OTES 



ON THE CROSSING OF THE BILL OF 

 THE CROSSBILL. 



It has long been known that, in the Common Crossbill {Loxia 

 curvirostra) the mandibles cross indifferently on either side 

 in different individuals. Recently, however, for a special 

 purpose, I desired to ascertain whether or not individuals 

 having the mandibles crossed to the right and to the left, 

 respectively, exist in nature in about equal numbers, and 

 (if not) what proportion of individuals has them crossed to 

 the right and what to the left. Hitherto, this point has 

 not been investigated, so far as I am aware. 



Accordingly, at the British Museum (where they have a 

 very extensive series of Crossbills), I examined the contents 

 of ten boxes, selected at random. These contained altogether 

 one hundred and seventy-one specimens, of all sexes, colours, 

 and ages (other than nestlings), coming from all parts of 

 the world to which the bird is indigenous. 



The result of my investigation was that there were eighty- 

 four specimens which were dextral, eighty -three which were 

 sinistral, and four which were doubtful. By a " dextral " 

 specimen, I mean one having the upper mandible crossing 

 the lower to the bird's right side (that is, to the left side of 

 an observer facing the bird) ; and by a " sinistral " specimen, 

 one having the upper mandible crossing the lower to the 

 bird's left side (that is, to the right side of an observer 

 facing the bird). 



Such close equality in the numbers of the two forms is 

 quite surprising. If the proportions of the two forms 

 observable in these particular specimens are fairly typical 

 of the proportions which obtain in nature (and I saw no 

 reason to doubt that they were), it appears that the two 

 forms exist in astonishingly equal numbers.* 



* There must surely have been some error of observation in con- 

 nexion with the statement (Mag. of Nat. Hist., 2nd ser., I., p. 165 : 

 1837), by my old friend the late Mr. Joseph Clarke, F.S.A., of Saffron 

 Walden, that " the mandibles of these birds are indiscriminately 

 crossed, though, in by far the greater proportion of those that came 

 into my hands, the upper crossed the lower one to the right." Mr. 

 Clarke adds, in a footnote, that " only three out of twenty-seven 

 that I examined [in 1835] were crossed to the left." 



