Far Eastern Curlew and Whimbrel prefer flying low: wind support and good visibility appear only secondary factors in determining migratory flight altitude

Background: In-flight conditions are hypothesized to influence the timing
and success of long-distance migration. Wind assistance and thermal uplift
are thought to reduce the energetic costs of flight, humidity, air
pressure and temperature may affect the migrants’ water balance, and
clouds may impede navigation. Recent advances in animal-borne
long-distance tracking enable evaluating the importance of these factors
in determining animals’ flight altitude. Methods: Here we determine the
effects of wind, humidity, temperature, cloud cover, and altitude (as
proxy for climbing costs and air pressure) on flight altitude selection of
two long-distance migratory shorebirds, far eastern curlew (Numenius
madagascariensis) and whimbrel (Numenius phaeopus). To reveal the
predominant drivers of flight altitude selection during migration we
compared the atmospheric conditions at the altitude the birds were found
flying with conditions elsewhere in the air column using conditional
logistic mixed effect models. Results: Our results demonstrate that
despite occasional high-altitude migrations (up to 5,550 m above ground
level), our study species typically forego flying at high altitudes,
limiting climbing costs and potentially alleviating water loss and
facilitating navigation. While mainly migrating at low altitude, the birds
also preferred flying with wind support to likely reduce flight costs, and
avoided clouds, which would help navigation and reduce the risks from
adverse weather. Conclusions: We conclude that the primary determinant of
avian migrant’s flight altitude selection is a preference for low altitude
with wind support as an important secondary factor. Our approach and
findings can assist in predicting climate change effects on migration and
in mitigating bird strikes with air traffic, wind farms, power lines, and
other human-made structures.