220 BULLETIN OF THE 



Individual Variation in the Size and Form of the Bill. — That con- 

 siderable variation occurs in the size and shape of the bill, in specimens 

 of the same sex and species living together at the same locality, is evident 

 from a glance at some of the preceding tables of measurements. The 

 variation in this organ is further illustrated in the accompanying plates 

 (Plates IV- VIII), in which are given figures of the bills of several 

 specimens of each of a number of species. Much greater differences 

 are here shown to exist in cospecific specimens of the same sex 

 and from the same locality than occur between those supposed to 

 be distinct, of which comparative figures of the bills have been pub- 

 lished with a view of demonstrating their specific diversity. In only a 

 few groups in fact, and mainly in the long-billed Grallce, is the bill 

 generally admitted to be too variable to afford an important basis for 

 the discrimination of species. 



The principal points of variation in the form of the bill eonsi.-t in 

 variations in its general size, without corresponding variations in the 

 general size of the individual, and in the details of its form in regard to 

 thickness and length. There are also other variations in respect to the 

 emargination or dentation of the terminal portion, especially in the vast 

 group of the insectivorous species, and in the "festooning" of the bill 

 in many of the hawks.* In respect to the size of the bill, it is a note- 

 worthy fact that birds specifically and sexually identical vary in such a 

 way that specimens much below the average size possess bills above 

 the average size for their respective species, and, conversely, that 

 specimens above the average size have bills much smaller than the 

 average for their respective species, the general proportions of the 

 bill in each case being essentially the same. In such cases, with 

 the increase or decrease in length, there are corresponding differ- 

 ences in the thickness of the bill, both in the vertical and trans- 

 verse directions. In other cases with the increase in length there 

 is no corresponding increase in thickness, such a differentiation thus 

 resulting in a relatively attenuated form of the bill. In other cases 

 the bill is shortened without a corresponding decrease in its thick- 

 ness, from which results a short, thick, or robust bill. The variation 

 in thickness is again sometimes relatively greater in the vertical 



* In respect to this point, see. Dr. Henry Bryant's paper on " Variations in the Plu- 

 mage in Buleo borealis auct. and B. Hwlani Aud.V" (Proc. Boston Soc. Nat. Hist., 

 Vol. VIII, p. 107 et $eq., Ifc6l, where the variation in this feature is especially noticed. 



