Sept., 1920 THE FUNCTION OF POWDER DOWNS IN HERONS 169 
beside the tip of the bill, in the powder downs, to feel the mandibles gently 
nibbling at the downy feathers and then to see the bill withdrawn with its sides 
covered with the grayish powder. Following this I observed as it was passed 
over other feathers. This process was repeated daily whenever | cared to see 
it. At the same time | discovered by examination that the uropygial gland, 
the usual source of oil for feathers seemed undeveloped and remained in a non- 
functional condition until the heron was practically grown. The bird in early 
life paid no attention to this gland but worked in either pelvic or pectoral 
down patches. The actual development of the oil gland I did not observe as 
the heron at this stage became so vicious toward others that I was forced to 
discourage its tameness until finally it left the laboratory. 
When attention was attracted to this peculiarity in the Great Blue Heron 
_I took oceasion to examine other young herons and found a similar condition 
existing in Snowy Herons, Black-crowned Night Herons and Bitterns. The 
accompanying illustration (fig. 36) shows well the relative size of the powder 
down tracts and the non-functional oil gland in a young Great Blue Heron 
(A. h. treganzart) about two-thirds 
grown. The oil gland is the small 
rounded object at one side be- 
tween the two pelvic tracts. In 
another cut (fig. 37) the young 
Great Blue Heron is shown secur- 
ing the greasy material from the 
powder down patch on one side of 
the breast. 
Parenthetically I may add that 
although on various occasions | 
examined powder down tracts in 
living and in dead herons I was 
unable to observe that these tracts 
Fig. 36. POWDER DOWN PATCHES AND UROPYGI- : i ; 
AL GLAND FROM GREAT BLUE Heron about Were luminous, in Spite of numer- 
TWO-THIRDS GROWN, DISSECTED FROM FRESH ous records on the part of others, 
IME S5 ELATIVE SIZE, P PEC- 
SPECIMEN TO SHOW R SIz i A ine contrary. 
TORAL POWDER DOWN TRACTS; Ff" PELVIC F ; 
TRACTS; U DEVELOPING UROPYGIAL GLAND Since making these notes on pow- 
(NOT YET FUNCTIONAL). der downs it has been my inten- 
tion to go farther into this subject and to study the development of these feath- 
ers and also of the oil gland. Asa matter of fact powder downs are so little un- 
derstood that ordinary definitions of them in text-books are vague and uncertain 
and their recognition in certain groups of birds is at times difficult. Where 
these downs are not segregated in definite tracts but are diffused through the 
pterylae it is possible, without careful attention, to confuse them with other 
feathers that have just begun to break their sheaths when in process of devel- 
opment. Similarity in function between powder downs and oil gland has sug- 
gested that these two organs may be homologous in origin. Should it prove 
that the powder down tract 1s a primitive arrangement from which the more 
complicated oil gland has developed then we may have a ready explanation 
for the occurrence of powder downs apparently at random in such diverse 
groups as Tinamous, the Whale-headed Stork, the Kagu, Sun Bittern, Mesite, 
herons and bitterns, the diurnal birds of prey (where they may be found in 
