LIFE HISTORIES OF NORTH AMERICAN GIILLS AND TERNS. 293 



The ground color shows a great variety of shades of olive and buff, 

 from " Dresden brown " to " clay color," and from " deep olive buff " 

 to " ivory yellow." The eggs are usually heavily marked with the 

 darkest shades of brown, " blackish brown." " seal brown." " bister," 

 and " mummy brown ;" sometimes the eggs are evenly sprinkled all 

 over with fine dots, but more often with larger spots and blotches 

 unevenly arranged. Frequently the markings, of either type, be- 

 come confluent in a ring around the larger end of the egg, and occa- 

 sionally large bold markings of different shades of brown produce 

 handsome effects. The measurements of 122 eggs, in the United 

 States National Museum, average 34 by 24 millimeters; the eggs 

 showing the four extremes measure 37 by 25.5, 35.5 by 26.5, 31.5 by 

 23 and 32 by 22.5 millimeters. 



In the southern portions of its range the black tern apparently 

 raises two broods, at least occasionally, for fresh eggs are frequently 

 found in May and again in July. As very few, if any, of the water 

 birds raise two broods in a season, it may be that these late sets may 

 be second or third layings of pairs previously robbed of their eggs. 

 Some good observers, however, seem to think that two broods are 

 regularly raised in some localities. The full set is almost always 

 three eggs, occasionally two, and very rarely four or even five. Per- 

 sonally I have never seen a set of four eggs, and think it is an ex- 

 ceedingly rare occurrence. Smaller sets than these are probably 

 second layings. Dr. E. W. Nelson, according to Dr. T. S. Koberts 

 (1877), says: 



I have seen the eggs of Sterna plumhea deposited on masses of floating 

 weeds in several instances, but only for the third brood, the bird having pre- 

 viously built two nests and deposited the eggs in both, which had been re- 

 moved by myself to ascertain how many they would lay. The result was 

 almost invariably as follows : First nest, three eggs ; second nest, two eggs ; 

 and the third, one egg. 



Dr. Frank M. Chapman (1904) has learned that the period of 

 incubation is 17 days. Audubon (1840) states that both sexes incu- 

 bate — a difficult matter to determine, as a black tern is seldom seen 

 on its nest. 



Young. — The young remain in the nest but a few days, and even 

 before that time they will leave it on the approach of danger. Doctor 

 Chapman (1904) gives an interesting illustration of this in his ac- 

 count of the home life of this species. He says : 



Three days later we visited the nest, expecting to see a pair of downy 

 young, but, to our surprise and disappointment, it was deserted. Evidently, 

 however, there was something not far away in which the terns were greatly 

 concerned. With piercing screams they darted at us, once actually hitting Mr. 

 Seton's hat. 



