THE EUROPEAN CUCKOO 267 



is the difficulty experienced to-day in obtaining a broody hen. Chickens, 

 during the past few years, have been selected chiefly for their egg- 

 laying capabilities and the great majority of chicks are hatched in in- 

 cubators. Without deliberate intention broodiness has been "bred out" 

 of most strains of domestic fowls, and if by chance a broody hen is 

 needed to rear a covey of wild partridges, a long tour in a car from 

 farm to farm is required in order to locate one. The common complaint 

 of twenty years ago that "the hens have stopped laying and have gone 

 broody " is now as much a thing of the past as a sirloin of beef or a 

 hansom cab. 



If a bird therefore embarks upon a series of chance layings in nests 

 other than her own, it is not difficult to see how a large number of eggs 

 will possess better survival value than the instinct for incubating and 

 brooding, and the latter characteristic will be speedily eliminated. 



Most parasitic birds are sexually promiscuous, either polygamous or 

 polyandrous, or both. This loose way of living, particularly when 

 it is associated with a loss of parental instinct on the part of the male, 

 seems to be connected with the development of the parasitic habit. In 

 some cases, for example in the cow-birds, it appears as a consequence 

 rather than a cause of the parasitic mode of life. Further information 

 is required about the biology of brood-parasites, but it seems certain 

 that the habit is almost always associated with various types of prom- 

 iscuous sexual relationships. 



Brood-parasites — by whatever routes they may have developed the 

 habit — seem to possess certain characteristics in common. Their eggs, 

 for example, always develop at a quicker rate than those of the host. 

 The advantage of hatching before the foster nestlings appears to be 

 extremely important, if not essential, for their survival. The European 

 cuckoo hatches two days before the host, and the cow-birds from one to 

 four days, even when the closely related species of cow-bird fosterer is 

 concerned. A widely spread habit among the females of brood-para- 

 sites is the removal or destruction of at least one egg of the foster bird. 

 The eggs themselves are frequently modified in certain well defined 

 directions. Thus, the shell is much tougher in the case of the parasite's 

 Ggg. This is even true of the two related cow-birds referred to above. 

 The size and weight of the eggs and the colour and pattern of the shell 

 frequently tend to resemble those of the fosterer. Some brood-parasites 

 lay eggs which are superficially indistinguishable from those of their 

 host. Well known examples of this phenomenon are recorded from 



