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THE QUARTERLY REVIEW OF BIOLOGY 



So much, then, for the cowbirds. In 

 the other groups of parasitic birds, other 

 factors seem to have been instrumental in 

 bringing about the parasitic breeding 

 habit. Too little is definitely known of 

 their biology to attempt an explanation, 

 but probably the habit arose differently in 

 each of the five families containing para- 

 sitic species. 



HOST SPECIFICITY IN THE CUCKOOS 



In the cuckoos, we have one clue which 

 indicates that territory had little to do 

 with the inception of parasitism. This 

 is furnished by the peculiar feature of host 

 specificity shown by some of the species, 

 especially in Europe and Asia, but to a 

 lesser extent in Africa and Australia as 

 well. In the classic case of the European 

 Cuckoo, Cuculus canorus canorus, it is now 

 well established that generally each female 

 deposits all her eggs in nests of a single 

 species. That is, one cuckoo may parasi- 

 tize only meadow-pipits, another may 

 lay its eggs only in nests of hedge-spar- 

 rows, while still another may victimize 

 reed-warblers exclusively. Each indi- 

 vidual has its own particular species of 

 victim to which it generally limits its 

 attention. The species Cuculus canorus 

 canorus lays its eggs in the nests of a great 

 number of different kinds of birds, but each 

 individual tends to use the nest of but one 

 kind. The parasitic habit in Cuculus 

 canorus canorus may therefore be said to be 

 characterized by individual host-specificity. 

 In the Indo-Malayan region there are a 

 great many genera and species of parasitic 

 cuckoos, some of which have carried this 

 specificity to an extreme with the result 

 that the great majority, if not all, of the 

 eggs are laid in nests of a single species or 

 group of allied species. Thus the Indian 

 Koel, Eudynamis honorata, lays its eggs 

 wholly in nests of crows and jays. In 

 British India it victimizes the Indian 



Crow, Corvus splendens, and the Jungle 

 Crow, Corvus macrorhynchus; in Burma it 

 foists its eggs upon the Burmese Crow, 

 Corvus insolens, and the Burmese Jay, Pica 

 sericea; in southern China the victim is 

 another Jay, Graculipica nigricollis. In 

 large districts in its range practically all 

 the individual koels victimize the same 

 species of bird. In other words, within 

 each of these districts the individual host- 

 specificity of each individual koel is the 

 same as that of every other one, and taking 

 into consideration the entire range of the 

 species the number of host species is so 

 small and the species so closely related 

 that the individual host-specificities of 

 all the koels are very similar. The 

 parasitic habit in Eudynamis honorata may 

 therefore be said to be characterized by 

 specific host-specificity. 



The development of specific from indi- 

 vidual host-specificity may readily be 

 accounted for by natural selection operat- 

 ing under conditions which would tend to 

 emphasize the value of small differences. 

 Thus, in the case of Eudynamis honorata the 

 bird (and its egg) is too large to be success- 

 ful with small fosterers. The crows are 

 everywhere common and their nests open 

 and plainly visible and the birds (and 

 their eggs) fairly close in size to the koels. 

 An abundant, accessible group of species 

 being everywhere available, the individual 

 koels having crows as their individually 

 specific hosts would rapidly increase and 

 gradually eliminate their less successful 

 fellows that depended on more precarious 

 and more uncertain specific hosts. In 

 time the entire membership of the species 

 Eudynamis honorata would be composed 

 of individuals parasitic on crows. 



During the course of my field work in 

 Africa I found that the various parasitic 

 cuckoos were ecologically isolated from 

 each other to a very considerable extent, 

 i.e., one species lived in dense forest, an- 



