Birds of a feather flock together, but only in similar climates

AUGUST 28, 2019

by Shi En Kim, University of Chicago Medical Center

One might assume that birds of flight are cosmopolitan travelers, and bird species should be distributed far and wide, spread across long distances—continents even. However, a study led by Alex White, Ph.D., a former University of Chicago graduate student now at National Museum of Natural History in Washington, D.C, shows that a bird has strong ties to the climate patterns of its habitat. As a result, the geographical distribution of birds may be more restricted than we think.

Bird's eye view

Scrutinize the distribution of birds across the globe and it is obvious that land birds, for example, have ranges that abruptly end at coastlines. You may not notice a similar turnover of bird species within continents, but in fact one is present at the freezing line, the boundary between the tropics and cooler, temperate areas. White's study shows that despite no significant physical barriers stopping them from spreading out, bird species are strongly confined to their habitats as demarcated by the freezing line.

Nowhere in the world does the freezing line loom as drastically as the Himalayas. Here, though, it is not the world's tallest mountain peaks that serve as the boundaries of avian habitats and movement. Instead, it is the freezing line, which cuts across the subalpine slopes at an elevation of about 1600 m, less than a fifth of the way up to Mount Everest's peak.

White conducted the study as part of his Ph.D. thesis at UChicago, working with advisor Trevor Price, Ph.D., professor of ecology and evolution. Focusing on the Himalayas, they examined the distribution of 305 species of open-habitat and tree-dwelling birds out of the known 621 species present in the region. The numbers of species were estimated from reported sightings and vocalizations across 38 sites in the Himalayan forests. This survey was performed over a ten-year period during the annual warm breeding months, when seasonal migrant birds were present and species numbers were at their highest.

Continued