1590 THE STORY OF THE UNIVERSE 



of the intruders. Thus sparrows usurp the nests of 

 swallows, and starlings those of woodpeckers. Pheas- 

 ant and partridge eggs are sometimes found in the 

 same nest, and the same has been observed in many 

 cases e. g., gull and eider-duck. When artificial 

 nests are forthcoming, birds are glad to be relieved 

 of the labor of construction, and different birds thus 

 sometimes share a common box. The resorts of birds, 

 when convenient nooks are available, are often ex- 

 tremely curious. 



It is a well-known fact that comparatively few 

 birds (at any rate outside of the tropics) remain in 

 the same place all the year round. They do not hi- 

 bernate, but migrate on the approach of cold. Some 

 we know as winter visitors, returning north again in 

 spring, most we know only in summer, for in autumn 

 they fly to the warmer south; a third set we call 

 "birds of passage" ; for these we only know somewhat 

 incidentally as they pass through on their way else- 

 where. 



Thus the swallow, the cuckoo, the nightingale, 

 etc., come to Britain in summer and breed there, 

 being winter residents further south; the fieldfare, 

 jacksnipe, bean-goose, redwing, and some others, 

 reach Great Britain in winter, being summer resi- 

 dents and breeders further north; while the little 

 sandpipers are familiar examples of the true birds of 

 passage which we know only for a short time as they 

 rest on our shores in the journey south in autumn, and 

 north again in spring. These three classes are 

 obviously only different cases of one fact of migra- 

 tion. Almost all birds are in some degree migra- 



