Following the wingbeats of migratory birds

    2008 marked a turning point in bird migration research – it was the year when miniaturised lightlevel geolocators were first deployed on small landbirds, Hoopoes, in Switzerland. What new insights have we gained since then?

    Geolocators are tiny and weigh less than 1 gram. They enable us to study the migration routes and migratory behaviour of small birds.
    Geolocators are tiny and weigh less than 1 gram. They enable us to study the migration routes and migratory behaviour of small birds.
    Photo © Swiss Ornithological Institute
    Geolocators give us surprising insights into the social behaviour of migratory birds. Data show, for example, that European Bee-eaters travel with “friends”.
    Geolocators give us surprising insights into the social behaviour of migratory birds. Data show, for example, that European Bee-eaters travel with “friends”.
    Photo © Bernd Skerra
    Alpine Swifts are true athletes of the avian world: they can stay in the air non-stop for up to 200 days, as we now know thanks to geolocators.
    Alpine Swifts are true athletes of the avian world: they can stay in the air non-stop for up to 200 days, as we now know thanks to geolocators.
    Photo © Marcel Burkhardt
    Contrary to what many people thought, Common Hoopoes migrate mainly at night and only fly short distances during the day.
    Contrary to what many people thought, Common Hoopoes migrate mainly at night and only fly short distances during the day.
    Photo © Marcel Burkhardt

    GPS trackers allow us to record the movements of large birds like eagles or storks around the clock. However, transmitting GPS coordinates uses a lot of energy, and a battery with enough power to last a whole year would be too large and too heavy for small birds. That’s why 15 years ago, the Swiss Ornithological Institute – in collaboration with the Bern University of Applied Sciences in Burgdorf and as one of the first institutions in the world – took up the challenge of developing geolocators for use in bird migration research. In early days, geolocators were relatively simple devices that measure ambient light intensities or light-levels at a given interval and store each recording together with a timestamp. This is also where the commonly used name – light-level geolocators – comes from. Later, measured day length is translated into latitude and solar noon time into longitude using astronomical algorithms, and this information can be used to determine the whereabouts of birds, their wintering and stopover sites, and migration timing.

    Unlike GPS trackers, geolocators store the data without sending them, which means they can make do with a small, light battery. If the tagged birds can be recaptured at the breeding site at the end of their journey, this technique allows us to trace the migratory pathways of small birds down to the size of a Whinchat.

    Rapid advances in the technology soon followed, revolutionising the potential applications of geolocators and our knowledge on bird migration. In addition to ambient light, more advanced types of geolocators can nowadays measure atmospheric pressure, acceleration and temperature, providing far-reaching insights into the lives of birds across full annual cycles. The newest generation of these geolocators, called μ Tag, provides a remote data download option via a VHF antenna. When combined with a solar panel, these tags can theoretically last for years providing data without the need to recapture the bird in hand.

    Surprising new insights into bird migration

    Geolocators have become an irreplaceable tool for researchers studying small-bodied birds (<100 g body weight), their migration patterns, distribution, behaviour, and interactions with the environment. Geolocators have brought light into the hitherto little-known lives of birds after they leave their temperate breeding grounds in Europe.

    As recently as 2009, there was only one ring recovery of a Hoopoe from sub-Saharan Africa, and we hardly knew anything about their migration. The application of geolocators has helped to map population-specific wintering regions and estimate migratory connectivity (i.e., linkage between breeding and wintering populations) of Hoopoes breeding across Europe. They have revealed that the Hoopoe – long assumed to be a diurnal migrant – in fact migrates predominantly during the night. Around 90 % of all flights of the tracked individuals were carried out in the dark, while the regular occurrence of short diurnal flights, and thus the potential for visual observations during the day, may have led to the false assumption that the species was a mostly diurnal migrant.

    Through studies with Tawny Pipits, we learnt that the ratio of time that the birds actually spend flying versus on the ground during migration is about 1 to 7. That means, for every hour of flight, Tawny Pipits need about 7 hours for resting, feeding, and accumulating energy reserves to power their next flight.

    Marathon flights

    The use of geolocators has also revolutionised our knowledge on how long-distance migrants cross ecological barriers, like the Mediterranean Sea and the Sahara Desert, where resting and feeding opportunities are scarce. Most trans-Saharan migratory passerines are nocturnal migrants, i.e., they travel at night and rest during the day. The predominant view up until recently has been that they do so also during the approximately 2,000 km long crossing of the Sahara Desert. Data from the three sensors – light, atmospheric pressure, and accelerometer – situated on geolocators all independently indicated that this might not be the case in most species. Instead, the data suggested that these birds regularly prolong their flights into the day and can fly uninterrupted for as long as 44 hours, as found in a Great Reed Warbler breeding in Kaliningrad, Russia. Such a marathon flight can carry the bird across the Sahara Desert in one go. Furthermore, data recorded on the pressure sensor allows us to estimate altitudes at which the birds are flying – as the sun rises, some Great Reed Warblers climb to astonishing 6,000 m above sea level. They likely do so to take advantage of favourable wind conditions in the upper layer of troposphere and/or to escape the sweltering daytime heat of the desert below.

    When it comes to marathon flights, no other group of birds takes it to extremes as swifts do. The application of geolocators on Alpine Swifts in a breeding colony in Baden, canton of Aargau, provided the first unequivocal evidence that swifts can stay airborne continuously for more than six months throughout migration and the non-breeding period in Africa. This means that all physiological processes, including resting, moulting feathers, and sleeping, are carried out while in flight. Coupling this information with data on atmospheric pressure, we could later reveal daily patterns of vertical airspace use by the Alpine Swifts. Interestingly, every evening and morning swifts appear to ascend several hundred meters higher into the air for about an hour before descending again. The reason behind these twilight ascents remains a mystery, but it may well be a part of social behaviour that we do not yet fully understand.

    Social interactions between individuals are an aspect of bird behaviour where geolocators have provided unforeseen information. Tracking of European Bee-eaters using multi-sensor geolocators revealed that some non-kin individuals remain together yearround, breeding, migrating, and spending the non-breeding period as a group – one could say “a group of friends”. These birds not only used the same non-breeding sites, but also displayed coordinated social foraging behaviour, often feeding together. Even more surprisingly, some individuals that did separate while migrating met again later in the non-breeding areas, having flown more than 5,000 km apart.

    International collaboration

    These are just some of the highlights from more than 100 geolocator studies the Swiss Ornithological Institute has been involved in during the last 15 years. Many of them are carried out together with international partners to uncover previously unknown migration routes, stopover areas, and non-breeding sites of little- studied species and populations. International outreach is also important for comparative studies and an improved understanding of large-scale migration patterns that link European breeding grounds with African and Indian non-breeding sites. It helps to understand how migratory birds interact with the environment, how their physiology and health status influence migratory decisions, performance, and individual survival. Since birds don’t know borders, international collaboration in this field is vital. Joining forces with other researchers helps to identify places along the flyways and on the non-breeding grounds that are important for species conservation. Joint efforts in fundamental research can therefore improve the protection of migratory birds, many of which are currently in steep decline.