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 signal information about individual quality. We evaluated these hypotheses with three 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 richness. These three features of flange color comprise much of the variation observed both within and among species. Neither our manipulation of apparent carotenoid- nor UV-richness shaped parents’ ability to transfer prey efficiently to nestlings, a proxy for how visually detectable nestlings were. We found some evidence that brightness shaped parents’ ability to transfer large prey items. 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, and that any role of brightness in offspring-parent interactions comes via detectability benefits. UV reflectance does not seem to shape parental care. 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.

Data are summaries of video recordings, scored by hand, of house sparrow parents interacting with nestlings. In three experiments, two nestlings were manipulated to appear i) yellow/pale (carotenoid-rich/carotenoid-poor), ii) bright or dull, and iii) control or UV reduced. The two files reflect different analyses conduced with these observations:

We counted prey items allocated to each nestlings overall and when both nestlings were begging (i.e., parents were presented with a binary choice). We measured the time it took parents to transfer each prey item, as well as the number of transfer attempts. Data in both files are organized by experiment. 

Data can be accessed using Excel or other csv reader.