_ 

 BIRDS AS REGULATORS OF OUTBREAKS. 79 



short-eared owl is one of our most valuable winter visitors, 

 arriving about October and leaving usually in March. It fre- 

 quents open moors, alights and secretes itself on the ground 

 in preference to trees, and feeds by day as well as in the 

 evening. In this winter of 1887-8 the moors were crowded 

 with these birds, it being no uncommon occurrence to start 

 two or more at the same time from the long grass ; the ex- 

 planation of their numbers no doubt being that the preceding 

 dry summer had been most favorable to the increase of the 

 animal life of the moors, which supplied ample food and in- 

 ducements for the birds to congregate." 



Mr. W. H. Hudson, in his fascinating book, " The Natu- 

 ralist in La Plata," gives a graphic account of the suppression 

 of an outbreak of mice on the pampas of South America. 

 These little creatures had increased to an enormous extent, 

 and animals of many kinds lived upon them. u ln the au- 

 tumn of the year countless numbers of storks (Ciconia mag- 

 nari) and of short-eared owls made their appearance. They 

 also came to assist at the general feast. 1 ' The mice were 

 soon reduced in numbers to a point far below their normal 

 limit. A similar abundance of birds is noticed, Mr. Hud- 

 son says, whenever other animals grasshoppers, crickets, or 

 frogs become excessively numerous on the pampas. He 

 explains the concentration of these birds usually seldom 

 seen upon the spot where food abounds by the statement 

 that when not breeding they are constantly travelling in 

 search of food, flying at great heights and covering a large 

 territory in their wanderings. " When the favorite food of any 

 one of these species is plentiful in any particular region, all 

 the individuals that discover it remain and attract to them all 

 of their kind passing overhead. This happened in the pampas 

 with the stork, the short-eared owl, the hooded gull, and the 

 dominican or black-backed gull, the leading species among 

 the feathered nomads ; a few first appear like harbingers ; 

 these are presently joined by new-comers in considerable 



