KEEL-BREASTED BIRDS. 293 
of Embryology,” Foster and Balfour ; ‘Comparative Embryology,” F. 
M. Balfour ; Yarrill’s ‘‘ British Birds” ; Samuels's “ Birds of New Eng- 
land,” etc. ‘‘The Auk” is the official publication (Cambridge) of the 
American Ornithologists’ Union. Other magazines are “ Nature,” 
“Science,” “American Naturalist,” “Popular Science Monthly,” etc. 
Good popular works are those of Brehm, Cassell, and Wood. 
Note.— The Migration of Birds.—The majority of the birds that 
breed in the northern and middle sections of the United States migrate 
to the South at the approach of cold weather, and return in the spring, 
thus making two long journeys every year. These flights are made by 
night and day, and small birds have been seen at night through a tele- 
scope at an estimated height of three miles. The great valleys, river- 
courses, and coast lines are generally followed, but numbers of our 
birds stop at Bermuda, showing that they either venture to sea, or are 
blown out. Flocks of birds alight at Tortugas, Florida, during the 
prevalence of northers, that must have flown across the Gulf of 
Mexico. Many of the European birds spend the winter in Africa, 
while those in the United States go as far south as Central America, 
the West Indies, and even South America. During these flights they 
often congregate at certain spots in vast numbers; thus, on the Island 
of Heligoland, that lies in one of these paths, hundreds of different 
species are often seen resting, or at night whirling about the light- 
house, dashing against the glass, so that their dead bodies are found 
piled in heaps in the morning. . 
The primary cause of migration is probably lack of food as cold 
weather comes on, while many other reasons are given. Tropical 
birds that breed at home do not migrate, and many of our birds, as the 
crow, English sparrow, and others, remain with us the entire season. 
Many of the birds of the Rocky Mountain country have a limited mi- 
gration, and some of the smallest birds make the longest journeys. 
Thus, the warblers (Dendroeca) and others, that breed as far north as 
Hudson Bay, winter in Mexico. Asa rule, birds return to their sum- 
mer homes with great regularity, many varying season after season only 
a few hours. 
For further information on this subject see report of the American 
Ornithologists’ Union ; ‘‘ Distribution and Migration of North Ameri- 
can Birds,” Baird ; the works of Wallace, Von Middeldorff, Hodgson, 
Giebel, Palmén, and Parker, and Newton’s article on birds in “ Ency- 
clopedia Britannica,” ninth edition, vol. iii. 
