204 BIRD STUDIES WITH A CAMERA 
north and the south, prevents them from joining 
many other birds en route. However, apparently 
most of the birds are warned at nearly the same 
time by a physiological change that the season has 
come for them to return to their nesting grounds. 
This is evidently in January, since in March a large 
number of the young on the island were found 
almost ready to fly, while some, as has been said, 
were already on the wing. There was, it is true, a 
great variation in the development of the young 
found, and indeed the birds were still laying, but I 
believe that the parents of these later broods had 
been robbed of their eggs by tourists. 
A careful count yielded a total of 845 nests, 
which had evidently been built during the season, 
but only 251 of them were occupied. Most of the 
vacant nests were on the ground, and had been 
deserted by their tenants, who were now running 
about the island. 
The 251 occupied nests contained eggs or young, 
as follows: 
55 nests with 1 egg each; 
Come “ 2eggs “ 
95 “ “ 49 “ “ 
23 3 
63“ “1 young each ; 
46 (73 6 D) (79 (%3 
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Incubation was found to be well advanced in 
eggs which were alone in their nest, showing either 
that one egg sometimes composes the set, or that the 
other eggs of the set had been destroyed. The fact 
that one nest was found with three young while 
twenty-three were found each containing three eggs, 
