OYSTERCATCHER AND NATURAL ENVIRONMENT. 291 



daylight. The accumulation of excremtnt on the high-water 

 mark of the night tide, when the latter reaches its highest point 

 hours after darkness sets in, is sufficient evidence of night 

 feeding, for the process of digestion and excretion in the Oyster- 

 catcher is extremely rapid. Darkness, however, has a restraining 

 influence on nocturnal flight ; for, whenever high water occurs 

 in Aberlady Bay near the time of sunset, the birds, if disturbed, 

 show unwillingness to fly to the refuge on Eyebroughty, and 

 remain over the bay, prosecuting an aimless flight until the tide 

 ebbs far enough to enable them to alight out of range. Further, 

 by night the birds remain, so far as known, in the central parts 

 of the feeding-grounds. They extend their feeding excursions to 

 the more distant parts of their territory only in bright moonlight. 



In winter the influence of variations in climatic conditions is 

 mainly an indirect one, for the Oystercatcher is remarkably 

 hardy, no extremes of temperature or humidity likely to occur 

 in the Forth area seeming to have any harmful effect. Torrential 

 rain inhibits all except compulsory movements, until the need 

 of satisfying hunger becomes paramount. Eeally hard weather 

 increases the food activities, at the same time suppressing all 

 motor signs of the pleasure state. The appearance of the birds, 

 however, never suggests a condition of " physiological misery." 

 With the coming of the new year, moderately cold weather, if 

 associated with bright sunshine, is favourable to early nuptial 

 developments, while the mild weather of autumn brings out 

 " autumnal genital activities." 



Climatic control is exercised mainly over the available food 

 supply. The Oystercatcher cannot open a Mussel when it is tightly 

 closed, unless the byssal cleft can be reached, or the Mussel is 

 small enough to be crushed. The former possibility rarely 

 occurs ; the latter seems to be dictated by necessity and not by 

 preference. Extremes of temperature and moisture (bright 

 sunshine, heavy rain, dry winds, hard frost) lead to an early and 

 firm closure of the shell-valves of Mytilus soon after the Mussels 

 emerge from the tide. The search for Mussels is then greatly 

 restricted. The climatic aspect of the problem is important, 

 for, so far from being a rare occurrence, weather conditions 

 affect the food supply on almost every tide. 



(To be continued.) 



