INTRODUCTION. Xlll 



of Roach and Gudgeon with which the tributary 

 Brent abounds.* They also find plenty of frogs 

 and water-rats along the brooks. 



In the spring and autumn, when the wind blows 

 from the east and south-east, we frequently find 

 Gulls upon the river and larger pools ; and flocks 

 of graceful Terns may be seen fishing at the re- 

 servoirs when they visit us on their way to and 

 from their breeding-stations. The subject of mi- 

 gration is a curious one, and the laws which govern 

 it are yet imperfectly understood; but to advance 

 here all that might be said in this regard would be 

 beyond the limits of the present work. It appears 

 highly probable, from their constitution, that most 

 birds incline to remain as much as possible in 

 the same temperature throughout the year, and 

 hence their gradual movements north and south 

 as they feel the effects of heat and cold. If a 



* The reservoirs here referred to are those at Kingsbury, 

 Elstree, and Ruislip, and a sketch of the first-named, or 

 rather of a portion of it, was selected as a frontispiece from 

 the fact that so many rare and beautiful birds have been at 

 various times observed there. 



b 



