Yolk testosterone levels are not consistently higher in the eggs of obligate brood parasites than their hosts

The survival of obligate brood parasitic nestlings depends on their ability to exploit hosts' parental care, mostly at the expense of the unrelated nestmates. Foster parents are characteristically manipulated by parasitic young through more intensive begging displays. For example, young of parasitic brown-headed cowbirds (Molothrus ater) beg more loudly and elevate their head higher than the chicks of most host species. What is the developmental basis of this difference between parasite and its host species? Previous studies showed that intraspecific differences in some maternally deposited egg-yolk steroid hormone concentrations were related to differences in the begging behaviors and growth of hatchling birds. The prediction was tested that interspecific variation in yolk testosterone concentrations was related to consistent differences in the begging of hosts vs. parasites. Contrary to this prediction, yolk testosterone levels in the eggs of brown-headed cowbirds were not consistently higher than in host species: cowbird testosterone concentration was higher than in eastern phoebes (Sayornis phoebe), lower than in yellow warblers (Dendroica petechia), and similar to red-winged blackbirds (Agelaius phoeniceus). These results show that variation in egg-yolk testosterone concentrations alone is not related causally to differences in the begging behaviors between parasitic and host bird species. Nonetheless, it remains feasible that interspecific variation in both maternally deposited testosterone concentrations and embryonic hormone-receptor distribution together function to shape phenotypic differences in behavior and growth rate between avian hatchlings of different taxa.