Abstract
BackgroundIn-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.MethodsHere 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.ResultsOur results demonstrate that despite occasional high-altitude migrations (up to 5550 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 preferring migrating at low altitude, notably in combination with low air temperature, the birds also preferred flying with wind support to likely reduce flight costs. They avoided clouds, perhaps to help navigation or to reduce the risks from adverse weather.ConclusionsWe 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.
Highlights
In-flight conditions are hypothesized to influence the timing and success of long-distance migration
Flight altitudes of far eastern curlew did not differ between day and night flights, whether it was over sea (Wilcoxon test, w = 16,715, p = 0.30) or over land (Wilcoxon test, w = 2551, p = 0.57, Fig. 2)
Contrary to the general perception that wind support is the most important factor determining the flight altitude of migrants (e.g. [6, 8, 15]), our results show that in these long distance migrants the primary determinant of flight altitude selection is a preference for flying at low altitude, which is modulated by wind support, air temperature and visibility
Summary
In-flight conditions are hypothesized to influence the timing and success of long-distance migration. The air through which birds, bats, and many insects travel may vary considerably in temperature, humidity, air pressure, visibility, wind speed and wind direction depending on the altitude at which these animals travel These factors affect the costs and the risks associated with long-distance travel, such as during migration [1,2,3,4]. Poor visibility (cloud cover) and precipitation may increase the costs and risks of migration and have been found to reduce migratory onset and intensity ([9] and references therein, [12, 13]) Given this range of potential effects of atmospheric variables on the energy and water balance of avian migrants and, the costs and success of migration, decisions on when to fly and at what altitude are probably of paramount importance to aerial migrants
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