6o 



BIRDS OF AMERICA 



1,'reater depth than do other Terns. When breed- 

 ing in colonies, the Cabot's Terns often place 

 their nests so close together that it is difficult 

 to avoid stepping on them while one is exploring 

 the premises. 



Some ornithologists attribute the remarkable 

 variation in the coloration of this Tern's eggs to 

 the fact that they are incubated alternately by 

 the male and the female, one bird being ready to 

 cover the eggs the instant the other leaves them. 

 Under these conditions the law of natural selec- 



tion cannot operate in such a way as to eliminate 

 an egg of conspicuous coloration, which is true 

 of many Terns' eggs. 



There is apparently reliable evidence that these 

 Terns mate for life, and return year after year to 

 the same nesting region, thou.gh not necessarily 

 always to the same spot. English observers have 

 noted that the birds change their actual breeding 

 ground from time to time, though apparently the 

 same general colony is likely to return to the 

 same island. 







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Photu liy H. K. Jub 



CABOT'S TERNS AND EGGS 

 Breton Island Reservation 



COMMON TERN 

 Sterna hirundo Liniucus 



\. O, V. Xuiiilier 71 



Other Names. — Sea .Swallow ; Wilson's Tern ; 

 Summer Gull : Mackerel Gull ; Lake Erie Gull ; Bass- 

 gull ; Red-shank. 



General Description. — Leuirtli. 15 inches. Color, 

 white with mantle of pale pearl-blue. 



Color. — Adults in Summer: Crown, lustrous green- 

 ish-black e.xtending to lower level of eyes; mantle, pale 



St-e Color Plate 7 



pearl-blue deepenin.^ on back, endin,g abruptly on rump 

 wliich. together with upper tail-coverts, is pure white; 

 throat, chin, and sides of head, pure white shading 

 insensibly to a much paler tone of color of mantle on 

 entire under parts ; outer primaries, grayish-black 

 strongly silvered ; secondaries, pure white shadin.g to 

 grayish-blue on end ; outer pair of tail-feathers, grayish- 



