52 U.S. NATIONAL MUSEUM BULLETIN 233 



suitable ones ..." and that some cowbirds seemed to show a trend 

 toward host specijficity on the phoebe "whether the nest site be shed, 

 culvert, rock ledge, or sugar shanty, in the latter case often following 

 the Phoebe deep into the woods." 



Not infrequently the cowbird deposits an egg in a nest before the 

 phoebe has laid any of her own, but this does not seem to affect the 

 latter. In at least two cases wherein I knew the cowbird had laid 

 fii'st, the nest was not forsaken; the phoebe laid her clutch of eggs as 

 though no strange eggs were present. On two other occasions, 

 however, I found cowbird eggs partly buried in the bottom of the 

 nest; in one case there were, in addition to the buried egg, another 

 cowbird's egg and 2 phoebe's eggs on top. Bendire, years before 

 (1895, p. 274), had also noted occasional attempts by the phoebe to 

 bury the cowbird eggs under a new floor in the nest. 



While some phoebe eggs are more or less speckled, most are un- 

 marked white and, as such, they are in strong contrast to the darker, 

 mottled eggs of the parasite. Crude experiments have been made to 

 test the latitude of egg coloration tolerated by the phoebe ; the result 

 was that all of the eggs which were tried — from the larger, bluish- 

 green eggs of the robin to the smaller, heavily dotted, cinnamon- 

 reddish eggs of the house wren — were accepted and incubated, and, in 

 the case of house wrens, were hatched and reared by the phoebes. 



Black Phoebe 



Sayornis nigricans (Swainson) 



A single instance of the nominate race of this flycatcher as a host of 

 the small, southwestern race of the brown-headed cowbird has come 

 to my attention. E. A. Stoner (1938) found a nest about a mile and 

 a half north of Manka, Solano County, Cahfornia, on June 26, 1937; 

 it contained 3 eggs of the black phoebe, 1 of the cowbird, and 3 of the 

 western flycatcher. The nest was obviously built by the phoebe 

 although at the time of discovery the western flycatchers seemed to 

 be in charge of it. While there is no certainty that the parasitic egg 

 was laid either before or after the change in ownership of the nest, 

 it appears that it was deposited after the western flycatchers were in 

 possession since there was a scanty lining of fine hairs over the phoebe's 

 eggs, over which, in turn, were the eggs of the western flycatcher and 

 of the cowbird. The case merits discussion here, nevertheless, be- 

 cause it involves the parasitism of the cowbird on the nest of a black 

 phoebe in spite of the fact that the occupancy of the nest, in the 

 meantime, had been taken over by another species. It recalls a case 

 I came upon many years ago in Argentina wherein a shiny cowbird 

 (Molothrus bonariensis) laid in the nest of a rufous ovenbird (Furnarius 

 rujus) although the nest had been taken over by a tree toad. 



