Far eastern curlew and whimbrel prefer flying low - wind support and good visibility appear only secondary factors in determining migratory flight altitude

Abstract

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 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. 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.

Assessing potential conflicts between offshore wind farms and migration patterns of a threatened shorebird species

“…We found significantly higher flight altitudes during spring compared with autumn, in accordance with the observation of flight altitudes in the vicinity of curlew wintering sites at the Wadden Sea, which were related to different wind regimes (Schwemmer et al, 2021). Galtbalt et al (2021) found median flight altitudes of 538 m over land and 156 m over sea for Far Eastern curlews Numenius madagascariensis during north-bound migration. Interestingly, this was similar to the range for spring migration in our study, while our study showed significantly lower flight altitudes during autumn.…”

Flight altitude dynamics of migrating European nightjars across regions and seasons

“…What our two scenarios have in common is that work during the climb is not wasted but invested in (and to a large degree recoverable from) the potential energy gained ( Pennycuick, 1969 ). We therefore urge caution when using ‘climbing costs’ per se as an explanation for low-altitudinal flights if these costs are not well formulated, as it may be misleading and could obscure the effect of other undefined factors ( Galtbalt et al, 2021 ). We hope our approach will inspire further analyses of the potential penalty of vertical movements and encourages researchers in attempting to enumerate such costs in future flight altitude-related studies.…”

“…Here, we broaden this definition to also include any larger to-and-fro movements when the bird is airborne at any stage of the migratory flight. Only focusing on the increased power output during climbing may lead to the assumption that extensive vertical movements would elevate the cost of migration and limit the possible benefits of a tailwind at higher altitudes ( Galtbalt et al, 2021 ). But, because of the potential energy gained during ascents, migrants are expected to preserve energy through gliding descents ( Baudinette and Schmidt-Nielsen, 1974 ), which in ideal circumstances could completely balance the climbing costs.…”

Compensation for wind drift prevails for a shorebird on a long-distance, transoceanic flight

“…We found that godwits generally appeared to seek out favorable wind conditions in their preferred direction within a limited vertical column extending from 100 to 3000 m.a.s.l. This often entailed low-level flights over the Pacific Ocean, a strategy that has been documented in other shorebirds making transoceanic crossings [ 56 ], as well as dynamic altitude-switching, which may incur lower costs for nimble, long-winged godwits than for other larger-bodied species [ 57 ]. Other factors not included in our models, such as predation risk or thermoregulation, may have also influenced flight altitude [ 58 , 59 ], though our models suggest that their effects are likely minimal over much of this flight.…”