As tagging devices get smaller and more sophisticated, they are revealing even more wonders of shorebird migration. We already know that Great Snipe are amazing – flying up to 7,000 km non-stop – but a 2021 paper by Åke Lindström and colleagues describes a striking daily cycle of altitude change during their long migratory journeys.
In their new paper in Current Biology, Åke Lindström and colleagues have used activity and air pressure data from multisensor dataloggers to show that Great Snipes repeatedly changed altitudes around dawn and dusk, between average cruising heights about 2,000 m (above sea level) at night and around 4,000 m during daytime. Most birds regularly flew at 6,000 m and one bird reached 8,700 m, an altitude that is just 150m short of clearing the top of Mount Everest! The same daily cycle was apparent everywhere – independently of climate zone, habitat and the height of the land being overflown. Wherever they are, as morning breaks migrating Great Snipes gain altitude – but why?
Great Snipe spend the winter in Africa, between 20 degrees south and 15 degrees north of the equator, heading north in spring to breeding areas in Scandinavia and northern Europe, as far east as 95°E (which is the same longitude as Myanmar).
The first paper to reveal the remarkable migration of Swedish Great Snipe appeared in Biology Letters in 2011, when Raymond Klaassen, Åke Lindström and colleagues revealed the tracks from three birds carrying geolocators. During these journeys, these individuals made long and fast autumn movements, covering between 4300 and 6800 km in two to four days and overflying suitable stopover sites that were used in spring. Ground speeds of 15 to 27 m/s are equivalent to between 54 and 97 km/h (33 to 60 miles per hour).
In 2016, using 19 tracks from four years of captures and recaptures, the same team published a paper in the Journal of Avian Biology. About half of the birds flew directly from the breeding grounds to sub-Saharan Africa, the others making a few shorter flights down through Scandinavia before embarking on a long trans-Sahara flight. Birds took advantage of wet seasonal conditions in this Sahel region for three weeks before moving south to the Congo Basin. Spring migration consists of a rapid movement across the Sahara, only a little shorter than the very long non-stop autumn fights, followed by slower movements through Eastern Europe. Birds arrived back on breeding areas in mid-May.
Great Snipes spend about eight months of the year in sub-Saharan Africa. In a 2017 paper in Wader Study, Edouard Debayle, Åke Lindström and colleagues analysed the moult and fattening patterns of over 3,000 hunted birds, to try to learn more about the phenology of migration. They discovered that:
- Adult males arrived in Africa from mid-August, having started and suspended the moult of the main flight feathers before arrival.
- Females on average arrived somewhat later and were about one month behind in the progress of flight feather moult.
- The adults of both sexes resumed primary and secondary moult immediately upon arrival and typically completed it by the end of November, in males, and the end of December in females. Juvenile Great Snipes arrived later than adults and did not moult their flight feathers in the first autumn/winter.
- Males apparently departed northwards between late March and late April, and the females about two weeks later. There is information about rates of fat deposition in the paper.
Several factors could influence the flight altitude of migratory birds. For example, how high is the land over which birds fly, what are the best temperatures and humidity conditions for efficient flight, at what height can a bird find the most helpful winds, can a bird use navigation landmarks and how can predation best be avoided?
Recent tracking of migratory birds of a range of species has shown that individuals change flight altitude more commonly and dramatically than previously thought but why? In their paper in Current Biology, Åke Lindström and colleagues reveal information from 25 tracked Great Snipe journeys and discuss the reasons that may lie between the patterns that they see.
Information about behaviour and flights was collected from multisensor dataloggers, consisting of an accelerometer for activity measurements, a barometric pressure sensor with internal temperature sensor, a light-level sensor, a real-time clock, and memory. The dataloggers weighed 1.4–1.7 g (about 1 % of a bird’s total body mass) and were attached to a plastic ring on the bird’s tibia.
In total, 107 dataloggers were put on Great Snipes between 2015 and 2019. Of these, 36 birds were retrapped one, two or three years later (but four birds had lost their loggers). This is an overall recapture rate of 34%, which is similar to the figure for ringed birds. In total, 25 out of the 32 retrieved loggers had functioned for some time, and 16 carried information on flight altitude for at least one of the long flights.
The methods section provides full details of how geolocator data were interpreted and altitudes were calculated and adjusted. Information on air temperatures at different altitudes and the topological features on flight paths provided a background against which to try to understand migration patterns.
The new data from a small number of multisensor data-loggers greatly enriched the migration story of Swedish Great Snipe, as revealed byÅke Lindström and colleagues. The key results in the Current Biology paper are:
Flight duration: There were three long flights, two legs on the way south and one on the way north. As noted earlier, northerly migration slowed once birds landed in Europe.
- On average, male Great Snipe left Sweden on 24th August and flew across Europe and the Sahara for 73.4 hours, before landing in the Sahel.
- The mean departure date from the Sahel was 24th September. An average of 23.2 hours later a bird would land in the Congo basin.
- Northerly flights commenced on 18th April, lasting an average of 82.4 hours and concluding in Europe.
Cyclic flight altitudes: There was an overall strong and consistent daily cycle in the altitudes used by the Great Snipes, in all three long flights. After a night at moderate to high altitudes the birds ascended to very high altitudes in early morning, stayed at these levels during the day, and descended again in late afternoon. They then repeated this cycle for one or two more days.
- The mean individual daytime flight altitude in the first Autumn flight was on average 4,549 m, compared to 2,126 m at night.
- For the In-Africa autumn flights, comparative figures were 3,874 m in daytime and 1,860 m at night.
- For the Spring flights, comparative figures were 4,114 m in daytime and 1,612 m at night.
These altitudes were estimated from air pressure readings and may be underestimates. For comparison, the highest point in the Alps is 4807 m.
Peak altitudes: Some Great Snipes occasionally flew extremely high and then always during daytime. Three birds in Autumn and two birds in Spring reached 7,000 m or more. Migratory waders are able to carry out flapping flight at such high altitudes due to several physiological adaptations of the heart, lungs and muscles. The single highest altitude estimate of 8,077 m was reached in Autumn. If air pressure is accounted for this bird may have been flying above 8,000 m for five hours, perhaps reaching an altitude of 8,700 m, and coping with an air temperature of -21.3 °C. Putting this in a local context for readers: Mount Everest is 8848m high and the high points in other regions of the world are: Africa (5895 m Kilimanjaro), South America Aconcagua 6959 m, North America Denali 6190 m and New Zealand Mt Cook 3754 m.
Ambient temperature, wind condition and humidity appeared not to influence the differences in day and night altitudes chosen by Great Snipes.
Discussing the results
The daily pattern of altitude changes for Great Snipes was very similar between Autumn, In-Africa and Spring flights, suggesting a common cause that is largely independent of climate zone (temperate or tropical), topography and landscape overflown (forest, savanna, farmland, desert or water). Altitude changes have been reported for other waders, such as Black-tailed Godwits, that have been linked to both ambient temperatures and finding more beneficial winds.
The authors discuss the possibility that landmarks are easier to see from a higher altitude when flying in daylight and suggest that predator avoidance may also account for higher day-time elevations. It would be interesting to know if a daily cycle is apparent in long flights over areas largely lacking bird predators, such as vast oceans.
There is no daily cycle in ambient air temperature or wind conditions at high altitudes that could explain the overall regular pattern of flight altitude selection found in Great Snipes but the authors discuss the theory that the warming effect of solar radiation may be countered by flying through colder, higher air. The temperatures at these heights would be too cold at night.
There are still few papers that provide altitude data for long-distance migrating birds but all of them report altitude changes and have revealed some surprisingly high flight altitudes. With more studies we may well find that migration is even more impressive than we already thought!
The full methods, results and discussion can be read in the paper in Current Biology.
Extreme altitude changes between night and day during marathon flights of Great Snipes Gallinago media: Åke Lindström, Thomas Alerstam, Arne Andersson, Johan Bäckman, Peter Bahlenberg, Roeland Bom, Robert Ekblom, Raymond H. G. Klaassen, Michał Korniluk, Sissel Sjöberg & Julia K. M. Weber.
WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience.