• CHANGING THE SENTRIES 279 



other species against man, as gunners in this country 

 well know in the case of the Curlew and Redshank, 

 while in India the Ruddy Sheldrake is the best 

 known sentinel. In the case of this bird the motive 

 is certainly not philanthropic, or rather philornithic, 

 for, as may -be seen in our parks, it cherishes an 

 extensive prejudice against all other waterfowl, 

 but being naturally wary and excessively noisy 

 to boot, it becomes automatically the watchman 

 for its neighbours. Flocking birds of the more 

 intelligent kinds very commonly have sentinels of 

 their own flock, and in the case of wild Geese the 

 process of changing sentry has been observed ; the 

 sentinel may be approached by one of the flock and 

 touched, after which it lowers its head and begins 

 to feed, while the relief bird assumes the watching 

 position — or it may, as has also been seen, force 

 another bird to take its turn by dealing it a hard 

 peck. 



In some cases, as is well known, sociability amongst 

 birds extends to social nesting, familiar to us in 

 the case of Rooks and Sand-Martins, though it is 

 more common among sea-fowl — ^in which it is 

 usual — than land-birds. The well-known Social 

 Weaver, the Republican Grosbeak of the old 

 naturalists {Phileteerus socius), carries the social habit, 

 usual among Weavers, so far as to construct a 

 common roof of grass, under which each pair con- 

 structs its own nest, and a somewhat similar habit 

 has been alluded to above in the case of the only 

 nest-building Parrot. As a general rule, however. 



