xx f INTRODUCTION. 



it is by these slight variations, which, when beneficial, are preserved by 

 natural selection, that birds adapt themselves to any changed conditions 

 of life. 



With birds, as with man, " when once a particular mode of building has 

 been adopted and has been confirmed by habit and by hereditary custom 

 it will be long retained, even when its utility has been lost through changed 

 conditions" (Wallace). Although many habits have long since ceased to 

 be of any service, they are retained. Witness the fact of the hole-building 

 Ducks covering their eggs like their congeners nesting in the open ; the 

 Jackdaws often elaborate a nest in a position where one even of the 

 slightest description is of small necessity ; and our domestic Swan adds 

 to its nest (undoubtedly a habit originally acquired for its protection from 

 floods) when that nest is far removed from the waters. Neither birds nor 

 men can change old habits suddenly. Witness how we still retain the 

 side-straps and the arms in our first-class railway carriages (a custom 

 handed down from the old coaching days), or the buttons on the backs 

 of our coats (which were formerly used to fasten up the long tails), and 

 many other cases which are now quite as useless as the instances noticed 

 among birds. Another instance, the Apteryx of New Zealand (Apteryx 

 australis) when it sleeps goes through the formality of placing as much of 

 its head as possible under its rudimentary wings *. With regard to birds, 

 however, these superfluous actions are in no way injurious to the species 

 performing them were they so, natural selection would assert its influence 

 and would eliminate those individuals who did not conform to their 

 changed conditions of life. 



It is thought a remarkable fact by some naturalists that species of very 

 wide range should build typical nests throughout their distribution. But 

 surely there is nothing extraordinary in this if the area of distribution is 

 continuous ! Cetti's Warbler (Cettia cetti] is a good instance. This bird 

 breeds from Spain and Algeria to Turkestan, and examples of its nest 

 almost from these two extremes do not differ in the least in their con- 

 struction ; but I do not see any thing remarkable in this, even though this 

 bird is not migratory, for it breeds " along the whole line/' and there is 

 nothing to prevent one style of architecture being common to the species f. 

 The Woodchat Shrike (Lanius rufus) is another good instance. 



One of the great points brought forward in favour of instinct is the 

 uniformity of the nests of the birds of each species, even though they be 



* Trans. New Zealand Institute, ii. p. 75 (1869). 



f It would be very interesting to know if those non-migratory species that are separated 

 by discontinuous areas of geographical distribution build typical nests throughout. This 

 is a subject of which we possess no information, and is well worthy the attention of 

 those observers suitably situated for studying this interesting question. 



