5 6 4 



THE QUARTERLY REVIEW OF BIOLOGY 



of the act, even though they had themselves to bring 

 up the unwelcome addition. It is likely, too, that 

 the greater abundance of new than of deserted nests 

 would favor the frequency of such mistakes until the 

 parasitic habit would have become established. 



This suggestion seems well founded and 

 possesses the virtue of being simple. 

 However even in this case, the actual 

 origin of the parasitic habit is not 

 explained. A possible method of evolu- 

 tion of the parasitic habit is suggested but 

 no indication is given as to why the birds, 

 if repulsed at a new nest, do not continue 

 nest-hunting until they find an unoccupied 

 one. Furthermore, birds that breed in old 

 nests of other species do not normally lay 

 the first egg on the same day that they 

 first occupy the nest, but usually the 

 possession of a nest seems to provide the 

 stimulus necessary for egg-production. 

 From this it follows that if a bird of such 

 breeding habits did try to occupy a new 

 nest it would either be repulsed by the 

 owners before it had a chance to lay, or 

 by the time it did lay an egg, the owners 

 would have forsaken the nest, leaving the 

 new occupant to care for its eggs, just as 

 if it had originally gone to an old nest to 

 breed. 



In order to explain the origin of the 

 parasitic habit we must first decide 

 whether it is the result of a change from a 

 normal nesting habit or whether it is a 

 phylogenetically original one. All the 

 evidence derived from a study not only of 

 the cowbirds, but of birds in general, 

 points unmistakably to the conclusion 

 that parasitism is an acquired habit and 

 not an original one. It is inconceivable to 

 think of a long line of parasitic birds 

 having no origin in normal nesting types. 

 Again, for the evidence behind this state- 

 ment I must refer the reader to my book as 

 space does not permit of its inclusion 

 here. It seems entirely safe and justifiable, 

 then, to assume that parasitism was not 



the original condition in the cowbird 

 stock. The problem, then, is not whether 

 the cowbirds were or were not always 

 parasitic, but how they lost their original 

 habits and acquired their present ones. 



LOSS OF PROTECTING INSTINCTS AS A FACTOR 

 IN THE ORIGIN OF PARASITISM 



To quote again from my book: 



We have seen that all five species of Cowbirds 

 establish breeding territories and that the distinctness 

 or the definiteness of the territory is most pronounced 

 in the most primitive, non-parasitic, Bay-wing, while 

 it is least definite, and at times, almost imaginary, in 

 the Shiny Cowbird and the North American 

 Molotbrus. In the Bay-Wing, and its off-shoot, the 

 Screaming Cowbird, the birds are practically always 

 strictly monogamous and only one pair is to be found 

 in a given territory. In the other two species, where 

 the parasitic habits are better developed, the terri- 

 tories are distinct chiefly in districts where the species 

 are not abnormally numerous; but where they are 

 unnaturally abundant, the territorial instincts arc not 

 strong enough to stand unmodified against the pres- 

 sure of Cow T bird population. Distinctness of territory 

 depends on the amount of what may be called 

 "territorial protection" displayed by the male. In 

 most birds the male establishes a breeding territory 

 and protects it from the inroads of other males of its 

 own species. The female sometimes has this instinct 

 as well. In the parasitic Cowbirds we see that the 

 birds have still retained the territorial desire but 

 have lost most of the instinct to protect their breeding 

 areas. The original factor involved in this loss is the 

 reversal of the usual method of territorial acquisition 

 in the most primitive of the Cowbirds. We have seen 

 that birds, usually the males, establish their territories, 

 and then choose a nesting site somewhere within 

 that territory. The Bay-winged Cowbirds however, 

 reverse this process. They leave the winter flocks in 

 pairs and, instead of staking out their "claims" they 

 look for old or even new nests in which to breed. 

 They fight for these nests if necessary, and when once 

 in occupation, they extend the territory radially 

 around the nest. In this way the territory, instead of 

 being the primary consideration, becomes a matter of 

 only secondary importance and with this reduction of 

 its significance, the instinct to defend it is correspond- 

 ingly lessened. In this way the amount of "terri- 

 torial protection" displayed by the male is decreased 

 and in the more recent, parasitic species, where the 



