170 LIFE-HISTORIES OF BIRDS 



trifling moment. The cry of hunger is similarly expressed, 

 with a slight modification in intonation. The young birds 

 have a rather peculiar note, expressive of hunger ; and 

 another, indicative of anger and disgust, which is represented 

 by a loud hiss. 



The question has often occurred to many minds, why 

 should these birds prowl about in the uncertain light of the 

 evening, or in the dusk of night, in quest of food? There 

 is certainly but little in the structure of the eyes and their 

 appendages that can interfere with diurnal missions of 

 this character. A careful study of the nictitating membrane 

 has acquainted us with the fact that the most dazzling light 

 can be confonted without disadvantage, and with the full 

 unprotected eye exposed. The only change manifested is 

 a contraction of the pupil proportionate to the quantity of 

 light received. In the twilight and in the dark of night, 

 there is a manifest dilatation of this organ, corresponding to 

 the diminution of light. These facts are incontrovertible. 

 The only explanation seems to be this : in the twilight and 

 in the night, the small mammals which constitute such a 

 prominent part of their diet, are abroad in the greatest num- 

 bers ; also the various small birds upon which they subsist 

 to a certain extent, are the more easily surprised and cap- 

 tured ; consequently, the habits of these- birds are modified 

 in conformity with the peculiar habitudes of the animals 

 upon which they feed. The sleepy look which they often 

 exhibit during the day, and the consequent bewildering 

 movements which characterize their flight, are certainly due 

 to the above conditions of their existence. Human beings 

 would act likewise under similar circumstances. 



In the selection of a place for nesting purposes, these 

 Owls vary in different localities. In Eastern Pennsylvania, 

 generally a hollow tree, chiefly an apple or an oak, is cho- 

 sen : but occasionally, a dilapidated and unoccupied barn ; 

 but more rarely, an occupied building in close proximity 

 to man. When the former situations are chosen, the hollow 



