Migration and the excess exposure of birds to human density in North America

Abstract

Migratory species must cross a range of landscapes that are increasingly modified by humans. A key question is how migrating populations are responding to human-induced environmental change. Here, we model the spring migration dynamics of 63 bird species in North America to quantify their exposure to human population density. We find that most bird species have a negative navigational bias, suggesting that they attempt to avoid human-dense areas during migration, and yet they experience far greater human density during migration as compared to breeding. Species that experience excess human density during migration share several key traits: they tend to be nocturnal migrants, they start migrating through North America earlier in the year, and they tend to migrate longer distances. These findings underscore that birds are especially vulnerable to threats associated with human disturbance during migration, with predictable exposures that are often elevated by 2- to 3-fold during migration.