OYSTEEGATGHER AND NATURAL ENVIRONMENT. 465 



(3) A distant or general feeding-ground resting on boulder- 

 clay or alluvium, and preferably cultivated, to which the adults 

 may resort for food, and from which they may bring a large 

 supply of large earthworms to the young. Auxiliary conditions 

 which have some value are : 



(4) The presence of a wet sand-beach in some part of the 

 breeding territory ; and 



(5) An abundance of Stonefly nymphs, along the river 

 margin, at the time when most of the young Oystercatchers 

 escape from the eggs. 



Eesume and Conclusion. 

 The general results of the inquiry point to the mud-flat in 

 winter, and the drift-river-valley in summer as optimum 

 habitats within the areas under observation. The presence of 

 a large human population has a pronounced effect on the dis- 

 tribution and numbers of the Oystercatcher. The Law of 

 Territory is shown to be valid for the Oystercatcher, both in 

 winter and in summer. The arrangement and extent of the 

 territories, and the general movements of the birds therein, 

 remain the same year after year. The theory of the biological 

 advantage, or necessity of conserving the food-supply for the 

 exclusive use of the settlement, by the maintenance of territorial 

 rights, is supported by observations made at one winter station, 

 where the food- activities of the settlement were found to strain 

 the reproductive powers of the shellfish of the area. In relation 

 to climatic conditions certain activities, normally successful, 

 become ill-regulated when they are elicited at an abnormal 

 period. The intangible associational barriers of Grinnell prove 

 to be fully applied to the Oystercatcher, both in winter and 

 in summer. They are (1) kind of food-supply afforded ; (2) pre- 

 sence of safe breeding-places, and (3) presence of places of 

 temporary refuge for individuals, when hard pressed by pre- 

 datory enemies.* The last condition, though long known, had 

 not previously been given the formal position its importance 

 deserves among the factors of distribution. In summer for the 

 young, and in winter for the settlements of the Oystercatcher, 

 places of refuge are an important factor controlling distribution 

 in the areas under observation. 



* Amer. Nat. 1914, vol. 48, p. 252. 



