22 U.S. NATIONAL MUSEUM BULLETIN 233 



is able to produce heat sufficient to keep 8 cc. of eggs at a given tem- 

 perature. The volume of an egg may be derived from its external 

 shell measurements, according to a formula worked out by Schonwetter 

 (1924). Using this method, the eggs of most vireos and small wood 

 warblers (not the larger Ideria or Seiurus), some of the smaller 

 buntings, and some flycatchers are found to have a volume of from 

 1.6 to 1.9 cc, while an average cowbird egg is just over 3 cc. Thus, 

 the volume of a 4-egg set of a smaU warbler would be about 6.4 to 

 7.6 CO., or shghtly to significantly more than that of 2 cowbird eggs. 

 If the nest of a warbler were parasitized so that it held 3 warbler eggs 

 and 1 cowbird egg, the clutch volume would be about 8 cc. If 2 

 of the warbler eggs were replaced by cowbird eggs, the resultant clutch 

 volume would be 9.3 cc. ; if 3 replacements were involved, the final 

 egg volume would be 1 1 cc. 



Hann (1947, p. 174) estimated that the ovenbird could incubate 

 successfully an egg volume of between 1.3 and 1.8 times the volume 

 of its usual laying of 5 eggs, and probably nearer the lower than the 

 higher figure. Hofslund's (1957) observations indicated that 1.3 

 times the normal clutch volume was the limit that a yellowthroat 

 could ordinarily incubate to successful hatching. 



It follows that, while a host species such as the rufous-sided towhee, 

 with eggs as large as, if not larger than, those of the cowbird, could 

 incubate successfully 4 or even 5 of the parasitic eggs in place of its 

 own, the smaller hosts could not. By and large, within passerine 

 species, the hatching potential is correlated, with some exceptions, 

 with the general size of the egg, which usually is related to the size 

 of the bird. 



Mutual Effect of Parasite and Host on Egg Production 



The exact number of eggs produced in one season by one cowbird 

 is still uncertain, and without this information it is diflScult to estimate 

 the overall percentage of egg success in the parasite to compare with 

 that of each of the common hosts. In my own field studies (1929, 

 p. 188) two cowbirds laid 5 eggs each and one laid 4, but it was not 

 estabhshed whether these numbers represented a whole season's laying 

 or merely the equivalent of a clutch, of which there might be more 

 than one in a season. As a matter of fact, I quoted one instance of a 

 cowbird in captivity reportedly laying 13 eggs in 14 days, a case that 

 could not be checked for its accuracy. Nice (1937b, p. 164) concluded 

 that the species laid three or four sets of up to 5 eggs per set, with an 

 interval of from 6 to 12 days between sets. D. E. Davis (1942) 

 rearranged the data and concluded that the interval between sets 

 should be shorter, about three days. He also made sections of entire 

 ovaries of several species of cowbirds (shiny, screaming, and bay- 



