cuckoos' eggs and evolution. 289 



In England, unfortunately, a large proportion of the Cuckoos' 

 eggs in collections have been supplied by dealers who, even if 

 they would not cheat intentionally, are themselves deceived by 

 the men who supply them, who are generally paid prices according 

 to the rarity of the foster-parent with which they are supposed to 

 have been taken. For this reason we have to dismiss from our 

 calculations about two-thirds of the eggs in private collections. 

 Outside the five best known fosterers, there are a few others well 

 authenticated which we may accept as normal fosterers, such as 

 the Sedge-Warbler, Spotted Flycatcher and one or two others. 

 As regards the rest, I cannot consider them as anything but 

 abnormal or casual. Dr. K. Williams, nearly forty years ago, 

 showed how certain Cuckoos which normally cuckolded Wagtails, 

 Robins, and Hedge-Sparrows, occasionally deposited their eggs in 

 the nest of practically any other small bird building within the 

 area they covered when they could not find a nest of the species 

 they really wanted. 



Perhaps no one in England has personally taken more Cuckoos' 

 eggs than Dr. Williams, whom I found to be possessed of a wealth 

 of knowledge on the subject. The greater number of his eggs 

 were given by him to Dr. Rey but I was so fortunate as to 

 obtain nearly 150 from him after he had ceased sending them to 

 that gentleman. From these most interesting sets, I find that 

 practically all the casual eggs laid in such nests as those of the 

 Bullfinch, Linnet, Greenfinch, Wren, Chiff-ChaflT, etc., can be 

 matched with those of Cuckoos which year after year deposited 

 their eggs in the nests of some one of the three birds which in 

 his vicinity provided the normal fosterers. 



Continental Cuckoos seem to have advanced considerably 

 further in evolution than have the individuals which yearly seek 

 our shores for breeding purposes. True, we cannot say that in any 

 instance complete or perfect evolution has been arrived at but 

 many instances show that elimination has already been very busy 

 and thus certain fosterers have been accepted through sufficient 

 generations to allow of all those Cuckoos who laid eggs in striking 

 contrast to their own to be completely exterminated. Thus we 

 find that the Cuckoos' eggs now placed in the nests of many birds 

 such as the Great Reed-Warbler, the Red-backed Shrike and a 

 few others, approach very closely both in colour and character to 

 the eggs of their foster-parents. The degree of resemblance varies 

 greatly, some eggs ai'e an excellent match and some are only fair, 

 whilst strongly contrasting eggs are in a very small minority and 

 even these, of course, may have been deposited by Cuckoos 

 normally cuckolding other fosterers. 



Perhaps the greatest advance shown by Continental Cuckoos is 

 the evolution of a blue egg to resemble those of the Redstart with 

 which they are placed. This blue egg seems normally to be laid 

 only by certain individuals which breed in the Baltic States and 

 East Prussia, more especially in Finland. Even here, however, 

 blue Cuckoos' eggs are not always placed with the blue Redstarts' 



