344 HERBERT FRIEDMAXX. 



female is extremely shy when on the nest and were it not for the 

 protection of the male it is doubtful if in many cases the female 

 would take sufficient care of the eggs. In Agelaioides the incu- 

 bation period is i2 l / 2 to 13 days. 



The genus Molothrus contains three species all of which are 

 parasitic but have the parasitic habit developed to unequal degrees 

 of perfection. The stages of perfection of the parasitic habit in 

 these birds agree with what seems to be the phylogenetic relation- 

 ships of the species. The most primitive member of the genus, 

 M. ntfo-a.rillaris, is parasitic on the still more primitive, non- 

 parasitic Agelaioides badius. Its incubation period is the same as 

 that of its victim and varies from i2 l /2 to 13 days. The second 

 species, -I/, bonariensis, is parasitic on a large number of small 

 birds but has the parasitic habit very imperfectly developed, wast- 

 ing large numbers of its eggs by laying in deserted nests, or even 

 on the ground. The incubation period of this species is ii*/2 

 days. The third species of the genus, M. ater, has the parasitic 

 habit best developed, lays its eggs in the nests of a great many 

 species of birds, and does not waste its eggs as does M. bonarien- 

 sis. This species has an incubation period of from 10 to iQ l /2 

 days. (No bird in the world is known to have a shorter incuba- 

 tion period; few have one as short.) ' L 



For the successful prosecution of a mode of reproduction such 

 as parasitism implies it is obviously of great importance to the 

 parasitic species to have a short incubation period as its egg may 

 be laid in a nest after incubation has been started in the other eggs 

 and yet must hatch as soon if not sooner than these others if the 

 young parasite is to survive. (As a matter of fact in the majority 

 of cases, irrespective of the species of bird victimized, the cow- 

 birds' eggs hatch first.) Consequently when we find that with the 

 perfection of the parasitic habit in the cowbirds (from the primi- 

 tive non-parasitic Agelaioides badius to the relatively perfect 

 Molothrus ater} there is a corresponding diminution of the period 

 of incubation, or in other words, an acceleration of the embryonic 

 growth-rate (amounting to about 20 per cent, difference between 

 A. bad ins and M. ater} it is difficult to interpret the case as other 



1 The evidence on which the phylogenetic relationships of the species of cow- 

 birds is based is contained in a manuscript now going the rounds of the publishers. 



