Flap or soar? How a flight generalist responds to its aerial environment

Abstract

The aerial environment is heterogeneous in space and time and directly influences the costs of animal flight. Volant animals can reduce these costs by using different flight modes, each with their own benefits and constraints. However, the extent to which animals alter their flight modes in response to environmental conditions has rarely been studied in the wild. To provide insight into how a flight generalist can reduce the energetic cost of movement, we studied flight behaviour in relation to the aerial environmental and landscape using hundreds of hours of global positioning system and triaxial acceleration measurements of the lesser black-backed gull (Larus fuscus). Individuals differed largely in the time spent in flight, which increased linearly with the time spent in flight at sea. In general, flapping was used more frequently than more energetically efficient soaring flight. The probability of soaring increased with increasing boundary layer height and time closer to midday, reflecting improved convective conditions supportive of thermal soaring. Other forms of soaring flight were also used, including fine-scale use of orographic lift. We explore the energetic consequences of behavioural adaptations to the aerial environment and underlying landscape and implications for individual energy budgets, foraging ecology and reproductive success.This article is part of the themed issue 'Moving in a moving medium: new perspectives on flight'.

Keywords: GPS; accelerometer; foraging ecology; orographic lift; soar; thermal convection.

Figure 1.

Spatial distribution of flight measurements. (a) Full spatial range of measurements from 15 May to 14 June including (b) measurements in and around the breeding colony. Each point is a measurement resampled to every 5 min, for all bird–years. Red, blue and yellow indicate flapping, soaring and mixed-flight modes, respectively (see colour scheme in panel (a)). The location of the breeding colony on Texel is indicated with a black star. The World Light Grey Base Map provided via the ArcGIS Map service is used as a base layer.

Figure 2.

Environmental envelope of soaring and flapping flight. Using a kernel density estimator, we show the 50 percentile for soaring (blue) and flapping flight (red) calculated independently for each flight mode across the two main environmental predictors boundary layer height (blh (km), y-axis) and time from 12.00 UTC (hour (h), x-axis).

Figure 3.

Fine-scale flight behaviour in relation to environmental conditions. (a) High resolution measurements (every 5 s) during part of a foraging trip by individual 805 on 25 May 2014 from 8.59 to 9.28 UTC (red, blue and green indicate flapping, soaring and mixed-flight modes, respectively), (b) 5 m resolution digital elevation model of the area (white areas indicate water), (c) orographic lift (ms⁻¹) at 9.00 UTC, with wind (4.5 ms⁻¹) blowing from the west. The area in (a) shown in subplots (b) and (c) is delineated by a white rectangle.