290 THE ZOOLOGIST. 



agent — have no apparent effect in the course of the season on 

 the general mass of the birds. Comparative immunity is due to 

 the wiidness of the birds and to their extreme sensitiveness to 

 the existence of potential dangers. Man, however, is himself an 

 important condition of the environment. He has modified the 

 local habits of the birds by leading to the establishment of 

 refuges, and by limiting the number and size of the available 

 feeding-grounds. The Peregrine is able to produce little more 

 than a momentary commotion in the ranks of the Oystercatcher : 

 no attack has been seen. The Sparrow-Hawk and the Kestrel 

 are ignored. The Great Black-backed Gull is treated with some 

 respect. The two species have not been seen very close 

 together. No attack has come under observation. Throughout 

 the winter the number of Oystercatchers in the area is not 

 appreciably diminished, though there must always be the loss 

 of a few. 



The activities of the Oystercatcher are controlled in winter 

 by the rhythm of the tides. The birds are slavish followers of 

 the tideline. For about four hours during each period of high 

 water the main supplies of food are inaccessible. As a conse- 

 quence, the periods of rest and activity are determined by the 

 tidal rhythm. There is also a rhythm of longer wave length 

 dependent on the fortnightly oscillation of the tides. This 

 rhythm is never manifest in early winter, but gradually appears 

 in consequence of human interference. It shows itself by day, 

 as a "reflex anticipation" of the greater height of the high 

 water of spring tides. It is not due simply to the tide carrying 

 the birds above the high-water mark of neap tides, for restless- 

 ness and the departure to the refuge occur long before the neap 

 high-water mark is reached by the tide. The basis of the 

 reaction is evidently complex. 



It is doubtful if a true tidal habit, diurnal or bi-weekly, is 

 ever established. No observations on this matter are possible 

 , on the shore. I have so far seen no sign of a tidal rhythm in 

 the habits of the birds when they begin to move up the rivers 

 where they breed. Diurnal rhythm is obscured in the winter 

 environment of the Oystercatcher, the activities of the birds 

 being regulated by the movement of the tides. Search for food 

 continues after dark, and is evidently carried on much as in 



