Associations of Synoptic Weather Conditions With Nocturnal Bird Migration Over the North Sea

Maja Bradarić,Rob Groenland,Ruben Fijn,Judy Shamoun-Baranes,Iris Manola,Willem Bouten

doi:10.3389/fevo.2020.542438

Abstract

The southern North Sea is part of an important flyway for nocturnal bird migration, but is also risky as it stretches over a large surface of water. Selecting nights with suitable weather conditions for migration can be critical for a bird’s survival. The aim of this study is to unravel the weather-related bird migration decisions, by providing a descriptive analysis of the synoptic weather conditions over the North Sea on nights with very high and low migration intensities and compare these conditions to the prevailing climatology. For this study, bird radar data were utilized from an offshore wind farm off the Dutch coast, in the North Sea. The study suggests that atmospheric conditions clear of rain and frontal systems, dominated by high pressure systems and tailwinds in spring and sidewinds in autumn are most suitable for nights of intense migration. Differences in temperature, relative humidity and cloud cover appear less significant between intense and low migration nights, suggesting that these variables exert only a secondary role on migration. We discuss how future developments in radar aeroecology and the integration of meteorology can help improve our ability to forecast bird migration.

Highlights

Twice a year large flows of terrestrial birds migrate over the North Sea during the night (Lack, 1963a,b; Hüppop et al, 2006; Hüppop and Hüppop, 2011; Shamoun-Baranes and van Gasteren, 2011)
Using 3 years of bird migration data measured off the Dutch coast, we revealed differences in the synoptic weather conditions occurring during high and low migration intensity nights in spring and autumn
The MSLP anomalies during low migration nights both in autumn and spring were not statistically significant, suggesting either a weak coupling between synoptic pressure systems and nights of low migration, or a combination of nights with unfavorable synoptic conditions for migration and nights of resting and feeding that might be decoupled from the synoptic pressure systems

Summary

Introduction

Twice a year large flows of terrestrial birds migrate over the North Sea during the night (Lack, 1963a,b; Hüppop et al, 2006; Hüppop and Hüppop, 2011; Shamoun-Baranes and van Gasteren, 2011). The crossing mainly occurs between the lowlands of Netherlands and Germany and Norway (cca 540 km), Denmark and Netherlands (cca 300 km) and between United Kingdom and Netherlands (cca 200 km) (Lack, 1959; Shamoun-Baranes and van Gasteren, 2011; Bradaricet al., 2020) To cross such distances, a bird with the average airspeed of 16 m/s needs between 3.5 and 10 h in still air. A bird with the average airspeed of 16 m/s needs between 3.5 and 10 h in still air Since such large water bodies are not suitable for resting and refueling for many terrestrial migrants, the trip is considered risky for migration, especially if at the same time birds have to cope with unfavorable weather conditions.

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