Volume 1. Number 13. 



SPECIALIZATION OF TAIL DOWN 

 IN CERTAIN DUCKS 



By C. William Beebe, Curator of Birds, and 

 L. S. Crandall, Assistant Curator. 



The careful examination of the birds which are constantly 

 added to the collection of the New York Zoological Park is made 

 for a two-fold purpose. First, of ascertaining their physical con- 

 dition, freedom from disease and their general health, and sec- 

 ond, to obtain data in regard to the characters revealed by the 

 living bird, especially as to the plumage; its moult, structure, 

 pattern and color in continuance of the experiments which have 

 been conducted in the Department of Birds in the past. Unex- 

 pected and interesting conditions are occasionally observed, 

 worthy of record, but not dealing directly with the evolutionary 

 problems which the Curator at present has under consideration. 



On examining several chicks of the American Wood Duck, 

 Aix sponsa (Linnaeus) hatched in the Zoological Park on July 

 12, 1912, it was noted that the down or plumaceous feathers rep- 

 resenting the rectrices at birth, were remarkably stiff and long 

 in comparison to the other feathers of this plumage. The left 

 central rectrice down was measured and found to be 29 mm. in 

 length. Careful watch was kept of a young bird selected for 

 experiment but it was not until July 22, the tenth day after 

 hatching, that an increase in length was noted. On this day, the 

 feather mentioned above had reached a length of 32.5 mm. It 

 soon became evident that this gain in length was due to the 

 pushing out of the rectrices of the juvenile plumage, carrying 

 the plumaceous down on their tips. This left central rectrice, 

 together with its burden of down, was measured carefully every 

 other day for a period of a week, the growth being as follows: 



