SOCIAL PARASITISM IN BIRDS 



565 



protecting instincts arc further reduced, the territory 

 of any one male is very apt to be invaded by other 

 males of its own species. 



Furthermore, in the Bay-winged Cowbird, we have 

 seen that the female has lost most of her protecting 

 instinct and seems to spend most of what she has 

 before laying her eggs, after which the eggs are largely 

 dependent on the male for protection. The female is 

 always quite bold and fearless when away from the 

 nest, but very shy and nervous when incubating. 

 Apparently she has the instinct to conceal her eggs in 

 a nest, usually not of her own building, but has very 

 little desire to protect them once they are laid. If not 

 for the protection of the male, (and very fearless he is) 

 she would probably be unable to care for her eggs, and 

 if the male should lose his protecting instinct the 

 result would be that the female would have the 

 instinct to lay Cor conceal) her eggs in nests but not 

 to care for them (or protect them). This would open 

 an easy path to parasitism. If we re-examine the 

 habits and instincts of the other Cowbirds we find 

 that this is exactly what has happened. The males 

 have lost their protecting instincts and we find that 

 the loss is more complete in Af . ater and M. bonariensis 

 than in M. rufo-axillaris. The very fact that we still 

 find these somewhat obscure, but yet real, stages in 

 the loss of the protecting instincts, only serves to 

 emphasize the downward path these instincts have 

 taken. So then, it may be said that the immediate 

 cause of the origin of the parasitic habit in the Cow- 

 birds was the loss of the protecting instinct of the 

 male. The fact that the female, still earlier in the 

 history of the group, lost most of her protecting 

 instincts cannot be called a causative factor because as 

 long as the male retained his instincts of defense, as 

 in the Bay-winged Cowbird today, the birds were not 

 parasitic. What caused the almost complete loss of 

 these instincts in the male we cannot definitely say, 

 but the factor which started the weakening, and 

 finally brought about their destruction was the 

 reversal of the territorial and nesting habits. When 

 the territory became of only secondary importance the 

 impulse to protect it was correspondingly weakened. 

 At the risk of seeming paradoxical it might almost be 

 said that the fact that the ancestral Cowbirds cared 

 more about the nest than the territory had much to 

 do with the origin of the parasitic habit. The com- 

 plex of reproductive instincts became unbalanced and 

 eventually collapsed. In other words, the birds were 

 more interested in a secondary than a primary con- 

 sideration with the result that the former suffered 

 much more than the latter. 



Fortunately we have a clue to the way in wbich the 

 male lost his protecting instinct. In order to fully 

 appreciate its significance it is necessary to digress for 

 a moment and consider the evolution of the Screaming 



Cowbird from the Bay-wing stock. As already 

 indicated in a previous section the Screaming Cowbird 

 is obviously a direct offshoot of the stock of which 

 the Bay-winged Cowbird is the living example. The 

 range of the Screaming Cowbird is wholly contained 

 within that of the Bay-wing and in general the 

 habitat or type of country occupied by the two species 

 is the same. I always found both species in the same 

 type of environment. It seems then that there could 

 have been no geographical or ecological isolation in 

 this case to preserve and differentiate the budding 

 form which in its present state we call the Screaming 

 Cowbird. Consequently the isolation necessary to 

 preserve the distinctness of the newly arisen species 

 must have been physiological. The physiological 

 isolation was probably that of differential breeding 

 seasons. Probably the original rufo-axillaris was an 

 early breeding bird (badius is a later breeder). 

 Although rufo-axillaris today is a late breeder, the 

 facts that its courtship season comes early in the 

 spring, and that it establishes its territories very 

 early, point to the conclusion that it once was an 

 early breeding bird as bonariensis and ater are today. 

 Inasmuch as the Bay-wing is nonparasitic and inas- 

 much as the Screaming Cowbird is a direct offshoot of 

 this stock it seems probable that originally the latter 

 species was also non-parasitic. In other words the 

 change between the normal and the parasitic mode of 

 reproduction occurred within the racial history of 

 M. rufo-axillaris. Assuming that in most ways the 

 original habits of the Screaming Cowbird were 

 similar to those of the Bay-wing we should expect 

 that the birds tried to breed in nests of ovenbirds, 

 woodhewers, etc., but tried to do so early in the 

 season. As elsewhere indicated the struggle for 

 nests is much greater early in the season than later on, 

 and the Screaming Cowbird, handicapped hereditarily 

 by a weakened territorial instinct, probably could 

 not succeed in this struggle. We have seen that 

 sometimes Screaming Cowbirds establish territories 

 in the spring, occupy them for considerable periods, 

 and then desert them without ever having utilized 

 them. This indicates very strongly that the weakened 

 territorial instinct of the male is often insufficient to 

 maintain its influence long enough to "make connec- 

 tions" with the somewhat more vernal development 

 of the egg-laying instincts of the female. In this 

 lack of attunement between the territorial instincts 

 of the male and the egg-laying instincts of the female 

 the parasitic habit probably had its origin. This 

 lack of attunement seems to have been caused by the 

 diminution of the protecting territorial instincts of 

 the male and this diminution seems in turn to have 

 been started by the reversal of the territorial and 

 nest-building instincts in the stock from which the 

 Screaming Cowbird evolved. 



