Groping in the Fog: Soaring Migrants Exhibit Wider Scatter in Flight Directions and Respond Differently to Wind Under Low Visibility Conditions

Paolo Becciu,Nir Sapir,Michele Panuccio,Giacomo Dell’Omo

doi:10.3389/fevo.2021.745002

Abstract

Atmospheric conditions are known to affect flight propensity, behaviour during flight, and migration route in birds. Yet, the effects of fog have only rarely been studied although they could disrupt orientation and hamper flight. Fog could limit the visibility of migrating birds such that they might not be able to detect landmarks that guide them during their journey. Soaring migrants modulate their flight speed and direction in relation to the wind vector to optimise the cost of transport. Consequently, landmark-based orientation, as well as adjustments of flight speed and direction in relation to wind conditions, could be jeopardised when flying in fog. Using a radar system operated in a migration bottleneck (Strait of Messina, Italy), we studied the behaviour of soaring birds under variable wind and fog conditions over two consecutive springs (2016 and 2017), discovering that migrating birds exhibited a wider scatter of flight directions and responded differently to wind under fog conditions. Birds flying through fog deviated more from the mean migration direction and increased their speed with increasing crosswinds. In addition, airspeed and groundspeed increased in the direction of the crosswind, causing the individuals to drift laterally. Our findings represent the first quantitative empirical evidence of flight behaviour changes when birds migrate through fog and explain why low visibility conditions could risk their migration journey.

Highlights

Atmospheric conditions are known to affect flight propensity, behaviour during flight, and migration route in birds (Shannon et al, 2002; Shamoun-Baranes et al, 2010, 2017; Sherub et al, 2016; Becciu et al, 2019)
The aim of this study was to examine the effects of fog on orientation and response to wind in soaring migrants tracked by a marine radar over a migration bottleneck, The study was carried out at the Strait of Messina in Southern Italy, where Honey Buzzards (Pernis apivorous) comprise approximately 95% of the tracked migrants (Becciu et al, 2018; Panuccio et al, 2018)
The mean flight direction of birds differed significantly between birds that flew under clear skies (96% of the tracks) and fog (4% of the tracks) conditions (Watson’s U2 test: test-statistic = 2.99, p < 0.001), with a mean direction of 57.3◦ under clear skies and 80.7◦ under foggy conditions (Figure 2)

Summary

Introduction

Atmospheric conditions are known to affect flight propensity, behaviour during flight, and migration route in birds (Shannon et al, 2002; Shamoun-Baranes et al, 2010, 2017; Sherub et al, 2016; Becciu et al, 2019). Migratory birds are adapted to fly under different weather conditions, but in general, they prefer to fly in favourable atmospheric conditions. Fog Affects Bird Orientation and Speed weak headwinds and is avoided under strong headwinds (Åkesson et al, 2016; Eisaguirre et al, 2018; Becciu et al, 2020; Santos et al, 2020; Nourani et al, 2021). Fog and low clouds may lower the visibility of landmarks and hamper orientation (Richardson, 1978, 1990; Chiaradia et al, 2007; Pastorino et al, 2017). Birds may benefit from such weather, the low visibility associated with fog may cause disorientation and avoidance of flight (Pastorino et al, 2017; Panuccio et al, 2019)

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