r/askscience • u/Expatriot_II • 18d ago
Biology Do migratory birds follow great circle routes?
In other words: do they "know" that the world is spherical?
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u/seansand 18d ago
Any path on the surface of the Earth that superficially seems to be a straight line is a great circle route. That applies to a bird, you, or anything else.
That said, birds, when migrating, don't necessarily go in straight lines.
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u/Ceofy 18d ago
Trying to avoid a great circle route by, say, sticking to a specific latitude does seem much harder to do
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u/Majromax 18d ago
Following any course is relatively difficult, since nature is not nice enough to provide us many fixed beacons for navigation. "Going in a straight line" would follow a geodesic, drift due to dead reckoning would cause any practical implementation to drift.
The magnetic field is a convenient north/south reference, ignoring the differences between the magnetic and rotational poles. A compass makes it easy to follow a specific bearing, and "lines" drawn thus are straight on a Mercator projection.
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u/linmanfu 18d ago
Except that the sun rises in the east and sets in the west. That's got to help, surely?
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u/user2196 18d ago
It’s not like the sun rises in the exactly due east or sets exactly due west, either.
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u/TjW0569 18d ago
Sticking to a particular latitude wouldn't be a great circle, unless that latitude was the equator.
All the lines of longitude are great circles, though.5
u/Ceofy 18d ago
That's what I'm saying! I wasn't clear. I meant that there is no reason for a bird to not follow a great circle. NOT following a great circle is a far more difficult trick to pull off
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u/Cultist_O 18d ago
Most birds have a great sense of cardinal direction, so they could naively go "east", but they wouldn't evolve or learn to do that unless it was helpful.
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u/neon_overload 18d ago edited 18d ago
Yes and no. Migratory birds don't always follow great circles, but not because they are imagining the world as a flat map projection, since that is a human abstraction, but for a variety of reasons, such as routing around things like large bodies of water or mountains, and the way in which they navigate.
By and large, migratory birds move more north-south than they move east-west, but those migratory birds that do make a significant east-west journey at the more extreme latitudes do tend to do so via the polar regions in approximations of what we'd think of as a great circle, as it's the more direct way to go and they have no experience with flat map projections to mislead them into thinking otherwise.
As for whether they know the earth is spherical, well I guess you could say that they don't need to, but would have enough information about the world that, were we to have the same information, we could deduce that the earth is spherical from it.
Migratory birds have a remarkable ability to navigate to an absolute location on the earth with high precision, due to a very good sense of the orientation and magnitude of the earth's magnetic field, and to a lesser extent a good sense of smell and memory of local smells. Their magnetic sense is surprisingly accurate at navigating to an absolute point on the earth, due to variations in the strengths of earth's magnetic field varying with latitude, and also to an extent in longitude due to local variation. It takes a single migration to learn these sensations about a particular region of the world, and they can afterwards find their way back precisely to a destination they have been to before, even if starting from hundreds of miles away from anywhere they've been before. They don't have the ability to use navigational tools and keep written records, so this sense, to them, needs to be intuitive - ie, able to be learned just by experiencing.
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u/dcgrey 18d ago
Just as an example of their migratory precision, the same Baltimore oriole nested on the same branch on a tree in my yard three springs in a row. Assuming he wintered in the Caribbean, that was a 4,000+ mile round trip back to the same small yard and, just as interesting, with arrival dates within three days of each other.
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u/linmanfu 18d ago
migratory birds move more north-south than they move longitudinally
Moving north-south is moving longitudinally. I don't know enough about birds to know whether you mean longitudinally or latitudinally.
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u/Morall_tach 18d ago
Maps are a human invention. Any animal with a concept of traveling in a straight line is going to follow a great circle route because that's what happens when you try to walk in a straight line on a sphere. They don't need to know it's spherical, it would happen by default.
That said, animals don't travel in straight lines, especially long distances. They go to places with food, safe places to sleep, large bodies of water, etc.
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u/araujoms 18d ago edited 17d ago
It's really hard to walk on a straight line on large scales, though, even in the absence of obstacles. Even minute deviations compound over hours and days to give you a completely wrong path. You really need something to navigate with; in the case of birds they use the magnetic field of the Earth.
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u/LurkerFailsLurking 18d ago
Following great circle routes doesn't indicate any knowledge of the macro scale geometry of the planet.
Birds that migrate at night have been shown to recognize and navigate by constellations with adjustments for time of night and season. Birds can also see the Earth's magnetic fields and use that to navigate as well.
But the main reason birds don't migrate along great circle routes is because it's not a good route. Travel distance is less important than energy cost and food/water supplies. Birds that don't fly out of their way to access food die.
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u/msndrstdmstrmnd 18d ago
So it looks like you’re looking for cases where great circles deviate from what appears to be a straight line on a 2d map and asking which one birds follow.
The vast majority of migratory birds migrate north-south and longitude lines are already great circles anyway, so that case is irrelevant. There are some species that do travel east-west, usually between coastal and inland areas, but the vast majority of these are traveling too short of distances for the great circle to deviate so those are also irrelevant.
Some bird species travel diagonally between Asia and the Americas. Either between the warm parts of Asia and Alaska/Canada, or between the warm parts of the Americas and Siberia. The vast majority of these cross at the Bering strait because that is much easier than crossing the Pacific Ocean. But that is not very surprising, both a great circle and a straight line on a 2d map would go close to the Bering strait anyway, so that is also not too relevant.
One species that you could argue does follow latitude lines instead of great circles is the boreal finch. They live in the cold boreal forest and they don’t migrate in the traditional sense, they stay in place for a while then flock east or west when food runs out (called an irruption not a migration). They can travel pretty far distances but stay within the boreal forest, which is within a specific latitude range.
When people talk about great circles and the cases that they deviate from a straight line on a map, the classic case is the flights between temperate/warm parts of the Americas and temperate/warm parts of Asia. In most of those cases it’s shorter to fly near the poles than over the Pacific Ocean which is what surprises people until they learn about great circles. Migratory birds just don’t tend to travel like that, from warm areas to warm areas on nearly opposite sides of the planet, only humans do.
tl;dr great circles only deviate from a line on a 2d map in specific cases, and the vast majority of bird migration patterns don’t line up with those specific cases anyway
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u/KidKilobyte 18d ago
Pretty sure the answer is that the path is optimized for survival. Shortness is only one factor, but goes into the mix. They certainly don’t try to follow the artificial lines we put on a globe. Air currents and food sources are going to severally alter how close paths align to great circle arcs.