OYSTEBGATGHEB AND NATURAL ENVIRONMENT. 381 



and Lapwing, has been observed to threaten only the very young 

 Oystercatcher. The effect of ground vermin is not known. But 

 no eggs or young under observation have disappeared without the 

 disappearance being accounted for, at least as well, in other ways. 



In the breeding season, so far as one is able to judge, the 

 succession of day and night is a factor, but not an important 

 one, in regulating the activities of the birds. The diurnal cycle 

 is rather complex, but it may be summed up as a periodic 

 alternation of rest and activity, the latter being of two kinds — 

 nuptial or social, and food activities. It should, however, be 

 understood that these periods are not simply periods of rest 

 and activity. All that can be said is that in one period 

 the resting phase predominates, in the next the active phase, 

 and so on alternately. The period of greatest darkness lasts 

 from about 11 p.m. to 1 a.m. Contrary to what one might 

 expect, it is not a period of rest but is rather a period of 

 the wildest activity, and that apparently not of food-getting, 

 but nuptial. Activity continues till 4 a.m., but it passes gradu- 

 ally into food-getting for both adults and young. After 4 a.m. 

 the sun begins to diffuse an appreciable warmth. Activities 

 lessen, and a quiescent period appears which lasts till 10 a.m. 

 During the earlier part of this period most of the " apparent " 

 sleep is obtained. Ten a.m. till 2 p.m. is a fairly active period, 

 devoted mostly to food-getting. From 2 p.m. till about 6 p.m. 

 the birds are mostly resting except for occasional and short- 

 lived storms of either kind of activity. After 6 p.m. the birds 

 are really active, at first in getting food for themselves and the 

 young, and then, by degrees, the feeding activity is interrupted 

 by the nuptial activities which later work up into the fully 

 developed midnight orgy. Such is in general outline the 

 diurnal rhythm of the Oystercatcher at an inland breeding 

 station. Exceptional features, of course, occur at any hour and 

 place, and very frequently the normal rhythm is entirely upset. 

 On very dark nights all activities, so far as one can then 

 determine, are suspended for the hour or so of greatest dark- 

 ness. On very clear nights many birds (? barren) leave the 

 breeding areas and make extended journeys into the hills, 

 where they are not seen in the daytime. 



It will be observed that the two periods of rest coincide, the 



