24 EVOLUTION OF BIRD-SONG 



shaking alarm by uttering his common call or note 

 of warning and occasional song-note tell ; and 

 here also the same increased rapidity of utterance 

 seemed to have the same result.^ On many occa- 

 sions I have heard a male blackcap, alarmed for the 

 safety of his young, repeat the ordinary alarm tack 

 (well named by Yarrell) so rapidly that a continuous 

 shaking cry, like the shaking alarms of the house- 

 sparrow and great tit, was produced. I have twice 

 heard long-tailed tits reiterate their rattling alarm in 

 the manner in which the call-note tuck of the brown 

 wren (a cry most often heard in autumn) is very 

 frequently prolonged, so that the utterance was 

 changed like that of the above-mentioned blackcaps. 

 On one occasion the tits were frightened by a pair 

 of sparrow-hawks flying low above them, and on the 

 other they were alarmed by the sudden appearance 

 of a strange dog. It is a matter of easy observa- 

 tion that the rattling alarm of the blackbird is 

 constructed of repetitions of one cry, which is some- 

 times uttered slowly at intervals of several seconds ; 

 but this is only when the bird is suspicious of some 

 concealed danger, and as the peril increases the 

 notes become more frequent, more acute, and con- 

 siderably higher in pitch. This incident affords a 

 ^ The female house-sparrow has both of these cries. 



