COLOR VARIATION AMONG NESTLING BROWN-HEADED COWBIRDS (MOLOTHRUS ATER) DOES NOT REFLECT DIFFERENTIAL SUCCESS WITH HOSTS IN TEXAS

Auk, The, Apr 2007 by Ellison, Kevin, Sealy, Spencer G, McGaha, Hope R

This is likely, because nestling discrimination is rare and presumably would result in increased costs if mistakes in discrimination were made (Lotem 1993; but see Langmore et al. 2003). The lack of strong selection for flange color is interesting, because Brown-headed and Bronzed cowbirds almost exclusively parasitized hosts with yellow- and white-flanged young, respectively. Nonetheless, with no clear difference in fledging success of either morph (as inferred from the occurrence of juveniles), flange color does not seem to reflect a character displacement at our site. Alternatively, we suggest that host use may reflect differential parasitism on the basis of host body size and possibly phylogeographic history (Friedmann 1929).

FLANGE-COLOR RATIOS

Because significantly more white-flanged Brown-headed Cowbirds were produced each year, local adaptation would be supported if hosts with white-flanged young were differentially parasitized at our site. A positively assortative pattern seems unlikely, given that Brown-headed Cowbirds, which produced predominantly white-flanged young, regularly parasitized species whose young had yellow flanges (Table 1). The fledging success of cowbirds of either flange color did not differ significantly; however, our sample size was limited.

Alternatively, hosts may differentially feed young with a nonmatching flange color (Rothstein 1978, Stevens 1982). Indeed, Stevens (1982) experimentally manipulated flange colors of Red-winged Blackbirds (Agelaitis phoenicens) and Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) and found that when yellow-flanged young were less common within a brood, they were fed more than their white-flanged nestmates. We lack the data to test either case directly and suggest further study involving manipulation of brood composition by recording host provisioning rates at nests that contain pairs of cowbirds with different combinations of flange colors.

We also have few data to address whether Bronzed Cowbirds fare as well with yellowflanged hosts, though Carter (1986) reported that this species fledged from nests of hosts of either color. Of 153 Bronzed Cowbird fledglings, only 50% were tended by species whose young have white flanges (Carter 1986). Moreover, the most frequent host recorded for Bronzed Cowbirds is the Northern Mockingbird (Minius polyglottos; Lowther 1995), whose young have yellow flanges. We suggest that little selection is currently exerted by hosts on cowbird nestling flange color, because survival of Brown-headed Cowbirds at our site seemed to be unaffected by flange color.

FLANGE-COLOR INHERITANCE

Stevens (1982) and Ward and Smith (1998) noted that flanges in the zone of hybridization are mostly yellow, which suggests that the allele for yellow flange color is dominant over the allele for white. By contrast, our analysis suggests that yellow may be a sexinfluenced recessive trait, but controlled crosses of cowbirds with flanges of known color are required to confirm the mode of inheritance. Flange-color inheritance may be elucidated by studies of Darwin's finches (Geospiza), among which a visually similar color polymorphism occurs. Eight geospizid species were recorded with color-free (referred to as pink by Grant et al. [1979]) and yellow external mouthparts (Fig. 1) in 82% and 18% of young (é; = 1,874), respectively. With striking similarity to those of Brown-headed Cowbirds in Texas, the flangecolor frequencies of Medium Ground-Finches (Geospiza fortis) were close to those expected under sex-linkage (Grant 1999). However, young produced by adults with known bill colors violated expectations under simple sexlinkage (Grant 1999). Therefore, Grant (1999) suggested that color was controlled by at least two loci with epistasis. Our observations of yellow-flanged cowbirds, some with yellowtinged plumage, further suggest multilocus control of color traits.