

 some rule as arbitrary as that of a sexual difference 

 in size. Now how would such a rule apply to our 

 own A. nisus? Why the difference between the male 

 and female of various specimens is very far greater, 

 as far as my experience goes, than that shown to exist 

 between M. hrachydactylus and M. hadius; and if 

 these were separated from each other, we should have 

 an equal right to call them two species. With regard 

 to M. sphenurus, Ruppell, it will be seen by reference 

 to the table in last notice, that the dimensions are 

 less than those given for either hrachydactylus or 

 hadius, and E/Uppell's figure, plate 2, drawn to a scale, 

 is even smaller than that from which the description 

 was taken. Both were males; what then was this 

 bird? The skins sent for it by M. Verreaux and M. 

 Lauretta have been figured, and their dimensions given 

 in the last notice. The slightest examination will 

 show how different are the two birds. 



Has then M. sphenurus Riippell, become mingled 

 with M. hrachydactijlus? Dr. Hartlaub, in a note to 

 M. hrachydactylus, in his "Ornithologie West Africa's," 

 p. 14, says he thinks they are distinct, and gives as 

 a reason that the cross bars on the tail feathers are 

 right through, while they are only on the inner web 



