204 BULLETIN 121, UNITED STATES NATIONAL MUSEUM. 



and size, but usually they are elliptical ovate, elongate ovate, or 

 elliptical oval. The underlying color of the shell is pale bluish 

 white, but it is usually completely or nearly covered with a thin 

 layer of white calcareous deposit. In some eggs thi^ deposit is 

 uneven, broken, or rough, but usually it has a smooth, clean, lustre- 

 less surface. 



The measurements of 40 eggSj in the United States National Mu- 

 seum and the writer's collections, average 59.4 by 40.2 millimeters; 

 the eggs showing the four extremes measure 65.5 by 41, 62 by 43.5, 

 52.5 by 40, and 66.5 by 34.5 millimeters. 



The male assists the female in the duties of incubation, but the 

 period of incubation seems to be unknown. The behavior of the birds 

 during the incubating and brooding period is fully described by 

 Doctor Chapman (1908a) as follows: 



One or both of the adults remain, as a rule, with the young. On March 

 9 the birds awoke at 5.15 a m., when for the ensuing 10 or 15 minutes there was 

 a subdued liind of quacking, and some birds were seen flying. At 5.30 several 

 hundred birds left the rookery in a body to go a-flshing, this being the first gen- 

 eral movement. Individuals returned at Intervals during the day and evidently 

 changed places with the bird left at the nest, which in turn went out to feed 

 and to gather fish for the young. There was no concerted return movement 

 until dusk, when flocks of birds came in from the sea, the last comers not arriv- 

 ing until after dark. In the meantime the man-o'-war birds had retired, and it 

 is not impossible that the boobies have acquired the habit of " staying out late " 

 to avoid being robbed of their food by the man-o'-war birds, which at times 

 attacked them as they approached the gay and forced them to disgorge. 



Sitting or brooding birds spend the night upon the nest with the mates 

 standing at their sides, but the close resemblance of the sexes rendered it im- 

 possible to distinguish them at this time. When the young is too large to be 

 brooded, it passes the night on the ground between the two parents, which stand 

 on either side, all three with their heads tucked under their scapulars. 



When perched on rocks about the border of the island, boobies showed a de- 

 cided fear of man and generally flew before one had approached to within 

 30 yards of them; but when on their nests they were conspicuously tame, the 

 degree of tameness being related to the advance of the nesting season. A. bird 

 with newly hatched young would not, as a rule, leave the nest unless actually 

 forced to do so, and it would strike so viciously at anyone approaching that it 

 was well not to venture within its reach. This was the extreme development 

 of parental instinct, which now gradually diminished as the young increased 

 in size. Evidently as a result of excitement caused by our presence, the birds 

 which remained to defend their young threatened us with their bills, picked up 

 bits of sticks or grasses only to drop them and pick them up again, and even 

 struck at their own young in a confused and aimless manner. The young 

 also had this habit. The report of a gun occasioned but little alarm among 

 the boobies, some of which, with their young near my feet, did not fly when the 

 gun was discharged. 



In spite of the apparent sociability expressed by their communal habits, the 

 boobies immediately resented trespass on their home site by one of their own 

 kind. Where the nature of the ground permitted, their nests were placed with 



