Hello! Have you ever wondered, do blind birds can navigate?
Welcome to monthly mailbag,
where we answer science questions from the comments section.
Seris Efthimiadis asked this one after watching Jim Al-Khalili’s Discourse on quantum biology.
It’s a fantastic question and I’m not sure there really is a definitive answer to,
but let’s dive in and see if we can peck away at an answer.
We all know birds are amazing navigators. Think of the massive migrations of Canadian
Geese, thousands of kilometres every year across North America. Or the homing ability
of pigeons.
In fact pigeons are probably the best studied and most impressive navigators, so lets focus
on them for today.
Pigeons can find their way back across hundreds of kilometres of totally unfamiliar territory.
How do they do it?
It turns out to be pretty complicated and scientists think pigeons use a whole toolbox
of senses and strategies, many, but not all, of which do rely on functional eyes. But first,
to understand what’s going on, let’s break down navigation:
There are two key parts of navigation: determining where you are in space, a map, and which determining
direction is which, a compass.
You then need to constantly update those two things to make sure you are where you think
you are and you’re going where you think you're going.
So let’s imagine dropping a bird off at the Eiffel Tower in Paris, 342km from the
Ri in London. How will it get home?
Our bird's never been to Paris, so there’s no chance of it navigating just by visual
landmarks it might recognise. Which is what we think pigeons usually do. So how is it
going to get home?
First, the compass we talked about. Birds use two major sources of information: the
position of the sun and the stars in the sky, and magnetoreceptors in their eyes. Both of
these mechanisms depend on working eyes at least to some extent, so it’s not looking
very good for Seris’ blind birds.
The sun can act as a compass for pigeons because they have a very accurate internal clock,
so they can tell which direction is which based on the position of the sun or the stars
in the sky. Scientists have been able to test this ability by artificially making birds
change time zones. They turn lights on in the middle of the night and they turn them
off during the day. Birds that have been time-shifted like this will get their directions wrong,
but not randomly, they'll get them wrong in a very predictable way.
For example, if home is due west and it is 6pm in their head, then they’ll expect the
sun to be, let’s say, 20 degrees above the Western horizon. So they’ll fly towards
it. But if they’ve been time shifted so that it’s actually 10am and the sun has
recently risen, they’ll fly East, in exactly the wrong direction.
But if it’s cloudy, then even time-shifted birds navigate just fine, so we think the
pigeon’s first option is the sun compass, and then they resort to using their other
senses, including magnetoreceptors in their eyes.
Now THIS is cool…
There’s a type of protein called cryptochrome that lives in the retinas of a lot of birds.
And it's reactive to blue light. So what scientists think happens is that when blue light hits
cryptochrome, it becomes “active” and will remain so for a little while until it
deactivates again. The amount of time that it stays active is affected by the strength
of the Earth’s magnetic field. And cryptochrome is also known to affect the sensitivity of
the retina. To see what that might look like to a bird, let’s go to the roof.
So what scientists think happens is that when a bird looks South, dark patches align over
their vision, kind of like this.
As they look around, the patches move too, giving a constant sense of compass direction.
We also have cryptochrome in our eyes, but the molecule that lets it stay active for
long enough to be affected by the magnetic field, superoxide, is toxic, so the antioxidants
in our eyes lock it down too quickly. Researchers think we’ve traded longevity for magnetovision.
So our pigeon in Paris is going to be able to orient itself pretty well, as long as it’s
not blind, Seris. It knows where North is, but how does it know that London is North?
How does it build itself a map? It’s still kind of an open question, but we do have some
more clues. It's either more magnetoreception or odour maps.
Apart from the cryptochrome in their eyes, pigeons also have magnetite, a biologically
formed, iron-rich material that reacts to magnetic fields, in their heads. Exactly where
is still a matter of some debate - it could be in the skin on the top of their beak, near
their nostrils, or between their brain and their skull. But in any case we’re pretty
sure it sends signals to the brain via the trigeminal nerve, which is the same one that
controls the eyes, so if Seris’s blind birds have nerve damage, this method won’t be
of much help.
Tiny particles of magnetite get aligned to the Earth’s magnetic field, just like needles
in this demonstration from our museum. The mini compasses will align to the strong magnetic
field imposed by this bar magnet. So imagine the pigeon’s beak is full of micrometre-sized
compasses, each a fraction the width of a human hair.
So that works fine as a compass, but it can also work as a map. The way they all get aligned
will tell the bird where it is on Earth, because of a concept called a gradient map. The intensity
of the Earth’s magnetic field varies with distance to the poles, being stronger near
the poles and weaker near the equator, so that gives latitude information. But the field
also varies in inclination, being vertical at the poles and horizontal near the equator.
And it's the combination of these two things where the bird can build a bicoordinate system.
Intensity and inclination vary across the world and there's a unique set of values for
any given point on Earth.
The bird builds up a feeling for the magnetic field at home and how it changes when it moves
in various directions. It then extrapolates that local map out, assuming the pattern will
continue to cover the rest of the Earth.
It turns out that this magnetite map probably only works on a very local level, up to 15km
or so, or at very large scales, 50km or more, because in small areas variation caused by
the metal content of underlying rocks is too localised to be extrapolated very far, and
in contrast the gradual change in the Earth’s magnetic field can only be reliably sensed
at very long distances.
There’s also some suggestion that pigeons can extrapolate familiar prevailing smells
in similar ways, to build up an olfactory map. But it’s unlikely that this can create
the kind of gradient map that could reach as far as Paris, so it’s probably restricted
to creating more of a pungent mosaic of the bird’s home town. Useful when the journey
is nearly over, but not so great for setting a course 300km away.
So to wrap this all together, let’s return to our brave pigeon’s voyage. He sets off
from the Eiffel Tower and starts looping around, getting his bearings. He quickly realises
he doesn’t recognise anything around him in this strange and foreign land. So he checks
the position of the sun against his internal clock and cross-references that with his perception
of the Earth’s magnetic field to calibrate his compass.
He has a sniff but doesn’t recognise the cheesy smells of France so extrapolates what
he knows about the large-scale gradients in the Earth’s magnetic field to place him
on a basic mental map, and sets off roughly in the right direction.
As he crosses the Channel small cues from his olfactory map confirm he’s heading in
the right direction as he catches the unmistakable pong of England. He speeds up. He hits the
Thames and can see the Shard poking up in the heart of London in the distance. He picks
up those familiar tinglings of the local magnetic field variations, and in no time he’s found
his way back to the warm embrace of the Royal Institution.
But he used his vision and his eyes an awful lot there. So to answer the question once
and for all, do blind birds can navigate?
No, probably not.
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