280 LIFE HISTORIES OF NORTH AMERICAN BIRDS. 



As already stated, trees seem to be preferred for nesting sites, and large 

 pines are oftener made use of for this purpose than other kinds. In certain 

 portions of Florida they resort to a considerable extent to the mangroves, 

 occasionally to live oaks, and in the West I have seen them nest in huge 

 cottonwood trees. 



The height from the ground varies considerably also, the extremes 

 being probably from 20 to 100 feet. Cliff and bluff sites are generally used 

 when no suitable trees are to be found in the vicinity, and nests on the 

 ground must be considered as unusual locations and of very rare occurrence. 



Incubation lasts about a month and both sexes take part in this duty. 

 The usual number of eggs laid by this species is two, rarely less, and very 

 seldom three; they are laid at intervals of three or four days. One of 

 them is always somewhat larger than the other, and occasionally this differ- 

 ence in size is quite marked. But a single brood is raised in a season. The 

 young at the end of the first year are considerably larger than the parents, 

 and were for sometime considered as a distinct species, and named by Audu- 

 bon Halicetus washingtonii — the Bird of Washington. In this plumage they 

 are dark brown throughout, mixed with dull fulvous; they do not attain 

 the adult plumage until the third year. 



The nests vary greatly in size and are usually almost flat on top. Some 

 are fairly well lined with dry grasses or seaweed. The eggs are pure white 

 in color, but frequently nest stained ; in very rare instances slight traces of 

 markings are observable of a pale buffy brown, and in the handsome series 

 of eggs of the Bald Eagle in Dr. Ralph's collection are two such specimens. 

 The shell is strong, granulated, and without any luster. The shape varies 

 from a rounded ovate to an ovate, the former predominating. Eggs from 

 the more northern breeding grounds are considerably larger than those from 

 Florida and the Gulf coast. 



Sixteen specimens in the U. S. National Museum collection, principally 

 from the Arctic regions, give an average measurement of 73.5 by 57.5 milli- 

 metres, the largest egg measuring 76.5 by 58, the smallest 69.5 by 56.5 

 millimetres. 



Forty-five Florida specimens, all collected by Dr. Ralph, and now in 

 his collection, give the following average: 69 by 53.5 millimetres, the largest 

 egg of this series measuring 74.5 by 55, the smallest 61.5 by 50 millimetres. 

 About a fair average would be 71 by 54 millimetres. 



The type specimen, No. 20697 (PL 9, Fig. 7), from a set of two, Ben- 

 dire collection, was obtained near Alden, Iowa, April 18, 1873, and was slightly 

 incubated when found. 



