4.1 Thermoregulation in migratory birds

Compared with mammals, birds have high basal metabolic rates, using energy at higher rates (Figure 21), and also higher body temperatures. Metabolic rates lie within a range of values between a minimum value, the basal metabolic rate (BMR), and the summit metabolic rate, when the bird is under the greatest physiological stress. Passerines tend to have higher BMR values than non-passerines. The smallest birds, hummingbirds, have the highest BMRs of all birds. In general, BMR is related to mass per unit surface area, with larger organisms, which have larger mass per unit area, expending less energy per unit mass than smaller ones.

In general, body temperatures of birds range from about 38 °C to 44 °C. Body temperatures of large flightless birds (e.g. emu and ostrich, Figure 22a) and some aquatic birds (e.g. penguins) are at the lower end of this range and within the range of mammals. The house sparrow (Figure 22b) has a body temperature of 43.5 °C.

Figure 21 Basal metabolic rate (as measured by oxygen consumption) in different groups of vertebrates.

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This figure shows generalised plots of oxygen consumption (per unit mass) against body mass for four vertebrate groups: the highest values are for passerine birds, followed by placental mammals, then lizards and just behind these are the amphibians. All four groups show a fall in oxygen consumption per unit mass with increasing body mass.

Figure 21 Basal metabolic rate (as measured by oxygen consumption) in different groups of vertebrates.

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Figure 22 (a) Ostrich (Struthio camelus). (b) House sparrow (Passer domesticus).

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This pair of photos contrasts the ostrich, the largest and heaviest of all living birds, with the house sparrow, a small bird weighing less than 40 grams. Part (a) shows two adult ostriches, a male and a female. They have a relatively small head, a long, bare neck, large body and long, sturdy legs. The male has black plumage with a white tail, while the female is grey-brown in colour. Part (b) shows an adult male house sparrow in breeding plumage. It is dark grey-black on the crown, around the beak and on the throat, and is chestnut brown at the back of its head. Its cheeks and underparts are pale grey and white. Its upper back and mantle are a warm brown colour with broad black streaks, while the rump is greyish brown.

Figure 22 (a) Ostrich (Struthio camelus). (b) House sparrow (Passer domesticus).

To maintain temperature within the thermoneutral zone requires energy which for most organisms is a precious resource. Migratory birds in particular have tight energy budgets; to cover long distances without food means that migrating birds need to assimilate and conserve energy as much as possible before and/or during migration. The birds should avoid both heat and cold stress, as these use energy needed for flight, and they do so in a number of different ways.

In cases where individuals travel for long distances without drinking - for example, when crossing oceans or deserts - both thermoregulatory and osmoregulatory (water regulation) adjustments are required in order to conserve energy and water. Before migration, birds increase food intake, sometimes doubling their mass (pre-migratory fattening). Much of this food is stored as lipid, which, when metabolised, produces carbon dioxide and also water (which can contribute to water balance).

Some migrants enter regions where temperatures can become extreme. When crossing deserts, heat stress can become a problem due to the increased heat produced from persistent muscle contraction. To avoid overheating, many birds pant and some species may increase flight altitude to reach cooler conditions. However, some birds, especially small passerines, are unable to cross the Sahara Desert fast enough for a non-stop flight and must land during the heat of the day and rest to avoid overheating and dehydration.

At high latitudes and altitudes, ambient temperatures can be very low and so before migration, changes in feather covering and fat layers may be needed, reducing the energy used for thermoregulation and thus making more available for migration itself.

Some small passerines reach stopover sites that are very cold so that individuals are at risk of hypothermia. Some species have been observed to huddle at stopover sites and it has been suggested that this behaviour reduces energy expenditure and contributes to thermoregulation.

There is now increasing evidence that during and before migration, birds are able to lower their body temperature below ambient in order to conserve energy for the flight. Migrating passerines at stopover sites are thought to lower body temperatures by 8-10 °C during the night, conserving energy. Measurements of daily changes in body temperatures in migrating blackcaps appear to support this. Using radio telemetry at a stopover site at Midreshet Ben-Gurion, Israel, Wojciechowski and Pinshow (2009) showed that while at rest during a stopover, blackcaps can lower their body temperature by up to 10 °C below normal resting temperature at night and become hypothermic (Figure 23).

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Figure 23 Body temperature of a blackcap on the night of 9-10 April 2007. The vertical grey bar indicates a period with relatively high nocturnal body temperature, possibly associated with a period of nocturnal migratory restlessness.

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This figure is a plot of blackcap body temperature against time of day between 16.00 (4 p.m.) and 08.00 (8 a.m.) the next day. The hours of darkness are from about 7 p.m. until 6 a.m. The vertical (body temperature) axis is marked from 37 to 43 °C. From 4 p.m. until about 7 p.m. body temperature fluctuated between 41 and 42 °C, after which it fell steeply to 38.5 °C just before 9 p.m. Temperature then rose steeply back up to 41-42 °C, but fell sharply again about an hour later. The brief period (about 90 minutes) of relatively high nocturnal body temperature is shown as a grey bar. By shortly after midnight, body temperature had fallen to 37.4 °C, which is the lower limit of normal resting body temperature (shown as a horizontal dashed line). There were some fluctuations in body temperature over the next 5 hours, with values never exceeding 40 °C and at 2.30 a.m. a temperature as low 36.5 °C was recorded. At about 5.30 a.m. there began a sustained rise in body temperature, bringing it back up into the 41-42 °C range by 7 a.m.

Figure 23 Body temperature of a blackcap on the night of 9-10 April 2007. The vertical grey bar indicates a period with relatively high ...

The effect of this is to reduce energy expenditure by as much as 30%. Blackcaps use protein as a fuel source during flight which is derived from the liver, gut, breast and leg muscles. Because of this, they arrive at stopover sites with atrophied liver and gut and during the initial stage of the stopover, their digestive efficiency is significantly lower than in the later part of the stopover. When they arrive therefore, birds may save energy by entering controlled hypothermia during cold nights until they are able to rebuild tissues and energy stores by foraging during the day.

It is now known that many bird species allow body temperature to drop below normal resting levels at night. For example, it has recently been suggested that lowered body temperature might be an important mechanism of energy conservation in migrating brent geese (Branta leucopsis, Figure 24).

Figure 24 Migrating brent geese.

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This is a photo of a flock of brent geese flying above the ocean. These birds have a white face and belly and black head, neck, and upper breast. The wings and back are silver grey with black-and-white bars.

Figure 24 Migrating brent geese.

However, none have been observed to lower it to the same extent as hummingbirds, which undergo deep, mammal-like torpor with body temperature decreasing by as much as 30 °C and BMR by a substantial amount, as shown in Figure 25a. Studies on rufous hummingbirds (Selasphorus rufus, Figure 25b) show that they use torpor to conserve energy that can be used later for migration. So, unusually, they go into torpor even when fat reserves are high and they also may go into torpor for periods during migration.

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Figure 25 (a) Rufous hummingbird. (b) Torpor in hummingbirds: oxygen consumption (red line) is high when hummingbirds are active during the day but may drop to one-twentieth of these levels at night.

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Part (a) is a photo of rufous hummingbird in flight. It has long, straight and very narrow beak, a white breast, reddish-brown face, upper parts, flanks and tail. Part (b) is a plot of oxygen consumption over the course of a day, from pre-dawn to post-dusk. Dawn, noon and dusk are marked on the horizontal axis and grey shading marks the two periods of darkness. The oxygen consumption scale along the vertical axis is non-linear and is marked from one to 50, in units of nanolitres of oxygen per gram per hour. In the hours before dawn, oxygen consumption is at a constant low level of one nl per gram per hour; it then rises rapidly, reaching 20 nl a couple of hours into the day. By noon it reaches 30 nl, and peaks at around 40 nl in the late afternoon. Then, just before dusk, consumption falls very rapidly and is back to the baseline level of around one unit as soon as darkness descends.

Figure 25 (a) Rufous hummingbird. (b) Torpor in hummingbirds: oxygen consumption (red line) is high when hummingbirds are active during ...

Activity 2 Studying the physiology of migration

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Homeostasis introduced the processes concerned with the regulation of body temperature, using migratory birds as an example. How can physiological information be obtained from birds that migrate long distances? Lucy Hawkes discusses her research on bar-headed geese with Brett Westwood, in an interview recorded for the BBC. Listen to the interview and makes notes about the environmental limitations that the bird faces and the adaptations in both behaviour and physiology that enable it to migrate.

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Transcript: Audio 1 Heart rate monitoring of migrating geese.

Interviewer

Now back in June on Saving Species we heard from Bangor University scientist Lucy Hawkes, who's studying the migration physiology of bar-headed geese, which summer in Mongolia and winter in India, crossing the Himalayas twice on their journey, which is no mean feat. The geese have proved to be superbly adapted to this extraordinary journey, with organs and tissues working at extremes they can endure altitudes and very low oxygen levels. Well, Lucy returned to Mongolia in July to retrieve heart rate monitors which had been fitted to 30 of the geese last year, and she's with me now.

So before we talk about the geese themselves and the heart rate monitor work, let's talk about Mongolia, what sort of landscape were you working in, Lucy?

Lucy Hawkes

Mongolia's an incredible country of just vast never ending rolling landscapes of tundra. It's probably one of the most overgrazed countries in the world, having said which there are huge numbers of flocks of sheep and horses and yaks, so the terrain is basically just comprised of very short grass, which also happens to be ideal forage for bar-headed geese.

Interviewer

How did you make your way through Mongolia, do you have fixers out there to tell you where the geese are?

Lucy Hawkes

We worked with a group called the Mongolian Academy of Sciences, and they set us up with drivers and technicians, and we've been working with them for four years now. It takes us two and half days to drive out from the capital and it's a very bumpy ride. It also is a very difficult terrain for surveying for animals as well, so we have to undergo enormous amounts of time in vehicles to actually manage to get anywhere near bar-headed geese.

Interviewer

Well the journey itself must be fascinating, but when you get to the places, are they vast lakes or small lakes in the tundra?

Lucy Hawkes

We specifically target one big lake called Terkhiin Tsagaan Lake in Western Mongolia, because it's huge. It's one of the biggest lakes in Mongolia. And bar-headed geese go to these lakes for one major reason. They're in Mongolia because they are breeding but they're also taking this as an opportunity to drop and regrow their flight feathers. Flight feathers get easily worn down over the course of a year, and so at some point they need to be replaced for nice new shiny firm ones. And during this time these birds can't fly, so they're very vulnerable to terrestrial predators. If you start hanging out next to a lake though, you can jump onto a lake whenever a terrestrial predator comes along and hopefully remain safe.

Interviewer

Give me a good description of a bar-headed goose, because not many of us have been lucky enough to go out to Mongolia and see them in their native habitat.

Lucy Hawkes

You may be lucky enough to see them in the UK. They're very pretty birds, so they tend to be kept in ornamental collections, and I know that there are some in Slimbridge as well. It's about two kilos in weight, so it's a good armful I suppose in size. It's got a lovely white head with two distinct black bars behind its eyes, bright yellow beak and bright orange legs.

Interviewer

And where does it live, what's its range in the world?

Lucy Hawkes

Its natural range extends from the southernmost tip of India all the way up to in the furthest north Russia. You might find it as far west as Kyrgyzstan and you might find it as far east as China.

Interviewer

So these populations you're studying, they're migrating from breeding grounds in Mongolia south to India, and that means they've got to cross the Himalaya Mountains.

Lucy Hawkes

Indeed, which is an unbelievably challenging feat. One thing that our tracking has revealed is that the birds actually cross the mountains at the same time of year when humans try to make their ascents of Everest. Both humans and geese seem to be waiting for this ideal weather window when conditions become the most benign as the time that they choose to cross.

Interviewer

We've heard stories of terrific feats of bar-headed geese supposedly being the most, the highest altitude migrants out, but do these birds, as far as you know, do they go over the top or do they go through the Himalayas?

Lucy Hawkes

It didn't seem to make an awful lot of sense to us that these birds would be flying over the peaks of say Mount Everest, when you could perhaps just go through the valleys next to them. And we've satellite tracked 62 of them so far, and all of them have been following the valleys rather than the peaks of the mountains. The maximum altitude we've recorded from any of these birds was 6,540 metres. Which is still incredible, that would be plenty enough to make us feel really quite dizzy and a little bit sick. Without sufficient acclimatisation we'd actually have trouble surviving at that altitude.

Interviewer

Well, talking of dizziness, that brings us to your work really. You're studying bar-headed geese to find out how they make these journeys and how they use oxygen, how they conserve resources on those journeys, so can you sum up what your work has been about?

Lucy Hawkes

So our work is trying to find out what strategies maybe the birds might use to kind of play the conditions to their advantage. The big finding that we've had so far is that the birds actually wait for a specific weather window to cross the Himalayas. So during the day, as the sun heats the ground, the air starts to rise, because obviously heat rises, and that means that you eventually have a very strong uphill wind blowing. And this is true if you go to the Andes or the Alps or the Himalayas or any mountain region on earth.In the Himalayas this means an uphill wind blows really reliability every single day. You might think bar-headed geese would use that as an opportunity to go, we actually found the opposite.

Those winds are quite strong and they're also quite turbulent, so a little bit dangerous flying in those winds, so the bar-heads actually wait for those winds to stop.It might sound a little bit crazy, but at night it's also an awful lot colder. Colder air is denser, it's got more molecules of all the various component gasses within a particular volume, that also means there's going to be a little bit more oxygen there, and we think they go at night perhaps because they're both risk averse and also because they're trying to use conditions where more oxygen is present.

Interviewer

Yeah, so this oxygen use is absolutely critical to the way that they migrate. What have you discovered about that? Have you discovered that they're physiologically adapted in any particular way to undergo these strenuous journeys?

Lucy Hawkes

We've been very fortunate to work with a big team of colleagues who've made some of those findings for us. For instance, we know that bar-headed geese have a mutation in their haemoglobin that makes it a little bit more hungry for oxygen, it will take it up a little bit more readily. They've also got a lot more blood circulating to their flight muscles than other birds perhaps do, and also they've got slightly larger wings compared with other geese as well, so these are some sort of what might appear very minor adaptations but taken together they actually make these geese into really amazing athletes.

Interviewer

So when you went back in July, you were trying to retrieve heart rate monitors that you'd put on 30 of the geese, how did you get on?

Lucy Hawkes

We managed to retrieve nine of them, and we think that's a pretty good return. This is the Tibetan Plateau after all, this is one of the largest land masses on earth, we're literally looking for a needle in a haystack, so we couldn't believe that we were able to get almost a third of those loggers back. Catching the geese, however, is quite something else!

Interviewer

So how do you go about it? I mean if they're moulting, they're flightless, are they?

Lucy Hawkes

They are, which is obviously a massive advantage for us. In brief, we set the nets into basically sort of a funnel on the bank of a lake. We get onto the lake in kayaks. And the thing to do really is just be where you don't want the geese to be. So we basically herd them into the nets but we do it ever so gently and ever so carefully, and eventually the geese will decide they've had enough of being on the same lake as us and they decide to get out. Our job is to try and make them get out into our nets, and at that point we can close a little gate, surround them on all four sides and capture them.

Interviewer

So this is gradually corralling them in kayaks and then paddling furiously to try and persuade them to go onto land and catch them in the nets. How did you get on?

Lucy Hawkes

We managed to catch 100% of the birds. We did five catches altogether, and every single one we caught all of the birds that we aimed to catch, which was fantastic. We've come a long way in that regard. We caught just over 400 birds altogether over the three week trip to Mongolia, and those included those nine birds carrying heart rate loggers.

Interviewer

Do you have any results at all from those heart rate loggers? Can you draw any conclusions from it, or is it too early at this stage?

Lucy Hawkes

So the heart rate loggers altogether contained over 14 Gigabytes of data, so an enormous amount of analysis needs to be done. We've made a start. Those have shown some really interesting things. For instance some of the geese's heart rate actually dropped to about 40 beats per minute overnight. That's really, really low; you wouldn't even expect to see that in a human. But the really astounding thing we saw was a 25 hour long flight where the bird's heart rate was sustained, and in addition to having heart rate we also had something called an accelerometer, and an accelerometer not surprisingly measures acceleration. Well it measures it forwards, backwards, left, right, up and down, and that tells us a little bit about how the animal's moving, and by looking at that data we could see that the animal was actually consistently flapping for that whole 25 hour period, so we know that bird made an entire 25 hour migration, which is just astounding.

Interviewer

Well it's an astonishing feat, even in normal conditions, but at high altitude it must demand enormous energy resources.

Lucy Hawkes

Absolutely, and we think that at the beginning of the flight it would have started on the Tibetan Plateau, and its heart rate was quite a lot higher, probably because the oxygen is so much more scarce there, because you're at high altitude. As the flight went along, the heart rate dropped and dropped and dropped, and towards the end we think the bird probably was just basically gliding down and was having quite an easy time of it, and its heart rate dropped down to about 240 beats per minute, which is high for us but not very high for a flying bird at all.

Interviewer

Well no. As you followed these remarkable birds, you must have learnt a lot about their migration strategy and you must have learnt an awful lot about the habitats that they're using en route as well. What's the bigger picture for conserving bar-headed geese? I mean do they need conserving, are they in danger at all or are they doing quite well?

Lucy Hawkes

For wide ranging species like the bar-headed goose, a real conservation conundrum is being able to actually measure if the population is increasing or decreasing. That's easy if they only occur over a small area and you can count them all, you can figure it out one year, count them again the next year and you know if they're going up or down. Bar heads occur over such a massive area it's really difficult to make those counts. So our tracking is enabling us to direct our efforts to count birds in specific places and in specific times of the year when we believe that we may have captured large portions of the population. As yet, those counts have not been made and they need to be made over several years in order to be robust, so we don't know if bar-heads are threatened or not.

One very important thing regards bar-headed geese is that they are thought to be vectors of avian influenza. They occur in several places where bird flu is known to be present, in for instance poultry farms, so they've been suggested to be suitable vectors. We've been swabbing bar-headed geese for avian influenza as part of our work as well, and we've never yet detected one positive result.

Interviewer

If you stand in a certain place in the Himalayas and look at a particular valley, will you see large numbers of bar-headed geese travelling down there, or are they diffuse. Do they travel down lots of different areas; do they have their favourite fly ways?

Lucy Hawkes

I think one of the problems is that they'd be flying at night, so you probably wouldn't see them, but you probably would hear them. Apparently bar-heads call while they're flying, and I don't think that's so much to do with communication as the fact that they're flapping their wings so hard it's expanding and contracting their lungs, and it's probably a bit like Monica Seles playing tennis, you know, just 'huh!' just comes out. But I believe that there must be tens of thousands of bar-heads crossing particular valleys in the Himalayas, and that that would be most likely in March and also in October. So anybody who's just been to the Himalayas this month or is just about to go may well see some.

The other thing that's really interesting about bar-heads migrating at high altitude is that they are performing a type of locomotion that requires huge amounts of oxygen but huge amounts of oxygen aren't available, so how do these tissues manage to cope in very low oxygen conditions? Well some of the ramifications of that have importance when we think about conditions when we as humans suffer very low oxygen conditions, such as when we're having a stroke or perhaps a heart attack, so some of the biomedical implications of how the system manages to cope may in fact one day lead to some discoveries that may help us to recover from those kinds of events.

Interviewer

And, as far as other geese are concerned, do bar-headed geese show adaptations beyond, over and above what most other geese show, that they are definitely adapted to this particular high altitude migration?

Lucy Hawkes

We believe that to be the case, although I think there are a large number of different species that can cope very well, probably at high altitude and also in low oxygen conditions. One of the world records for birds being found at extremely high altitude is held by the lowly mallard, a mallard duck was sucked into an aeroplane engine at about, I think it was about 1200 metres. They I guess scraped the bits off later and confirmed that it was a mallard. So there are probably other species of birds up there. I'm also aware that a pilot at cruising altitude spotted a flock of swans flying in formation as well.

Interviewer

So bar-headed geese don't necessarily hold the crown when it comes to high altitude flying?

Lucy Hawkes

I think they hold the crown for the most regular high altitude migrants, but I'm not sure that they hold the absolute Guinness world records.

Interviewer

Thank you very much Lucy.

Thank you Lucy Hawkes from the University of Bangor for sharing the information of bar-headed geese, and may your work continue.

End transcript: Audio 1 Heart rate monitoring of migrating geese.

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Audio 1 Heart rate monitoring of migrating geese.

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The bar-tailed godwit undertakes a very long migration. How does the bird prepare for the journey? Phil Batley studies migration and has been tagging the birds to follow their progress. He discusses his research in an interview with Philippa Forrester, which you should listen to now. What particular physiological strategy do the birds adopt to prepare for the flight?

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Transcript: Audio 2 Satellite tagging of migrating bar-tailed godwits.

Phil Battley

So this is D-zero, the first of the satellite-tagged birds tonight. So he's ready for release, he's strolling off down the beach towards the tide edge, then he'll take stock of his surroundings, and he'll be heading off.

David Steemson

And next stop Alaska?

Phil Battley

Yeah, in a manner of speaking.

David Steemson

Well, next stop the Yellow Sea.

Phil Battley

Yeah, next stop the bit of shelving just over the creek.

Reporter

Well, that was back in February. Our reporter David Steemson joined scientist Phil Battley and his team at Miranda in New Zealand where a group of Alaskan bar-tailed godwits were fitted with electronic tags so we could follow their migration north. At the end of March the godwits left New Zealand and flew a mere 6,000 miles to Asia. After stopping off to feed for several weeks, they left for their breeding grounds in Alaska, then at the beginning of July they moved to the Yukon Delta in South West Alaska to fuel up. Critical, because sometimes towards the end of August or early September these birds will set out on their journey south back to New Zealand and will attempt to do that with no stopover. However, after all that travelling, only one godwit is left with a transmitter which is still working, so all eyes are fixed on a bird named D8. So to find out how D8 and the other godwits are preparing for this epic journey I rang Phil Battley who began by explaining how feeding up results in both internal and external changes.

Phil Battley

Well, for a start, when they're feeding they're putting on a lot of fat so that's going to go in a thick layer under their skin, right around their body, and also inside their abdomen, so it'll pack around their internal organs, and it's going to cause the bird to get a lot heavier and, because they're heavier, then they're going to need to have bigger flight muscles and leg muscles to carry that. And they're also going to need bigger hearts to pump their blood around faster, their blood's probably going to get thicker as well, their lungs will probably get bigger, so sort of everything that's associated with exercise will get bigger in that bird, probably dramatically bigger. But the flip side of that, as we know from other birds, smaller ones called knots that have a very big stomach, a big gizzard, once they take off on a long flight things like the digestive system is just waste sort of ballast, so they may have the option of shrinking their guts down just before they leave, and saving themselves a few grams in weight along the way. We haven't had a conclusive demonstration of that in godwits but it's quite likely for a bird that flies this far that they may do something like that.

Reporter

Extraordinary changes, and physically would you be able to see that? Is there appearance change?

Phil Battley

Oh, it certainly does. Now when you see a bird that's not migrating, it's skinny, you can see lots of legs. By the time they're ready to leave, it's like a brick with wings, is one way I've had it described, so they're, you know, their backside is hanging out, their breast is bulging down towards the ground, so they are quite a different bird really.

Reporter

It doesn't really conjure up an image of a sleek, lean flying machine preparing for this journey of brick with legs.

Phil Battley

No, it doesn't really, but it's a very streamlined brick. It is a finely honed flying machine, it just happens to be a rather fat one at the time, but if you ever see a photograph of a bird flying in a wind tunnel it's a revelation. It looks like this animal is swimming underwater, its feathers are just so sleek on its body so you can take that image and put it on to this godwit, and imagine it, and you can see once they're up in the air they cut through it, you know like it's nothing, really.

Reporter

So what cues trigger that migration south? What makes them think right, now's the time to go, do we know?

Phil Battley

There is one quite strong correlate with wind as they leave Alaska and that's the presence of a large low pressure system in the sort of Northern Pacific and when they've done the modelling of the wind systems around it, it looks like they can probably get tail winds of the best part of 30 km an hour off those. Now these birds will be flying at something like 60-70 km an hour under their own steam, so a 30k tail wind is, you know, as much as half of your own airspeed so it's a real good boost. But the really amazing thing is that the juveniles, the young birds, which have never done this flight before, some of them will be ready to fly with the adults, but a whole lot of them will get left behind at the end, after the adults have gone. They will have to advance on a trans-Pacific flight of some kind, you know, without ever having done it before.

Reporter

Well, last year one bird, E7, stayed airborne for 7,150 miles, the longest non-stop flight of any bird. Energetically this is a massive challenge for a bird, so I asked Phil about the benefits of flying non-stop back to New Zealand.

Phil Battley

The benefit of it is that it gets them to New Zealand directly, it gets them here very quickly, and they don't have to go through Asia, so some of the benefits that may be there is that because they aren't stopping they're not exposing themselves to predation risk. Every time a bird goes somewhere new there could be new risks to it, or it could be that the islands that they have an option of landing on, and it's all really good for refuelling. The other thing for them is that they're likely to fly at quite high altitudes so if they come right down low, then they have to get up to altitude again and that's quite expensive for them. And also if they travel direct here then there's no risk of encountering any new parasites or pathogens, or something like that, so it seems extraordinary, but if they can do it, it seems very sensible.

Reporter

Well our own strategy seems to be quite high risk now, not because we've chosen it, but because we only have one godwit left to track. So I mean how high is the likelihood that we'll be able to follow them home, do you think?

Phil Battley

Well, it's 50/50 I'd say. If he's transmitting we will know when he arrives absolutely.

Reporter

So that is D8, you're our only hope!

Phil Battley

Exactly! Not quite... but, you know, one step better.

Reporter

Phil, thank you very much.

Phil Battley

My pleasure.

End transcript: Audio 2 Satellite tagging of migrating bar-tailed godwits.

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Audio 2 Satellite tagging of migrating bar-tailed godwits.

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One way of countering problems associated with energy expenditure and thermoregulation in migrating birds is to have good orientation and navigational skills. This does not necessarily involve finding the shortest way to a destination; and in fact sometimes physiological difficulties may be avoided by taking a roundabout route. For example, favourable winds and meteorological conditions can conserve energy by reducing the costs of thermoregulation and birds require exceptional navigational skills to take advantage of this. The next section will explore these skills.