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

Parent birds respond to a variety of cues from their offspring when provisioning them with food. Bright flange colours and large body size are two traits of offspring that many parents seem to favour, but little is known about how such traits interact. By experimentally altering the difference in mass between nestmates while concurrently blackening the flanges of either the large or small nestlings in the brood, we tested whether large body size could compensate for dull coloration. In three species of cavity nesters (northern flicker, Colaptes auratus, great tit, Parus major, and pied flycatcher, Ficedula hypoleuca), doubly disadvantaged offspring that were both small and dull performed the worst, whereas large and bright offspring received the most provisioning, consistent with the idea of additive effects. We used regression to show that, for flickers, dull (blackened) nestlings performed as well as bright nestlings when they were 20% larger than their bright nestmates. For passerines, dull nestlings had to be 30–40% larger than bright nestlings to receive equivalent amounts of food from parents. Within-brood differences in mass before manipulation were often greater than this size difference threshold, suggesting that hatching asynchrony has a stronger influence on food allocation than do colour differences among siblings.