344 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 9 



by finding her on a new-laid egg. Only one egg is laid and incuba- 

 tion is equally shared. Thus, for instance, the male would spend 2, 3, 

 or 4 days on the egg without quitting the burrow, then the female 

 would take over. On dark nights the bird at sea would return and 

 converse with the sitting bird for an hour or two, but would not 

 necessarily relieve it. Nor could we get any evidence that it fed its 

 sitting mate. We came to the conclusion that the sitting bird stuck 

 to the egg as long as hunger permitted, or as long as it could retain 

 possession of the egg against its mate's determination to brood it. 

 This at least appears to be the explanation of the irregular shifts or 

 watches by one or other of the pair on the egg on dark nights. 



On moonlit nights, however, this ardor to incubate was cooled by 

 what we presume must be the bird's fear of being seen and killed 

 by the predatory gulls and hawks which frequent the island, as 

 already mentioned. So when a period of moonlit nights intervened 

 the bird at sea never visited the bird on the nest at all. Thus for 

 5, 6, 7, and more rarely up to 10, and once 12 nights and days, when 

 the moon happened to be near or at the full and the skies cloud-free, 

 the sitting bird remained brooding but starving at the nest. We 

 even weighed some of these starving birds and proved an average 

 loss of a very small fraction of an ounce every 24 hours. Starving 

 is really the wrong word, though at the time it seemed appropriate 

 in our view. Now we have learned that a sea bird can easily endure 

 long fasts and no doubt this fact will help us to understand how the 

 sea bird survives long storms at sea, when the weather conditions are 

 such that feeding may be impossible and the bird's energies may be 

 entirely directed to fighting the storm. 



The incubation period of the shearwater we found to be a record 

 for a British breeding bird — 50 to 54 days. One parent remained in 

 the burrow by day to brood the downy chick for the first week of 

 its existence, but afterward it was only visited by night. Wlien the 

 moon was bright at night, the same thing happened as during incu- 

 bation — the burrow was not visited at all. The young chick thus 

 early had its first lesson in fasting. However, it was fat from the 

 day of its birth, and showed no perceptible sign of going back during 

 the occasional enforced fasts, in fact it seemed simply to sleep and, 

 so to speak, consolidate the position already gained. At any rate 

 this program of cramming interspersed with an occasional fast re- 

 sults in the chick becoming enormously fat by the time it is 60 days 

 old. To our surprise we now found that the parents deserted the 

 chick. They had spent 60 days busily gathering fish for it and 

 many nights feeding it from the supply of semidigested food stored 

 in the parental crop. Now suddenly and completely they gave up 

 all this, and stopped visiting the burrow, and probably the island. 



