4^ 



The In'sli Naturalisl. 



strain") that the whole hckl knows at once what is up. 

 The signal of the white uj)per tail-covert is, in fact, generally 

 a property of moderately sociable birds that feed, in rather 

 scattered companies, and that either have not very powerful 

 voices or (as in the case of the Jay) have strategical reasons 

 for not always (^xercising their vocal powers when they are 

 changing their ground. 



I pass now to birds \\'hose plumage is marked in such 

 a way that we can't help calling it ornamental, though it 

 is a difficult question to decide whether ornament is really 

 the principal function it has to serve. 



Provisionally, we may include among instances of 

 ornamental plumage not only such brilliant birds as the 

 King-fisher and Bullfinch, and such wearers of elaborate 

 plumes and patterns as the Great Crested (irebe and the 

 Ruff, but also any marked contrasts of colour like the grey 

 and black of the Hooded Crow and the black and white 

 of the ]\Iagpie. To the same class we should refer all 

 obviously nuptial tints — I mean colours that are assumed, 

 whether by one sex or by both, for the nesting season only, 

 like the black throat and bright yellow breast of the Grey 

 Wagtail, or the deep brown hood assumed for only a few- 

 months by both sexes of the Black-headed Gull. Along 

 with these go the numerous cases in which the cock bird's 

 plumage differs in some marked manner from that of his 

 bride, though he may retain that difference all the year 

 round. Familiar instances are the Blackbird, Stonechat, 

 Chaffinch, (ireenfinch. Bullfinch, Reed-Bunting, Sparrow- 

 Hawk, and Kestrel. ()\n- commonest Dublin bird, the 

 Sparrow, presents us with another example, and a 

 ])articularly interesting one, as J ho})e to sliow by-and-bye. 



By far the best known explanation of most of these 

 cases of bright ])lumage is that which ascribes them to 

 what is called s(;xual sele( tion. That is to say, when birds 

 select their mates in s])ring, thev are supposed to gi\'e 

 ])referenc(.' to those candidates who show the m^arest 

 approach to a certain standard of beauty ; and so that 

 standard is iir>t imj)ro\('(l from generation to generation, 

 and afterwards maintained at tlie liigli le\-el at which it 

 is regarded as jMactically })erfect. 



