NESTLESS BIRDS AND ANNEXERS 53 



habit is, comparatively speaking, a recent one, as it 

 only prevails in one or two families of highly specialised 

 birds. Whether the habit will become more widely 

 prevalent it is, of course, impossible to say. Should 

 birds, however, follow human example in the growing 

 tendency to shirk the responsibilities of offspring, then 

 we may safely say that such will be the case. The 

 manner in which this very exceptional habit amongst 

 birds has arisen is a very fascinating question for 

 ornithologists to solve, and notwithstanding the many 

 plausible explanations of the phenomenon that have 

 been suggested, it is still largely enshrouded in 

 mystery. We can of course presume that parasitism 

 may be the retained habit of some ancestral form of 

 the species practising it at the present time, and 

 acquired during conditions of existence of which we 

 can have no possible conception nowadays. We can 

 also suggest in its explanation that the habit may 

 have prevailed more widely during earlier epochs of 

 avine existence. The fact that every detail and 

 condition of the habit is so marvellously perfect 

 seems to suggest its long continued duration. The 

 choice of nest is not the least important condition of 

 success, for a species must be selected capable of 

 bringing the alien young bird to maturity. Then the 

 coloration of the eggs of parasitic birds is another 

 important factor, this varying much or little according 

 to the number of species selected by the parasites 

 and the degree of variation reached by their eggs. 



