Stochastic atmospheric assistance and the use of emergency staging sites by migrants

Abstract

Numerous animals move vast distances through media with stochastic dynamic properties. Avian migrants must cope with variable wind speeds and directions en route, which potentially jeopardize fine-tuned migration routes and itineraries. We show how unpredictable winds affect flight times and the use of an intermediate staging site by red knots (Calidris canutus canutus) migrating from west Africa to the central north Siberian breeding areas via the German Wadden Sea. A dynamic migration model incorporating wind conditions during flight shows that flight durations between Mauritania and the Wadden Sea vary between 2 and 8 days. The number of birds counted at the only known intermediate staging site on the French Atlantic coast was strongly positively correlated with simulated flight times. In addition, particularly light-weight birds occurred at this location. These independent results support the idea that stochastic wind conditions are the main driver of the use of this intermediate stopover site as an emergency staging area. Because of the ubiquity of stochastically varying media, we expect such emergency habitats to exist in many other migratory systems, both airborne and oceanic. Our model provides a tool to quantify the effect of winds and currents en route.

Navigating the flow: individual and continuum models for homing in flowing environments

“…IBM models based on principles similar to those here have been extensively applied to bird and insect navigation (e.g. [13]), and it would be intriguing to adapt our model to accommodate the peculiarities of these applications: for example, by incorporating 'resting phases' in response to unfavourable atmospheric conditions, or flight adaptation towards favourable winds.…”

“…Lagrangian-based particle models are often employed for the IBM, with each individual indexed by its instantaneous position and velocity: navigation can be incorporated via a directional bias according to orienteering cues. Currents can be obtained from widely available datasets and models based on these principles have been applied to understand movement dynamics across aquatic and airborne populations: the advection-dominated movement of fish larvae [8]; the role of current-directed movement in jellyfish blooms [9]; the influence of directed movement on turtle drifting within ocean currents [10,11]; the Atlantic movements of eel larvae [12]; how wind influences the choice of staging sites during red knot migration [13]; the exploitation of favourable winds by high-flying insects [14]. For many further references and examples, see [15,16].…”

Ecophysiology of avian migration in the face of current global hazards

“…Some bird species, in fact, are believed to be unable to complete their migratory journey without substantial wind assistance [104]. Wind conditions are stochastic, and some species have been found to use intermediate stopover sites for emergency refuelling when confronting unexpected headwinds [105]. Although we lack predictions of how wind patterns may change in response to global change processes, such factors must be taken into account when considering global change consequences on flight range.…”

Unexplained altitude changes in a migrating thrush: Long-flight altitude data from radio-telemetry

“…Small (,100 g) migrants represent the largest gap in our understanding as they cannot carry GPS transmitters, which can provide researchers with detailed information about in-flight behavior such as altitude (Bridge et al 2011). Information about flight altitude and associated atmospheric conditions can be used to inform many different aspects of avian migration biology; recently, for example, authors have used altitudinal information collected from GPS transmitters to predict when migrants will use 'emergency' staging sites (Shamoun-Baranes et al 2010) and to better understand the physiological capabilities of migratory birds (Bishop et al 2015). Flight altitude may also affect a bird's probability of dying: If an individual flies too low, it may strike a building, radio tower, or other man-made structure (Longcore et al 2013).…”