HOST RELATIONS OF PARASITIC COWBIRDS 79 



as a cowbird fosterer. No reason exists, however, for assuming the 

 bird will be very different in this respect from the better known and 

 more completely documented P. caerulea. This should be true 

 despite the fact that Hanna (1934, p. 89) has suggested that the 

 earlier breeding season of P. melanura (the extreme egg dates at 

 Riverside being April 10 and May 30) may help it to escape excessive 

 parasitism, especially early in the season. He also suggested that 

 the usual habitat of this gnatcatcher, the dry bush-covered hillsides 

 or dry gullies, might have an isolating effect so far as cowbirds are 

 concerned, but this is not at all certain. 



Baird, Brewer, and Ridgway (1874, p. 157) noted that at Cape 

 St. Lucas, Baja California, Xantus found cowbhd eggs in nests of 

 the black-tailed gnatcatcher, the local race of which is P.m. margaritae. 

 This is all that has been recorded for the race. 



The Arizona-northwestern Mexican form. P.m. lucida, is known 

 as a cowbu'd host from the following records. G. Bancroft informed 

 me that he collected two parasitized sets of eggs, one at Santa Eulalia, 

 Chihuahua, and one at Guaymas, Sonora. A. R. Philhps (in litt.) 

 noted a recently fledged cowbhd being attended and fed by a male 

 black-tailed gnatcatcher near Granados, northeastern Sonora, on 

 August 10. Swarth (1905, p. 79) found a fledgling cowbird being 

 attended and fed by a black-tailed gnatcatcher in the Santa Rita 

 Mountains, Arizona. At Sacaton, Arizona, Gilman (1915, p. 88) 

 found a parasitized nest, and at Alamo Ranch, near Tucson, Brandt 

 (1951, pp. 80, 133, 684) reported two more parasitized nests. Monson 

 (1949, p. 248) found a fledgling cowbird being attended by one of 

 these gnatcatchers, at Tucson, Arizona. Brewster (1882, p. 77) 

 reported a parasitized nest which was found at Yuma, Arizona, by 

 Stephens. W. J. Sheffler informed me that in Aiizona he found 

 many parasitized nests of this host; in fact, at times he was led to 

 wonder how the gnatcatchers were able to withstand the pressure 

 of cowbird parasitism and to raise enough of their own young to 

 maintain their population. 



Bent (1949, p. 371) quoted Rowley, who observed that "along 

 the Colorado River area, cowbirds parasitize the nests of these birds 

 rather abundantly ..." and who noted that he had "found a female 

 setting on three eggs of a cowbhd and none of her own, with many 

 nests containing one or two cowbird eggs." 



For the California race. P.m. calif ornica, there are fom* records: 

 Woods (1930, p. 126) saw a pair of these birds feeding an almost 

 fully grown cowbird at Azusa in June 1928; Cl5^de L. Field found 

 a parasitized nest at National City, April 24, 1929; N. K. Carpenter 

 found another in San Diego County; Hanna (1934, p. 89) found 

 still another at Riverside in May 1933. 



