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

METHODS

FLANGE-COLOR RATIOS

We recorded flange colors of young cowbirds and their hosts at Fort Clark Springs, Kinney County, Texas (29°18'N, 100°43'W; Fig. 3). This site is dominated by lawns, huisache (Acacia minuata), and honey mesquite (Prosopis glandulosa), as well as riparian habitats of pecan (Carya illinoensis) and live oak (Quercus virginiana). To assess the frequency of each flange color, we searched for cowbird eggs and young during four breeding seasons, 1999-2002. Both cowbird species were common at the site, and each year more than 50 eggs of each species were found within the 40-ha primary study site (see Ellison 2004). Young Bronzed Cowbirds were distinguished from Brown-headed Cowbirds by appearance and vocalizations (Friedmann 1929, Carter 1986). Almost all young were found within 1 km of the area where cowbird eggs were collected for another study. Having assigned maternity using microsatellite DNA markers (Ellison 2004), we measured flange color and assigned sex for the young of at least six Brown-headed Cowbirds within the 40-ha area each year. However, it is likely that we sampled the young of more females, because the genetic analysis involved young from the smaller primary study area and we captured 192 females. Of the 126 females that were color banded, 28 were resighted in the 40-ha area of the primary site and likely bred locally.

K.E. visually assigned flange color as yellow or white (sensu Rothstein 1978) for cowbirds observed in nests or hatched in an incubator. We used a general dichromatic categorization because no cowbirds were detected with intermediate flange colors and intermediates have rarely been observed in other studies (i.e.,

To better assess broadscale geographic variation among flange-color ratios, we summarized published and unpublished data. Also, we present data only from locales where flange color of more than four birds was reported by Rothstein (1978; his sites 1,13-16, and 20-21; see our Fig. 2; sites 1, 9, and 13-14). Given the pattern of introgression of M. a. obscurns and M. a. artemisiae documented by Fleischer and Rothstein (1988), we report data from only their two most distant sites: Bishop (n = 93) and Lake Edison (n = 60) on the east and west slopes of the Sierra Nevada Range, California, respectively (Fig. 2, sites 2-3). We also used data from Boulder County, Colorado (Ortega and Cruz 1992). Unpublished data were obtained from Mandarte Island, British Columbia (48°38'N, 123°17'W; J. N. M. Smith); San Juan Basin Research Center, La Plata County, Colorado (37°14'N, 108°2'W; C. P. Ortega and J. C. Ortega); Delta Marsh, Manitoba (50°12'N, 98°12'W; S. G. Sealy); Konza Prairie Reserve, Kansas (39°5'N, 96°33'W; W. E. Jensen); Fort Hood, Bell County, Texas (31°8'N, 97°33'W; K. Ellison and S. G. Summers); and the Rob and Bessie Welder Wildlife Foundation Refuge, San Patricio County, Texas (28°0'N, 97°5'W; H. R. McGaha).

FLANGE-COLOR INHERITANCE

If sex-linked, a dominant trait would be more common among males because female birds are heterogametic. To determine flange-color frequencies by sex, we used DNA extracted from blood (see Alderson et al. 1999) to sex young incapable of sustained flight. Sex was determined through polymerase chain reaction (PCR) amplification of chromo-helicase-DNA binding (CHDl) genes from the avian W and Z sex chromosomes electrophoresced on 2% agarose gels (Griffiths et al. 1998); CHD-analysis resulted in two distinct bands for females, one for males, and none when PCR amplification failed. We verified the accuracy of this technique with DNA from five adults of each sex. Also, we used wing length to identify the sex of most juveniles. Hill (1976) established that wing length could be used to assign sex of juvenile cowbirds. Because of geographic variation in body size, we used the genetic technique (for 17 juveniles at our site) and gonadal inspection (270 juveniles collected at Fort Hood, Texas; see Summers et al. 2006) to establish a wing-length criterion for assigning sex for the birds at our site. Using the criterion that birds with wing length >99 mm were males, we assigned only two males (0.7%) to the incorrect sex. Thus, we were confident that the criterion was valid and that birds with wings >99 mm did not need to be sexed genetically.

RESULTS

FLANGE-COLOR RATIOS