47. How do migratory birds make use of the earth’s magnetic field? →

There is plenty of evidence that ‘experienced’ migratory birds (ones that have already made at least one migratory round trip) can cope with massive geographical displacements. They can, for example, successfully complete their journeys, even after being ‘kidnapped’ by scientists and taken to places that are totally unfamiliar - which may be thousands of kilometers away from their normal route.

This is a really astonishing feat. In the same circumstances we humans would be completely at a loss but for our maps, sextants and compasses - or nowadays, GPS.

Some experts believe that these birds are ‘true’ navigators, capable both of determining their present position and of selecting the right course to follow to reach their goal. I’m not so sure.

Well, how do they do perform this feat? It looks increasingly likely that their ability to ‘read’ the characteristics of the surrounding geomagnetic field is the key (though exactly how this sense actually works remains uncertain).

In my book, Incredible Journeys/Supernavigators I mentioned some intriguing evidence that reed warblers might be using changes in magnetic declination (the angular difference between the directions of true north and magnetic north) to estimate their longitude. Such a skill would help them compensate for large scale east/west displacements from their normal migratory routes, though it would not enable them to determine their position.

A fascinating new study from Dmitry Kishkinev and his colleagues now casts doubt on this hypothesis, but it sheds new light on the role that magnetoreception plays in avian navigation.

Kishkinev et al. have now shown that experienced Eurasian reed warblers exposed to altered magnetic fields (characteristic of completely different and unfamiliar locations far from their normal migratory routes) can successfully reorient themselves. However, they can do so only when all three components of the geomagnetic field (declination, inclination and intensity) are present and match a real place - not when declination alone is altered (as in the earlier experiment).

The crucial thing about the latest experiment (like earlier ones from the same team) is that the birds were not physically moved. The changes to which the birds were subjected related only to the surrounding magnetic field. All the other cues available to them (whether based on sight, sound, or smell etc.) remained unaltered.*

One possibility is that these birds have access to some kind of ‘cognitive map’ based on gradients in the three magnetic parameters. (Claims for such ‘maps’ are, I have found, seldom justified by the evidence presented though they make good headlines!)

Such a ‘map’ would presumably be based on the birds’ experience of how the magnetic field varies along their migratory route. Such a map might extend beyond the confines of their standard journey if - and this is a very big if - the birds could somehow infer their new position by extrapolating the spatial variation of the various magnetic cues from those they have observed along their normal migratory corridor.

There are plenty of experts who doubt whether that is even theoretically possible, and it is unlikely that birds could generate positional information from the geomagnetic field that is anything other than very coarse-grained.

However, as the authors of the new paper rightly point out, there is no need to postulate such an extravagant navigational system.

Rather than using their mysterious magnetic map to ‘fix’ their position and set a course to their destination, there is a much simpler and more plausible explanation.

Perhaps the detection of magnetic fields that differ markedly from those they are familiar with warns birds that they are off course. The size and nature of these differences may additionally tell them in which direction they have been displaced. They might then be able to make course adjustments that return them to the correct migratory corridor. Once they are back on track, they should have a good chance of reaching their destination.

A set of skills like that would certainly be very valuable as a way of dealing, for example, with the effects of strong cross-winds. Whether, as the authors seem to imply, it counts as ‘true navigation’ is debatable.

The authors also cautiously observe that their findings do not support ‘a strong role for other environmental cues’ in the navigational tool-kit of the reed warbler.

It will be very interesting to see whether their results can be replicated in other bird species.

*What the researchers actually observe is the direction in which the caged birds attempt to fly under the altered magnetic fields generated by the coil systems that surround them. This technique is widely believed to be a good proxy for actually tracking where the birds go, though some experts question whether it is reliable.

21.Mother and father goose lead the way…

You’ve probably seen geese flying in a tight V-formation, and you may have wondered why.

It seems that the upwash from the wings of the lead bird gives a bit of extra a lift to the birds following on either side, and thus reduces their energy consumption - an effect that continues down the line to the tails of the V. That really matters on a long migratory journey, but the lead bird obviously has to work a bit harder than the rest. So it came as no surprise when some recent studies showed that birds took turns in taking the lead, thus sharing the extra burden.

But something slightly different emerged from a fascinating new study.

Researchers attached lightweight GPS trackers and accelerometers to wild white-fronted geese belonging to four family groups setting out on their long migration from the Netherlands to northern Russia. This meant that they could not only see exactly where each bird was in the formation, but also how fast they were flapping their wings.

It turned out that only the parents ever took the lead. Usually the father, but sometimes the mother - depending on the wind conditions. The mothers had to flap their wings faster when they were out in front, but the fathers apparently did not need to.

It makes good sense for the parents to take on this leadership role, because it increases the chances of their offspring surviving the rigours of the long journey - and thus having a chance to pass on their genes to future generations (so-called ‘kin selection’).

But this system probably also helps the young, first-time migrants to learn the route they need to follow in future, where best to stop for refuelling, how best to avoid dangers - and indeed how to fly most efficiently.

In groups of homing pigeons, less experienced birds follow more experienced flock members, and this helps them to learn routes. Further experiments will be needed to see whether this is also true of young migratory geese.

19.Young cuckoos are even cleverer than we thought…

A couple of weeks ago I wrote about the amazing journeys undertaken by adult Mongolian cuckoos - like Onon in this picture.

Here I talk very briefly about a fascinating new study, just published in the journal Nature. It reveals that young cuckoos - making their very first migratory journey all the way from Russia to Southern Africa - can somehow tell when they have been moved 1800 km to the east of their proper route. And, just like experienced adult birds, they can change course to compensate for this displacement.

It’s the first time such an ability has been clearly demonstrated in young birds and it leaves us wondering how they solve a problem that we humans only cracked a few hundred years ago.

18.My interview for the Stratford Literary Festival

I was lucky to be interviewed by the novelist Clare Clark who asked extremely good questions. I hope my answers measured up to them!

17.Amazing cuckoos

There’s no better way of getting a sense of what migratory birds can do than to look at the maps that record their journeys.

The Mongolian Cuckoo Project has been using tracking devices to follow the amazing journeys of cuckoos returning home from Southern Africa in recent weeks. Do please visit their excellent website to see the achievements of Onon, Bayan and the other birds. It’s a real model of public engagement in science.

Quite apart from the extraordinary distances the birds have been covering, I’m struck by the close similarity between the routes they have been following. It would be fascinating to know exactly how they perform such impressive navigational feats.

Presumably, like many other migratory birds, cuckoos have a sun and star compass as well as a magnetic one. And when they have made their first migratory journey, it’s safe to assume that they use familiar landmarks to help them retrace their route. But of course there are no landmarks over the ocean!

And cuckoos face a special problem. Their unusual lifestyle means that when they first head south, they must do so alone - because their parents will have left before them.

So how on earth do they find their lonely way over thousands of miles of land and ocean to the areas in Africa where they pass the winter months? Some kind of genetic program must be involved. But how does that work? We just don’t know.

One last thing: why not lend your support to this brilliant project by following this link?

14.How birds steer by the stars - again, this time with the help of a globe!

If you found the explanation I gave in my last post a bit hard to follow - and I certainly wouldn’t blame you if you did - I hope this may make things a bit clearer. Do let me know either way!

13.How birds steer by the light of the stars...

Polaris (aka the Pole Star) stands in the sky vertically above the North Pole (not quite exactly, but near enough). That means that if you were at the North Pole during the long Arctic night and looked directly overhead you’d see Polaris. And if you were anywhere else in the northern hemisphere, facing towards Polaris, you would be looking due north (true, not magnetic). Though, as I explain in the video, thanks to the phenomenon of ‘precession’, this would not always have been the case, nor will it be in the not-too-distant future.

If you know where true north is, you can set a course in any direction. No wonder Polaris used to be called ‘Stella Maris’ - or the ‘star of the sea’. In the Middle Ages, the same term (which can also be translated as ‘the star of Mary’) was applied to the Virgin Mary, whose sky-blue cloak is emblazoned with a star in many medieval paintings. Mary was likened by the theologian Alexander Neckham (1157-1217) to the Pole Star, standing at ‘the fixed hinge of the turning sky’ by which the sailor at night directs his course. As I wrote in SEXTANT, ‘Polaris must have seemed a perfect symbol for the Mother of God, the immaculate spiritual guide and intercessor’.

https://upload.wikimedia.org/wikipedia/commons/5/5a/Star_Trails_Shoreline.jpg

But, as I explain in this video, birds are not human and they don’t use Polaris. Instead, they wisely attend to the rotational pattern of stars around it - something on which they can always count. To illustrate this, here’s a long-exposure photograph of the northern sky in which you can see the circular paths traced by each star. Polaris itself appears as a stationary dot at the centre of the pattern. The same principle would also work in the Southern Hemisphere, which is handy because there is no southern equivalent of Polaris: the southern celestial pole lies in a rather blank patch of sky, at least for the present!

12.The amazing journey of the blackpoll warbler...

Here is the first of what, if I have the energy, will be a series of short videos introducing some fascinating aspects of animal navigation. It relates to a tiny American bird: the blackpoll warbler.

These birds normally only weigh about 12 g and even when they fatten themselves up for their long migratory journey they only put on another 5 g. I think you’ll be as amazed as I was to discover what they can do.

The original research on which this story is based can be found here: DeLuca, W. V., Woodworth, B. K., Rimmer, C. C., Marra, P. P., Taylor, P. D., McFarland, K. P., ... & Norris, D. R. (2015). Transoceanic migration by a 12 g songbird. Biology letters, 11(4), 20141045.