Abstract
The distributions of birds during migratory stopovers are influenced by a hierarchy of factors. For example, in temperate regions, migrants are concentrated near areas of bright artificial light at night (ALAN) and also the coastlines of large water bodies at broad spatial scales. However, less is known about what drives broad-scale stopover distributions in the tropics. We quantified seasonal densities of nocturnally migrating landbirds during spring and fall of 2011–2015, using two weather radars on the Yucatan peninsula, Mexico (Sabancuy and Cancun). We tested the influence of environmental predictors in explaining broad-scale bird stopover densities. We predicted higher densities in areas (1) closer to the coast in the fall and farther away in spring and (2) closer to bright ALAN and with lower ALAN intensity in both seasons. We found that birds were more concentrated near the coastline in the fall and away from it in spring around Cancun but not Sabancuy. Counter to our expectations, we detected increased bird densities with increased distance from lights in spring around Sabancuy, and in both seasons around Cancun, suggesting avoidance of bright areas during those seasons. This is the first evidence of broad-scale bird avoidance of bright areas during stopover.
Highlights
Migrating birds travel long distances between their breeding and nonbreeding grounds, facing conditions along the way that could hinder or halt their journeys
Geographic position in relation to bright lights and the coast is the major driver of broad-scale stopover distributions of nocturnally migrating birds along the Yucatan Peninsula coast and elsewhere
These results contradict the pattern of broad-scale attraction of migrants to bright areas observed at more northern latitudes during both spring and fall, raising the question of how and why migrant response to light changes between seasons and between temperate and tropical latitudes
Summary
Migrating birds travel long distances between their breeding and nonbreeding grounds, facing conditions along the way that could hinder or halt their journeys. Light pollution is a pervasive pollutant with multiple environmental effects [16,17,18] such as the disorientation and attraction of nocturnally migrating birds to bright ALAN, a phenomenon that we have known about for over a century [19]. Bird stopover densities during migration at the landscape scale are unexpectedly high in urban areas [10,21], a result of broad-scale attraction of night migrants to ALAN [11,22]. This association is stronger during the fall migration season [22], possibly because juvenile birds are more prone to ALAN-related disorientation [23]. Given the confluence of migrating birds around large water bodies where light pollution occurs, bird distributions at temperate and sub-tropical latitudes are related to geographic and ecological factors [10,12], and to light pollution [11]
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