SOCIAL PARASITISM IN BIRDS 



By HERBERT FRIEDMANN 

 Department of Biology, Amherst College 



IF ONE were to enumerate the main 

 features characteristic of birds, the 

 chances are that the habit of nest- 

 building would be among the first 

 to be mentioned. This indicates in no 

 uncertain fashion the universality of this 

 habit in this large group of vertebrates, 

 and in turn, this very universality imme- 

 diately focuses our attention on those 

 relatively few species that neither build 

 nests nor care for their eggs or young. 

 These birds lay their eggs in occupied 

 nests of other species, to whose care they 

 are left, and because of this habit are, for 

 want of a better term, said to be parasitic. 

 The habit is not true parasitism in the real 

 biological sense, and may be called social 

 or breeding parasitism. Few problems in 

 the study of animal behavior have aroused 

 more interest for a longer period of time, 

 and from Aristotle to the present time 

 there is an unbroken series of attempts to 

 explain the origin of this peculiar habit. 

 In the early days of biological science this 

 question was limited to a single species, 

 the well known European cuckoo, Cuculus 

 canorus, and it was of this bird that 

 Aristotle wrote, ending his discourse with 

 the cautious sentence, "People say that 

 they have been eye-witnesses of these 

 things." Since his time a great many 

 individuals have also claimed to have been 

 eye-witnesses of these and similar things, 

 but it is only within the last century 

 that accuracy and precision have been 

 brought into play in these observations 

 and the facts separated from the interpre- 

 tations. Less than two centuries ago it 



was found that many cuckoos in Asia were 

 also parasitic, but the habit was still 

 supposed to be confined to the one family 

 of birds. 



In the early days of the last century it 

 was discovered that the cuckoos were not 

 the only birds with parasitic breeding 

 habits, and that the cowbird of North 

 America, Molothms ater, a bird belong- 

 ing to an entirely different order, also 

 exhibited this remarkable mode of repro- 

 duction. Later, workers in southern 

 South America found that some of the 

 neotropical cowbirds were likewise para- 

 sitic, and observers in Africa announced 

 that the habit was also found in some of 

 the honey-guides. Quite recently a few 

 of the African weaverbirds were shown to 

 be parasitic, and just a few years ago a 

 South American duck, Heteronetta atri- 

 capilla, was found to possess this habit as 

 well. At present, this manner of repro- 

 duction is known to occur in five widely 

 separated and distantly related families of 

 birds: the cuckoos (Cuculidae), the hang- 

 nests (Icteridae), to which group the 

 cowbirds belong, the weaverbirds (Plo- 

 ceidae), the honey-guides (Indie at oridae), 

 and the ducks (Anatidae). Of the cuckoos 

 about seventy species are known to be 

 parasitic; of the hangnests, only the cow- 

 birds and the rice grackle, half a dozen 

 species in all; of the weavers, only three; 

 of the honey-guides, all the species of 

 whose breeding habits we have any knowl- 

 edge, less than half a dozen; and of the 

 ducks, a single species. The entire 

 number of parasitic species forms but a 



554 



