228 BIRDS OF THE PLAINS 



of a mammal on a cold day. I have never succeeded 

 in seeing a bird's breath, so am of opinion that the fowls 

 of the air do not exhale so much moisture as mammals 

 do. But even allowing for this, a considerable amount 

 of moisture must be given out in expiration, so that 

 it seems probable that young birds require more mois- 

 ture than they obtain in their food. Drops of water have 

 to be administered to hand-reared birds. Many birds 

 fill up the crop with food and then discharge the contents 

 into the gaping mouths of their young. In this con- 

 dition the food must be mixed with a considerable 

 quantity of saliva and possibly with water. The crop 

 of a bird is a receptacle into which the food passes 

 and remains until actually utilised. There seems no 

 reason why water should not be stored for a short 

 time in this receptacle just as food is. Perhaps birds 

 " bring up " water as they do solid food, and thus assuage 

 the thirst of their young. Such a process would be very 

 difficult to detect ; it would be indistinguishable from 

 ordinary feeding to the casual observer. I hope that 

 some physiologist will take up the matter. A quantita- 

 tive analysis of the air exhaled by a bird should not be 

 very difficult to make. 



