Parents take both size and conspicuousness into account when feeding nestlings in dark cavity nests

► We studied food allocation according to nestling size and flange colour. ► We experimentally altered brood weight ratio and painted flanges black. ► Small, dull nestlings were most disadvantaged, suggesting additive effects. ► Dull nestlings could overcome their disadvantage when 20-40% larger. ► Intrabrood size differences probably drive parental feeding in many broods.