226 BULLETIN OF THE 



cited. The Grallce have already been referred to as presenting re- 

 markable examples of bill variation. In some of the Anatidce, how- 

 ever, it is scarcely less, whilst it is especially great among many of the 

 Longipennes. Hence some authors evidently attach too high impor- 

 tance to the exact form of the bill in these groups. 



All the illustrations referred to above have been drawn, with one or 

 two exceptions, from fully adult specimens. One of these is a speci- 

 men of Passerculus savanna (Plate V, fig. 18), which is a bird of the 

 year, killed in Labrador in Augu.-t, before it had quite completed its 

 first moult. Another is the smallest billed specimen of Chrysomitris 

 trisfis (Plate V, fig. 10), which is also evidently a bird of the year. The 

 other is an autumnal specimen of Dendrceca striata (Plate IV, fig. 

 15). They all, however, would be ordinarily considered as adult in 

 size. 



Variations in the Size and Form of the Bill, Wing, etc., 

 resulting from age. 



In the foregoing remarks on the variations in general size, in propor- 

 tions, and in the form of different parts, exclusive reference has been 

 had to adult specimens. It is easy, however, to confound difference 

 depending upon age with those strictly resulting from individual differ- 

 entiation. The form of the bill is especially subject to variation 

 by age in specimens that upon casual inspection would seem to be 

 full-grown In long-billed birds the bill increases in length for several 

 months after the bird is full-fledged, and even after it has once 

 moulted. In short- and thick-billed birds, the bill increases considerably 

 in thickness as well as in length after the individual seems to have 

 acquired its adult size and proportions. As a general rule, then, " birds 

 of the year " possess a relatively shorter and thicker bill than those 

 fully adult, or three or four years of age. In old age an abnormal 

 elongation of the upper mandible occasionally occurs, especially in 

 species in which the tip of the upper mandible is decurved and projects 

 slightly beyond the lower, as in Corvus, Quiscahts, Vireo, Tta-dus, 

 Larus, etc. Since, however, great differences occur in the form of the 

 bill in specimens of the same age, in birds of the year as well as in those 

 unquestionably adult, it is sometimes difficult to determine how much 

 of the difference in certain cases is to be considered as due to age and 

 how much to individual variation. 



