FOREWORD 13 
birds retain this color very well in captivity, even after 
several moultings. 
The source of the copper has long been uncertain. In 
nature the birds are fruit-eaters, and their diet in cap- 
tivity consists of bone meal, zweibach, corn meal, wliite 
potatoes, eggs and carrots — foods that are usually 
assumed to contain no copper ; and no copper utensils are 
used in the preparation of the food. Dr. John Marshall, 
however, writes me that all the common cereals contain 
minute quantities of copper; and Dr. Leon A. Ryan, 
University of Pennsylvania Medical Bulletin, June, 1907, 
states that copper may be found in animal tissues. Dr. E. 
P. Corson-White has found traces of copper in the bone 
meal used at the Philadelphia Zoological Garden. The 
copper in the red color of the touracou's wing therefore 
comes from the food. 
It is probable that a bright and cheerful bird house 
does not influence the color of birds by the direct action 
of light on the color as much as indirectly by improv- 
ing the health and spirits of the birds. Coloration 
in birds is a very complicated proposition. It depends 
upon age, sex, season, health, light, heat, moisture, mode 
of life, and food. No one bird house can combine all the 
conditions necessary for the retention of natural colors 
in every species. The desert species from a habitat of 
intense light and dryness require for their color a dif- 
ferent environment from the forest species, from a 
habitat of shade and moisture. The suppression of 
sexual feeling in captive birds may influence color. In 
nature the finest colors are attained by mating birds. 
It may be said that all animals — except those of noc- 
turnal habit — have a feeling of joy and well-being in fine 
weather and bright surroundings that reacts favorably on 
the general health. 
The variability of the breeding period induced by 
captivity in many animals may be mentioned with 
sterility. It was shown some years ago in the Philadel- 
