MUSEUM OF COMPARATIVE ZOOLOGY. 233 



" those from Eastern Labrador and Newfoundland," he says, appear- 

 in^ " to have decidedly broader, stouter, and more convex bills than those 

 from the Hudson's Bay and more northern countries."* In the writings 

 of various authors on the birds of Southern Mexico, Central America, 

 Southern Asia, and Northern Africa, frequent mention is incidentally 

 made of the larger size of the bills of southern representatives of north- 

 ward ranging species. Although such statements record what have 

 been apparently regarded as only isolated facts, their frequency indicates 

 that the increase in the size of the bill to the southward is not confined 

 to the birds of Eastern North America, nor exclusively to those of 

 temperate and sub-tropical countries, but that it is a general geograph- 

 ical law, similar to that of the variation with locality in the general bulk 

 of the individual. 



Geographical Variation in Color. — Geographical variation in color 

 in birds may be regarded as of two kinds, which may be termed, from 

 their different geographical relations, latitudinal variation and longi- 

 tudinal variation. The first is coincident with differences in latitude, 

 and the second with differences in longitude. Both are due, however, 

 to climatic peculiarities, and are hence, strictly speaking, climatic. The 

 latitudinal is perhaps at present the best known, and will be first con- 

 sidered. 



(a) Latitudinal Variation. — ■ In those species of North American 

 birds whose breeding range extends over a wide range of latitude, the 

 southern-born specimens are, as a general rule, appreciably darker or 

 brighter, or more intensely colored, than northern-born ones of the 

 same species ; in many instances the difference being so great as to im- 

 press even the casual observer. Dark colored birds, like the Quiscali, 

 Agelceus phceniceus, etc., become blacker towards the southern limit of 

 their respective habitats, where tho.-e with metalic reflections have the 

 iridescence more intense and of a darker hue, greenish and bronzy re- 

 flections changing to purple. The slaty, ferruginous, and olive tints, and 

 the various shades of red and yellow of others, become also far more 

 intense. In species barred transversely with dark and light colors, the 

 dark bands, as a general rule, become broader, and the light ones 

 narrower. Those with white spots on a black ground have the spots 

 reduced in size and number, the smaller ones becoming ob-olete. 

 White bars on the wings and terminal white spots on the tail feathers 

 * Birds of X. Amer., p. 634. 



