Barometer logging reveals new dimensions of individual songbird migration

Abstract

Recent advances in tracking technology are based on the use of miniature sensors for recording new aspects of individual migratory behaviour. In this study, we have used activity data loggers with barometric and temperature sensors to record the flight altitudes as well as ground elevations during stationary periods of migratory songbirds. We tracked one individual of red‐backed shrike and one great reed warbler along their autumn migration from Europe to Africa. Both individuals performed their migration stepwise in travel segments and climbed most metres during the passage across the Mediterranean Sea and the Sahara Desert and least metres during the first flight segment in Europe. The great reed warbler reached its highest flight altitude of 3950 m a.s.l. during the travel segment from Europe to west Africa, while the red‐backed shrike reached 3650 m a.s.l as maximum flight altitude during its travel segment from Sahel to southern Africa. Both individuals used both lowlands and highlands for resting periods along their migrations. Furthermore, temperature decreased with increasing altitude during migratory flights for both individuals, highlighting the potential to determine flight duration from temperature measurements. Finally, we discuss how barometric data could be used to investigate birds’ responses to changes in air pressure as a cue for departures on migratory flights. This new technique, i.e. using a miniature data logger with barometric pressure sensor to estimate flight altitudes and ground elevations, will open up new avenues for research and importantly advance our understanding on how small birds behave during migratory flights.

Integrating light‐level geolocation with activity tracking reveals unexpected nocturnal migration patterns of the tawny pipit

“…Therefore, we performed a second classification based on atmospheric pressure recordings and cross‐validated the two approaches. Atmospheric pressure is not expected to change rapidly when the bird is stationary (weather related changes) but can vary considerably as the bird takes altitude or covers substantial distance in flight (Liechti et al , Sjöberg et al ). Because background variation in air pressure linked to weather rarely exceeded 1 hPa 30 min −1 , we used a threshold of atmospheric pressure change > 1.5 hPa 30 min −1 for identifying flight phases within the dataset.…”

“…State‐of‐the‐art multi‐sensor tracking devices that integrate measurements of ambient light, activity and barometric pressure have the potential to bridge this gap in our knowledge and to provide novel insights into species‐ and individual‐specific migratory behaviour along their entire migratory journeys (Bäckman et al , b, Dhanjal‐Adams et al , Liechti et al , Sjöberg et al ). Here we use multi‐sensor individual tracking to describe migratory patterns and flight behaviour over the entire journey of tawny pipits Anthus campestris breeding in Central Europe.…”

Urban areas affect flight altitudes of nocturnally migrating birds

“…Furthermore, we also found a positive relationship of temperature with the difference of flight altitudes between urban and non‐urban areas at the lowest quantiles of the distribution in both seasons (Figure ), suggesting that higher flight altitudes are related to higher temperatures over urban areas, potentially in relation to the nocturnal heat island effect (Oke, ). Increasing altitude as a response to high temperatures during migration has been recorded at the individual level in one shorebird species (Senner et al, ) and two songbird species (Sjöberg et al, ).…”

“…New technologies such as low‐weight multi‐sensor tags (e.g. Sjöberg et al, ) will provide finer resolution data to test some of the ideas posed here. Nevertheless, we acknowledge that the differences in flight altitudes between urban and non‐urban areas may also respond to other urban‐related factors such as air pollution, volatile organic compounds, noise, electromagnetic radiation and/or polarized light pollution.…”

Light‐level geolocator analyses: A user's guide

“…There are several recent technological advancements that will perhaps even increase their appeal. The most important among them is probably the combination of light recording with other en route data, such as air pressure, acceleration, magnetism and temperature (Bäckman et al, 2017;Dhanjal-Adams et al, 2018;Sjöberg et al, 2018). These additional data can facilitate the refinement of location estimates (see step 4 in the analysis).…”