SOCIAL PARASITISM IN BIRDS 



567 



other in open country, and among species 

 of different genera living in the same type 

 of country, one species restricted its 

 parasitism to open, arboreal nests, while 

 others laid only in domed nests either in 

 low trees, or on the ground. The ecologi- 

 cal factors affecting the ranges and habitats 

 of the various parasitic cuckoos neces- 

 sarily limit the number of host-species 

 available to each species of cuckoo. In 

 the tropics the number of species and of 

 individual birds is very large and the 

 resulting struggle for existence more in- 

 tense than in the more lenient regions to 

 the north and south. As a result of the 

 keenness of the competition we find that 

 similarity in habits survives side by side 

 where those habits do not affect the same 

 species. That is, a habit such as the 

 parasitic one could survive far more easily 

 in many species in the same region if they 

 did not conflict with each other than if 

 all were parasitic on the same group of 

 host-species. So then, in the bush veldt 

 of Africa we find that the little golden 

 cuckoos, Lampromorpba, victimize weaver- 

 birds, grass-warblers, and a few other 

 types of birds, chiefly limiting their 

 attention to the weavers and Cisticolas. 

 Most, (almost all) of their victims build 

 domed or covered nests, some of them on 

 the ground. In the same districts we find 

 that the crested cuckoos, Clamator, confine 

 their visitations to open, arboreal nests, 

 such as the golden cuckoos never molest. 

 However, with a fair number of species 

 to choose from there is no environmental 

 reason why a certain individual parasite 

 should further limit its range of activities 

 by tending towards extreme host-specific- 

 ity. It is not of any particular obvious 

 benefit to the parasite to be still further 

 restricted in this way. 



The only way to arrive at a proper 

 understanding of the way in which host- 

 specificities might have begun is to study 



individual birds as well as species. In 

 working on the reproductive habits of 

 birds one of the first things to be deter- 

 mined is the extent and definiteness of the 

 individual breeding territories. Chance 

 and others have done this for the European 

 Cuckoo, Cuculus canorus canorus, with 

 splendid results. In the case of the 

 African species of parasitic cuckoos I 

 found that all of them establish definite 

 breeding territories to which they adhere 

 during the egg laying season. The males 

 of some species, such as Lampromorpba 

 caprius, Chrysococcyx cupreus, and Cuculus 

 solitarius, are very faithful to their terri- 

 tories. The breeding territory in the case 

 of a parasitic bird is based not upon a 

 sufficiency of food for the young but upon 

 an adequacy of nests for the eggs. As 

 stated above the small golden cuckoos 

 parasitize weaver-birds QPloceus, Hyphan- 

 tornis, Otyphantes, etc.) very frequently. 

 A great many species of these weavers are 

 arboreal and build their nests in large 

 colonies, often as many as a hundred or 

 more nests in a single tree. I found 

 that in several cases a pair of didric 

 cuckoos, Lampromorpba caprius, had estab- 

 lished their territories around trees con- 

 taining colonies of weavers and in at least 

 four cases the territories were entirely 

 restricted to single trees. These weaver 

 colonies very seldom contain more than a 

 single species of weaver, at least in my 

 experience. In such cases the individual 

 cuckoos, by restricting their territories to 

 single trees, automatically limit their 

 parasitism to single species. These 

 weaver colonies are very common all over 

 the African continent south of the Sahara 

 and the didric cuckoos are also common 

 and widespread. Therefore it seems very 

 likely that individual host specificities are 

 being formed in many individual cuckoos 

 in the way just mentioned. It is impossi- 

 ble to imagine any cuckoo as originally 



