Colour of the upper mandible: An adaptation to reduce reflectance

Abstract

Reflectance from the upper mandible of willow flycatchers ( Empidonax traillii) was altered by painting the mandible with white nail polish. Sham-experimentals were painted with clear polish and controls were untouched. Light-billed flycatchers adjust to increased reflectance from light bills by foraging from shaded perches, a result similar to the frequent use of shade by naturally light-billed wood warblers (Emberizidae, Parulinae). When in sunlight, the proportion of commuting flights (‘give-up flights’) was significantly higher among white-billed flycatchers than among dark-billed conspecifics, suggesting a lower foraging success in sunlight for white-billed flycatchers. The results are consistent with the hypothesis that dark mandibular coloration is an adaptation to reduce visual interference.