Mar., 1910 
HABITS OF THE BLACK CABT VIREO 
71 
however, includes only one adult female (there are eighteen adult males) if the sex 
has been correctly determined in all cases, shows that the sexes are alike and that 
the presence of black on the head or its extent is probably a matter of age. The 
grayer headed specimens invariably have the whites of the under parts less pure, in 
this respect being more or less like young birds in their first autumn which lends 
probability to the theory that the relative age is the true explanation of the varia- 
tions noted.” 
As for myself, however, I think that Mr. Ridgway lackt sufficient material for 
examination, and has been led astray by the mistakes of collectors in determining 
the sex correctly in some of the specimens. I spent three weeks with the Black- 
caps at their breeding time, when they were in full plumage, and had ample 
time and opportunity to study 
them. I watcht them build 
their nests, incubate their eggs 
and feed their young. I dis- 
sected over thirty of them, and 
can say that the sexes are 
not alike , and that the female 
docs not have a distinct black 
cap at any age. This matter, 
as to whether the sexes of 
Vireo atricapillus are differ- 
ent in plumage or not, has 
been discust by several writers 
since the year 1878, and both 
conclusions have been reacht. 
For several years I have been 
of the opinion that the sexes 
arc different, and after an ex- 
amination of nearly forty 
skins I am firmly convinced 
that such is the case. 
The material on which I 
base' the following conclusions 
numbers nineteen specimens, 
six adult males taken in July 
1901, seven adult males and 
five adult females in May, 
1902, all taken in Blaine 
County, Oklahoma; and one 
adult male taken in Comanche County, Kansas, in May, 1885. 
When compared with the males, it is at once apparent that the females have a 
distinct buffy tinge on the under parts, strongest on the breast, and nearly, if not 
entirely, absent from the throat and abdomen. Above, the olive-green tint of the 
back averages duller in color than in the males, tho this character is not strongly 
noticeable in some instances. The wing bars and the light edgings to the tertials 
are also paler — nearly white — in place of the yellow of the males. The same is 
true, to a less extent, of the light greenish edgings of the rectrices. In the males 
the head and neck — except the throat, loral streak, and an orbital ring of white 
— are clear black, while the same region in the females is slate-gray. (See 
Fig. 24.) The white of the loral and orbital region is clearer in the male, tho 
Fic 
. 24. DORSAL SURFACE OF MARE AND FEMALE BLACK 
CAPT VIREO, ILLUSTRATING SEXUAL DIFFERENCES 
IN COLORATION AND MAKINGS 
