28 BULLETIN 86, UNITED STATES NATIONAL MUSEUM. 



both above and below, after the breeding season ; and the sometimes 

 more deeply ochraceous or tawny of the light markings in freshly 

 plumaged birds. 



Molt. — The natal plmnage is naturally of brief duration, and with 

 the beginning of the bird's growth in size passes directly into the 

 Juvenal plumage, which, in turn, through a practically continuous 

 molt gives way to the first autumn plumage by at least some part of 

 September, at which time the bird is full grown. During this process 

 the wing-quills and rectrices are grown only once, but the contour 

 feathers of the juvenal plumage are wholly or in large part replaced. 

 This livery is worn until January or February, when by a complete 

 molt the fully adult plumage is assumed. 



The adult birds after the breeding season molt usually the contour 

 feathers, but not the wing quills and rectrices, this taking place be- 

 tween the middle of July and the middle of September, mostly 

 between July 15 and August 15. The birds are thus, with wings and 

 tail intact, even though molting other feathers, free to start on their 

 migration. During January or February another molt, this time 

 complete, takes place, so that by the time the birds are ready for 

 their long northward journey they have an entirely new suit. These 

 different plumages have already been described. 



Individual variation. — The differences exhibited by various indi- 

 viduals of the same sex and age is very great, with respect to both 

 size and color, in all of the forms of this species. This variation in 

 color affects chiefly the amount of light mottling on the upper sur- 

 face ; the fineness or coarseness of these markings ; their color, which 

 in some subspecies is so decided that it amounts to two phases; the 

 width of the dark barring on the abdomen; the width and number 

 of dark bars on the crissum, these in some individuals being heavy 

 and close together, in others narrow or far apart ; and the length of 

 wing and tail. In only one or two of the subspecies are there really 

 two color phases, gray and ochraceous or tawny, for such differences 

 are usually subspecific. There is, moreover, usually not much purely 

 individual variation in the shade of the dark brown or blackish 

 areas, though occasionally such instances crop out in some of the 

 races. 



Geographical variation. — For a bird so highly migratory, Chordeiles 

 virginianus has responded rather remarkably well to the climate, 

 environment, and other modifying influences of the various parts of 

 its breeding range. The number of subspecies that now appear 

 recognizable is nine, an increase of two over the previous number. 

 Two of these are West Indian, the others continental. Nearly all of 

 them correspond very well to the characteristics exhibited by races 

 of other species which range over areas of similar physiographic 

 conditions, the principal exception being Chordeiles virginianus hes- 



