House Sparrows Passer domesticus increase in body size and dorsal feather insulation along a Himalayan elevational gradient

Elevational gradients impact organisms in diverse ways because of lower temperatures and oxygen levels at higher elevations. Birds adapt to these constraints through changes in body size, plumage, metabolism, and behaviors such as elevational migration. But whether species show variation across multiple axes of adaptations along a single elevational gradient is not well demonstrated in the literature. The broad elevational distribution of the House Sparrow Passer domesticus in the Himalayas provides a rare insight into whether morphological and plumage modifications can simultaneously arise within a species across its elevational distribution. Drawing insights from eco-physiological patterns, we hypothesized that sparrows at higher elevations would exhibit larger body sizes (Bergmann’s rule), shorter extremities (Allen’s rule), and a more insulative feather structure. We sampled sparrows at 13 sites (n = 169) from 300 m to 3500 m along an elevational gradient in the western Himalayas and found that sparrows at higher elevations were significantly larger following Bergmann’s rule, and had more downy dorsal feathers likely giving higher-elevation sparrows thermal advantages at low temperatures. We show that House Sparrows may show convergent modifications in response to temperature clines whether they are elevational or latitudinal.