Yellow and Black
European Starlings (Sturnus vulgaris) exhibit seasonal bill morphology in that during the breeding season, the beak is an ostentatious yellow (likely by unconjugated carotenoids) that seems to be recognized as the Starling’s signature, while at the onset of post-breeding molt, the bill keratin becomes a less striking black in color by deposition of melanin. Typically seasonal color change observed in the bill of many species serves as a sensory cue involved in sexual and social interactions.
Bonser and Witter (1993) proposed a mechanical function in order to explain the seasonal change observed in bill coloration. An increase in wear and abrasion resistance and surface hardness of industrial materials is possible by optimimally designed particulate composites. Based on this, Bonser and Witter hypothesized that the embedded melanin granules of the non-breeding morph of Starling bill would serve to increase wear resistance of the bill keratin or rhamphotheca. Assuming that wear resistance is proportional to microindentation hardness for keratin (as is the case for metals and polymers), they tested the hypothesis by comparing hardness measured by microindentation of melanic and non-melanic sections of rhamphotheca. Microhardness values for melanic keratin were found to be nearly twice (at least 1.7x) that for non-melanic bill keratin.
While the study leaves much unresolved, I am in praise of this article in all of its brevity. Previous studies of bill morphology had dealt solely with nutritional aspects (e.g. acquisition of dietary protein) or environmental and behavioral aspects (e.g. the practice of bill wiping as a possible response to change in growth rate). This work, on the other hand, exemplifies well the contribution of mechanical insight to the understanding of avian ecology and biology.
One possible explanation for the seasonal bill morphology is that the energetic cost of carotenoid-colored bill keratin, while vital for spring activities, need not be afforded during the non-breeding, winter months.
Or, based on the argument of mechanical utility, deposition of melanin granules, which increases hardness of bill keratin, may be less costly ultimately than the accelerated rhamphotheca growth rate that would be requisite to compensate for increased wear during foraging activities, for example, of winter months.