316 Wonders of the Bird World 



small size, extreme variability of colour, as well as the thick- 

 ness and weight of the shell, by which it can always be told 

 with certainty from the eggs of the other birds with which it 

 is placed. Dr. Rey, who has for years made a special study 

 of the Cuckoo and its nesting-habits, has come to the con- 

 clusion that the birds inhabit a very limited area, to which 

 they return year after year. Each female lays about 

 twenty eggs in the course of the season, and these are laid 

 on alternate days, and each female Cuckoo lays similar 

 eggs throughout its life, Only one egg is deposited in a 

 nest, and when a second egg is found, it must be the work 

 of another female. Dr. Rey confirms my idea that each 

 Cuckoo is parasitic on a particular species of bird, and only 

 puts its eggs in another bird's nest when that of its chosen 

 foster-parent is not to be found, or it is not ready to 

 receive it. Although the number of Cuckoos' eggs which 

 do not agree with those of the foster-parents is very large, 

 yet the resemblance of the parasitic egg to that of the 

 birds in whose nest it is placed, is remarkable, and in some 

 cases the likeness is absolute, though as yet there has been 

 no approach in the colour of the Cuckoo's egg to those of 

 such birds as the Wrens, Hedge-Sparrows, or Willow 

 Warblers. In nearly every case the egg deposited in the 

 nest of the Redstart is found to be blue like that of the 

 latter bird. 



Cuckoos, however, are by no means the only species 

 which are parasitic on other birds, and the Cow-birds 

 (Molothrus) present us with still more curious phenomena 

 in the way of nesting, especially as some of their ways 

 would appear, at first sight, to be highly detrimental to the 

 interests of the birds themselves. As regards the common 

 North American Cow-bird (Molothrus pecoris] the late 

 Captain Bendire has published full details in his 'Life 

 Histories of North American Birds,' and some of his facts 

 are very interesting when compared with those recorded of 



