Nest-building, Incubation, and Migration. 247 



species becomes manifest. It is wonderful, says Mr. 

 Hudson, that the screaming cowbird should lay only in the 

 nests of the bay wing; but the most mysterious thing is 

 that the common Argentine species indiscriminately para- 

 sitical on a host of species, never, to his knowledge, drops 

 an egg in the nest of the baywing unless it be forsaken. 



It is clear that, though the interesting parasitic habits 

 of the cowbirds throw side lights upon the parasitism 

 of the European cuckoo, and though some of their features 

 may fairly be ascribed to natural selection, yet they leave 

 the origin of parasitism itself unsolved. "We have to fall 

 back on conjecture. Mr. Hudson is inclined to believe 

 that the Argentine cowbird lost the nest-making habit by 

 acquiring that semi-parasitical habit, common to so many 

 South American birds, of breeding in the large covered 

 nests of the oven birds (Dendrocolaptidas), and he adduces 

 evidence that this piratical habit does tend to eradicate the 

 nest-making instinct. He further suggests that a diminu- 

 tion in the number of birds that build domed nests would 

 involve the cowbird in a struggle for nests in which it 

 would probably be defeated ; and supposes that the origin 

 of the parasitical instinct may have been in the occasional 

 habit common to so many species, of two or more females 

 laying together, as with the baywings. The young of 

 those birds that most often abandoned their eggs to the 

 care of another would inherit a weakened natural instinct ; 

 the whole race would, by intercrossing, degenerate, and 

 could only be saved from final extinction by some indi- 

 viduals occasionally dropping their eggs in the nests of 

 other species. 



Little need be said concerning the instinct of incubation 

 in its normal development. It is partly based on physio- 

 logical grounds ; and may fairly be claimed by upholders 

 of natural selection as explicable on their principles. 



