Directed flight and optimal airspeeds in gulls

Submitted by Johan on 16 February 2016.

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Figure 1: A Lesser Black-backed Gull in flight, sporting a high-resolution UvA-BiTS data logger. Photo: C.J. Camphuysen

Diagnosing in-flight adjustment to wind by birds is challenging, since they may follow landmarks, take detours, soar in thermals or become wind drifted.

Gulls are noted opportunists and flight generalists. When his University of Amsterdam PhD supervisor Judy Shamoun-Baranes first showed James McLaren trajectories of GPS-tagged Lesser Black-Backed Gulls (Figure 1) flying directly from England to their breeding colony on the Dutch coast, he initially thought "What on earth can I do with these?" He then realized that these high-resolution GPS and accelerometer data provided a rare opportunity to assess optimal flight theory and reaction to wind in the absence of landmarks.  

The tracks and concurrent wind data revealed that the gulls saved energy by adjusting their orientation to both horizontal and vertical wind patterns, and increased flight speeds in headwinds and crosswinds. James also computed predicted and theoretically optimal trajectories using techniques from aeronautics (Figure 2). Predicted trajectories often resembled actual flight but flight speeds were consistently over-predicted. This seeming inefficiency may reflect an overriding adaptation to more common flight activities (e.g. foraging), or alternatively that the current estimates of flight costs are too low. This study also reveals the importance of properly accounting for wind drift when assessing flight costs, which could affect for example estimates of the impact of climate change on migration.

Figure 2: Four trajectories of GPS-tagged Lesser Black-backed Gulls (solid magenta lines), together with simulated trajectories based on optimal orientation given horizontal winds (solid blue lines), full compensation for wind drift (dashed red lines) and allowing full wind drift (dot-dashed green lines). Co-incident wind vectors are shown in grey.


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