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Young animals often solicit care from their parents using behaviors and morphologies collectively termed begging. Many nestling altricial birds add to their familiar postures and vocalizations by displaying colorful mouthparts, hypothesized to enhance their visual conspicuousness and/or be favored by choosy parents. We evaluated these two hypotheses with experimental manipulations of the flange tissue of house sparrow (Passer domesticus) nestlings. We allowed parents to interact with nestlings that appeared to have either carotenoid-rich or carotenoid-poor flanges, and then with nestlings that appeared similar in carotenoid-richness but differed in either brightness (total light reflected) or ultraviolet (UV) richness. These three features of flange color comprise much of the variation observed both within and among species. None of our three manipulations shaped parents' ability to transfer prey efficiently to nestlings, a proxy for how visually detectable nestlings were. Parents preferentially allocated prey to nestlings that appeared carotenoid-rich, but did not bias allocation when we manipulated brightness or UV reflectance. These patterns suggest that nestlings displaying carotenoid-rich flanges benefit from parental favoritism, but that variation in brightness and UV reflectance do not independently shape parental care, at least within the range a house sparrow parent might encounter in nature. Efforts to understand mouth color in the context of offspring-parent communication and visual signal evolution may benefit from considering the component parts of color, as they may function, be constrained, and evolve independently. [ABSTRACT FROM AUTHOR]

We explored the responses of monogamous house sparrow parents to deviations in their mates' contributions to nestling provisioning. Following 1-2 days of baseline measurement of parental food delivery rates, we applied small lead fishing weights to the tail feathers of either male or female parents. Weighting had much greater immediate impact on male parental care than on female care, but the handicapping had little long-term effect on either male or female provisioning behavior. When parental performance of handicapped males was most impaired, their mates did not show significant increases in parental care as compensation, nor did females mated to handicapped males reduce their provisioning as their mates recovered from weighting. Similarly, males mated to handicapped females did not respond to their partners' recovery with declines in their own efforts; paradoxically, these males showed a sustained elevation of provisioning throughout the post-treatment interval, despite no significant reduction in provisioning by weighted females. The apparent insensitivity of both males and females to changes in their mates' parental behavior, and the ineffectiveness of current partner behavior at predicting an individual's provisioning effort, fail to conform to assumptions of biparental care models that require facultative responses to partner deviations in effort. Instead, the remarkable consistency of each individual's behavior supports the notion of “sealed bids” and suggests that variation in nestling provisioning is largely attributable to factors that are independent of the mate's current behavior, such as differences in individual quality. [ABSTRACT FROM AUTHOR]