How Migratory Birds Solve The Longitude Problem
Migratory songbirds solved the longitude problem long before humans came down from the trees, yet we are only beginning to understand how birds deal with this incredibly difficult problem that tormented and killed people for millennia
Although we rarely think about it now, the greatest scientific challenge of the 17th and 18th centuries was longitude — determining one’s east-west location — and this conundrum was particularly pervasive when traveling across vast oceanic expanses.
The most common way to deal with “the longitude problem” was to sail along the coast to the latitude that passed through a ship’s eventual goal, then sail along that line of latitude until reaching the intended destination. Hopefully. This strategy, known as “westing” (or “easting” when traveling east), significantly lengthened oceanic voyages by days or weeks, leading to poor nutrition, scurvy and other health issues, or starvation for crew members, and increased risks to the vessel itself.
But westing was not fool-proof — and it was dangerous. After a number of spectacular shipwrecks, the British, French and Spanish governments independently established prizes, each worth millions of dollars in today’s money, to be awarded to whomever could solve the longitude problem. Such international focus upon a particular challenge made longitude into one of the largest scientific projects in human history.
Eventually, the longitude problem was solved by a self-educated British carpenter and clockmaker, John Harrison, who invented the maritime chronometer, an extraordinarily accurate and precise clock that served as a portable standard designed to keep the time for a known fixed location, in this case, Greenwich Mean Time (GMT). Using this device, navigators would determine the difference between GMT and local time, and use this information, along with a little spherical trigonometry, to calculate longitude whilst at sea.
But as unknown numbers of people struggled and sank and died in the cold, murky depths of the world’s oceans, migratory birds successfully navigated the skies above their heads, and have done so for millennia, demonstrating they solved the longitude problem long before humans came down from the trees. Yet how birds deal with the longitude problem remained a mystery, even to this very day.
How migratory birds detect longitude was a scientific mystery
We know that birds do not inherit a birdie version of GMT that they can compare to their local time (ref), but they still can accurately correct for east-west displacements. Thus, it is reasonably obvious that migratory songbirds solve the longitude problem in a different way than human mariners do — but how?
Other animals apparently have solved the longitude problem, too, so perhaps they provide some clues for how migratory birds detect longitude? For example, research indicates that sea turtles living in the North Atlantic Gyre system rely upon a slanted grid scheme that coordinates location based upon magnetic inclination versus magnetic intensity (ref) — a system that probably is also used by “homing” pigeons (ref). But this strategy is difficult, at best, to use in areas where magnetic inclination and magnetic intensity isolines run nearly parallel, as in North America and western Europe (Figure 2).
In these areas, birds may instead rely upon another method to detect longitude: magnetic declination. Magnetic declination is the angle between magnetic north (which is detected by a compass) and true(geographic) north (the shortest line to reach Earth’s North Pole, which lies almost directly below the North Star, Polaris).
“As magnetic declination mainly varies along the east-west axis, it provides the possibility to measure longitude,” said Nikita Chernetsov, director of the Rybachy Biological Station and professor of vertebrate zoology at the Saint Petersburg State University, in a press release.
But magnetic declination is variable and for this reason, it often is not very informative for determining longitude in many parts of the world — but it can work quite well in North America and western Europe. For example, in Europe, the difference between magnetic north and geographic north increases from east to west. So if birds can precisely detect that difference, they can use that information to successfully extrapolate their longitude.
An international team of researchers, headed by Professor Chernetsov, has been investigating this phenomenon in Eurasian reed warblers, Acrocephalus scirpaceus, for a number of years.
Reed warblers are small rust-and-buff-colored songbirds that migrate long distances between their marshy or waterfront breeding territories throughout Europe and western Asia, and their winter range in Sub-Saharan Africa (yellow; Figure 2).
Based on previous work, the researchers knew that reed warblers navigate by sensing Earth’s magnetic field and — somehow — combine that information with celestial cues to inform and guide their autumnal migratory route (ref & ref). But how? Is it possible that reed warblers can actually measure magnetic declination? This was the hypothesis proposed by the research team.
To test their hypothesis, the researchers captured 15 adult reed warblers during autumnal migration at the Rybachy Biological Station, a popular stopover for migratory birds on the Baltic Sea in northwestern Russia. The songbirds were housed in outdoor cages made from wood and netting that allowed them to see the night sky. The cages also had sloping funnel-shaped floors that recorded the birds’ movements as they tried to scramble up the steep incline and into the night sky.
The cages were placed within a magnetic coil system that can be used to very accurately change magnetic field parameters (ref). Initially, the caged warblers were allowed to orient themselves under starry skies according to the magnetic field at Rybachy. They tried to migrate west-southwest to Gibralter, which is the seasonally appropriate direction for this species.
After the researchers established the direction the birds wanted to migrate, they used this coil to rotate the magnetic field by 8.5 degrees counter-clockwise whilst keeping everything else unchanged. This recreated the magnetic field that the birds experience in southern Scotland — 1,450 kilometers (900 miles) away. Surprisingly, the warblers housed in the rotated magnetic field abruptly changed their migratory direction to east-southeast by 151 degrees — as if they had suddenly been relocated to Aberdeen, Scotland.
But this response was only seen in adult warblers: when the researchers subjected 25 young-of-the-year birds to the same treatment, the naive warblers were unable to reorient themselves properly, and were “perplexed and became disoriented”, according to the authors (ref).
Based on this finding, the researchers concluded that birds inherit a simple compass mechanism from their parents, and they integrate this with the mental map of the world that they build during their first migration.
“Reed warblers seem to learn the large-scale spatial pattern of the declination gradient during their annual movements, just like they learn other gradients, inclination, and total intensity,” Professor Chernetsov explained.
Reed warblers are the first animal known to use magnetic declination to address the longitude problem
“We’ve shown for the first time that magnetic declination may be a component of the magnetic navigational map, at least in some long-distance migratory birds,” said Professor Chernetsov.
“It seems that a bird as unassuming as the reed warbler, may have a geographic map or memory that enables it to identify its longitudinal position on the globe, only by detecting the magnetic north pole and its variance from true north,” elaborated a study co-author, Richard Holland, a Senior Lecturer in Animal Cognition at the University of Bangor, in a press release.
Do reed warblers integrate their biological compass with other cues, such as star patterns, as they learn their migratory route? Probably.
“This, combined with other external cues, which may include the strength of the magnetic field, star positions or smells enables it to locate its current position and orient itself during a long migration,” said Dr. Holland.
These findings make me wonder whether other long-distance or oceanic travellers — such as albatross, which wander hundreds of thousands of kilometers annually over Earth’s oceans — rely upon magnetic declination to determine their location? Might these little brown birds provide important hints for how other animals navigate over hundreds or thousands of kilometers? And of course, let’s not forget practical applications, such as the possibility that magnetic declination could be a useful navigational tool that we might use, too.
“We humans do not use the magnetic map for our navigation, but we might want to look into this option,” proposed Professor Chernetsov.
Nikita Chernetsov, Alexander Pakhomov, Dmitry Kobylkov, Dmitry Kishkinev, Richard A. Holland, and Henrik Mouritsen (2017). Migratory Eurasian Reed Warblers Can Use Magnetic Declination to Solve the Longitude Problem, Current Biology, published online on 17 August 2017 ahead of print | doi:10.1016/j.cub.2017.07.024
Dmitry Kishkinev, Nikita Chernetsov and Henrik Mouritsen (2010). A Double-Clock or Jetlag Mechanism is Unlikely to be Involved in Detection of East–West Displacements in a Long-Distance Avian Migrant, The Auk 127(4):773–780 | doi:10.1525/auk.2010.10032
James L. Gould (2008). Animal Navigation: The Longitude Problem, Current Biology 18(5):R214–R216 | doi:10.1016/j.cub.2008.01.011
Dmitry Kishkinev, Nikita Chernetsov, Alexander Pakhomov, Dominik Heyers, and Henrik Mouritsen (2005). Eurasian reed warblers compensate for virtual magnetic displacement, Current Biology25:R811–R826 | doi:10.1016/j.cub.2015.08.012
Manuela Zapka, Dominik Heyers, Christine M. Hein, Svenja Engels, Nils-Lasse Schneider, Jörg Hans, Simon Weiler, David Dreyer, Dmitry Kishkinev, J. Martin Wild and Henrik Mouritsen (2009). Visual but not trigeminal mediation of magnetic compass information in a migratory bird, Nature 461:1274–1277 | doi:10.1038/nature08528
Read more about migration:
GrrlScientist. “Can We Save Europe’s Migratory Birds?”, Forbes, 7 November 2016. (Medium link.)
GrrlScientist. “Watch: Songbirds Return to North America”, The Guardian, 4 July 2015. (Medium link.)
GrrlScientist. “Butterbutt biology: what mitochondria teach us about bird migration”, The Guardian, 3 October 2013. (Medium link.)
GrrlScientist. “Wise old birds teach migration route to young whooping cranes”, The Guardian, 2 September 2013. (Medium link.)
GrrlScientist. “Watch: Discovering dragonflies that cross oceans”, The Guardian, 6 April 2011. (Medium link.)
GrrlScientist. “Fly Me to the Moon: The Incredible Migratory Journey of the Arctic Tern”, ScienceBlogs, 13 January 2010. (Medium link.)
GrrlScientist. “Will The Great Animal Migrations Disappear Forever?” ScienceBlogs, 30 July2008. (Medium link.)
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Originally published at Forbes on 22 August 2017.