HABITS OF THE GREAT CRESTED GREBE. 459 



throughout nature. Moreover, as the eggs would only be laid — 

 after a full indulgence of the sexual passion — in the last nest, 

 the incubating instinct might gradually restrain the birds — now 

 somewhat sated — from pairing on that one ; whilst the others, 

 being used for that purpose only, would tend more and more to 

 be built for it only, too. With regard to the multiplication of 

 nests, we have the Wren as a familiar example of the habit, 

 whilst my last year's observations on these same two Grebes 

 record it in this species. Peewits are another instance, for they 

 make a number of hollows, in all respects similar to the one in 

 which the eggs are finally deposited, though, from their strange 

 manner of doing this, another question arises, which I shall 

 shortly bring forward. None of these birds are at all closely 

 allied to the Bower-birds of Australia, but in the Thrush and 

 the Blackbird we, at any rate, get a good deal nearer to them. 

 With regard to the Blackbird, I have seen one clear instance of 

 an apparently quite capricious abandonment of an almost finished 

 nest in order to build another ; nor is it in the least likely that I 

 happened here — any more than in the case of the Grebes — to 

 come upon a pair of very exceptional birds. It is the rarest 

 thing, I think, speaking generally, to meet with a real exception. 

 The appearance of it, in nine hundred and ninety-nine cases out 

 of a thousand, marks but our ignorance. There remains the 

 Thrush, and to this bird I paid some attention this spring, and 

 was surprised at the number of nests which I found in different 

 stages of construction, and which were not afterwards completed. 

 That birds have, as a rule, any particular — or, at least, any 

 clearly defined — object in building more than one nest, I do not 

 myself believe ; but, be that as it may, such a habit, joined to the 

 one of pairing on the nest, appears to me to offer just that sort 

 of foundation out of which such a state of affairs as we have with 

 the Bower-birds might eventually arise. 



But now another question arises. If a certain structure — 

 the nest — is habitually made use of by any species of bird for 

 pairing as well as for laying eggs in, which of these two uses are 

 we to consider as the primary, and which the secondary one ? 

 In other words, has the bird built a thalamum which has become, 

 in time, a nest, or a nest which has become a thalainum? This 

 brings us to the origin of nest building, which need not, 



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