Any elevational trend related to air density and oxygen supply may be altered or reversed in macro-scale comparative analyses because overall gradients in flight adaptations may reflect foraging ecology, lifestyle, or seasonal mobility (Figure S1). For example, mountaintops are colder with more open habitats and fewer flying insects; thus, a higher proportion of montane species (or trophic guilds) may be terrestrial (e.g., ground tit), whereas lowlands are warmer with more flying insects, increasing the diversity of aerial insectivores (e.g.,Cypsiurus parvus).
