184 BRITISH BIRDS. [vol. xv. 



I gave them other fresh eggs which I got from another marsh 

 three miles away and left them to sit. In most cases these 

 were hatched and reared, in any way I finally left the nests 

 in good order and condition. In certain cases I found that 

 after the eggs had been sat on for a week or ten days the 

 nests became empty, and I am strongly of opinion that the 

 Cuckoo revisited these nests and upon landing her egg missing 

 removed the rest and caused the fosterer to build another 

 nest for her subsequent use. This sort of thing I have 

 noticed for the last five years in connection with Cuckoos 

 that have used Reed- Warblers as fosterers. I watched it 

 more closely this year and can prove on four occasions that 

 my Cuckoo did deposit an egg in the rebuilt or second nest 

 of the fosterer. The sixteenth egg was laid in the rebuilt 

 nest that had received the fourth, the seventeenth occupied 

 the second nest of the fosterer that had received the fifth, 

 and the same thing occurred with those that contained the 

 twelfth and sixth and the nineteenth and ninth respectively. 

 It is well known that as soon as the nest of a pair of birds is 

 destroyed they immediately set to work to build another. 

 It is so with the Reed-Warbler : if the first nest meets a tragic 

 fate they commence to pull it to pieces and build another 

 with the same material a yard or so away. Three pairs had 

 their second nests built and clutches of four eggs in eight 

 days, which is remarkable, and the amount of work accom- 

 plished in the building of a second nest during the stay of an 

 hour or so on the marshes is surprising. The use of the 

 material from number one no doubt facilitates the work on 

 number two and relieves the bird of a considerable amount 

 of hunting for suitable material. In all cases where the 

 Reed-Warblers hatched and reared their young they only 

 had the one brood, but where for an3'^ reason the first nest 

 was destroyed a second nest was proceeded with at once. 



This theory of the Cuckoo destroying nests for her subse- 

 quent use is very interesting as the same thing has happened 

 this year in the case of another Cuckoo using Wagtails as 

 fosterers, first nests being destroyed in the same way. I have 

 never been able to ascertain accurately what the Cuckoo does 

 with the Reed-Warbler's egg she removes when depositing 

 her own. In cases where she removes one I have never been 

 able to find a trace of it, but in cases where she removes two 

 I generally find one lying on the duck-weed or on the thick 

 black ooze in the ditches. On the face of it it looks as though 

 she takes one away at any rate, which being fresh she no doubt 

 relishes ; but the second egg in cases where she removes two 



