Barnacle geese in flight.
Birds of a feather
9 March 2009
Every year barnacle geese embark on an epic 6000-kilometre migration from Scotland to the Arctic and back again. Tamera Jones meets Steve Portugal to find out how.
Before embarking on an epic 3000-kilometre journey from Svalbard in the Arctic to Caerlaverock in Scotland, you'd think that barnacle geese would do everything they possibly could to build up their fat reserves. On the contrary, research in captive geese suggests that around six weeks before migration, the geese mysteriously lose about a quarter of their body mass.
Dr Steve Portugal at the University of Birmingham spent his PhD working out why the geese lose weight at such a crucial time. It turns out that almost everything a barnacle goose does is exquisitely timed to the seasons. And losing weight so soon before migration is closely related to another important event in the goose's calendar - growing new wing feathers.
Steve Portugal's supervisor Professor Pat Butler has monitored the autumn migration of six geese by satellite.
Their heart rates rocketed to around 250 beats per minute in flight compared with a resting rate of around 50 beats per minute.
Most of the geese stopped periodically, probably along the coast of Norway.
Some particularly fit birds completed the journey in just four days, flying non-stop for 24 hours in some cases.
One large population of barnacle geese spends the majority of the year over-wintering at the Solway Firth in Scotland. The geese feed on winter stubble crops to bulk up, reaching their heaviest around the start of February before migrating to the Arctic around April.
Migrating is a risky business and requires a lot of effort and energy, so much so that it acts as a key population control for barnacle geese. 'Having fledged their nests, young geese are more likely to perish on their first migration than at any other time of the year, often being blown off course by strong winds,' says Portugal. Even so, many birds migrate, so there must be something in it.
Birds migrate in response to the availability of food and to find the best breeding grounds. This improves their chances of producing a large clutch of chicks - or goslings if you're a barnacle goose. Geese consider Svalbard one of three ideal Arctic spots for their summer residence. Food is widely available and the conditions are ideal for breeding because there are fewer land predators.
The Svalbard archipelago lies in the Arctic Ocean well within the Arctic Circle. It is midway between the north of Norway and the North Pole. The sun stays above the horizon from late April until late August, giving continuous daylight.
The geese leave Scotland for the Arctic in April. They follow spring up the Norwegian coast to take advantage of the new foliage growth. The availability of daylight and food in Svalbard gives the geese the perfect opportunity to build up their fat reserves and breed. They put on a lot of weight during this time, reaching their summer peak around the end of June.
By June, the ice has melted, so the birds can feed to their hearts' content. Piling on the pounds in June and July is crucial for the birds. The annual wing moult - when old wing feathers fall out and are replaced by new ones - starts in July. Shortly afterwards, they quickly lose weight.
Steve Portugal, researcher in the School of Biosciences, University of Birmingham.
Once feathers have finished growing, they - like hair - are dead. Over time they become worn and abraded so need replacing.
Barnacle geese go through a fairly rapid, simultaneous wing moult from the start of August to mid-September before migrating south in October. They are completely flightless during the moult, making them particularly vulnerable until new feathers grow back.
During this time, foraging for food is a struggle and escaping from predators, like arctic foxes or polar bears, is considerably trickier. In contrast, passerines - much smaller perching birds - lose their wing feathers one at a time over a period of months. This doesn't hamper their ability to fly. It seems to make sense then for the wing moult to pass as quickly as possible for barnacle geese.
'Simultaneous wing moult has only evolved in birds that feed and avoid predators while flightless. Barnacle geese still have access to grasses and mosses, and can use water for safety,' says Portugal.
Many other waterfowl lose weight during wing moult, but until now it wasn't clear exactly why. Some researchers have suggested that a lighter bird would be more able to fly sooner - once its new feathers had grown - than a heavier bird.
Migrating is a risky business and requires a lot of energy.
To figure out why the birds lose weight, Portugal checked the rate of oxygen consumption in captive geese. He wanted to monitor their metabolic rate - as well as behaviour - at different times of the year.
The captive geese had constant access to food, no predators and importantly, don't migrate. So if changes in metabolic rate, weight and behaviour are important for wild geese, Portugal surmised that he might see similar changes in the captive geese.
Portugal found that captive barnacle geese lost weight at the same time - during the wing moult, even when they have unlimited and unrestricted access to food and no predators to worry about. Intriguingly, their metabolism also shot up a massive 80 per cent during the wing moult.
'Growing a whole bunch of new wing feathers uses up a lot of energy, so metabolic rate and oxygen consumption both rise to take account of this. The energetic cost of moulting means the birds need more nutrients to make feathers. Amino acid metabolism goes up, as well as blood volume and heat loss, all of which need energy,' Portugal adds.
He noticed that the birds tend to rest and stay put a lot more than before the wing moult. 'It's as if the birds sense their vulnerability when they're unable to fly and they put as much energy as they can into moulting to get it over with as quickly as possible,' he says. This, together with hampered foraging opportunities, means the birds inevitably lose weight.
Intriguingly, their metabolism also shot up a massive 80 per cent during the wing moult.
It seems binging before wing moult starts is crucial. Being in the best possible condition is likely to help the geese grow feathers quicker than birds not in tip-top condition. Indeed research has shown that birds in good condition grow feathers quicker than birds in poor health. The flightless period is shorter in birds that are in good condition, which means they are vulnerable for a shorter period of time. Barnacle geese that have failed to put on enough weight and are in poorer condition are less likely to weather the weight loss during wing moult.
Once the new wing feathers have grown, the geese set to work to replace the weight they've lost. With only six weeks before their 3000-kilometre trip back to Scotland, it's a race against time. The most effective way the geese can put weight on in a short period of time is by going into a state of anapyrexia - they decrease their body temperature by nearly five degrees Centigrade. Maintaining a higher body temperature uses energy, so keeping it lower helps them conserve valuable energy for their imminent migration.
'Geese that have been in captivity for more than five generations and have very different environmental cues compared to their wild counterparts still appear to carry on as if they were summering in the Arctic and migrating like wild geese,' says Portugal.
Arriving back in Scotland in October, they have just over half a year before they have to repeat the remarkable journey all over again.
Steve Portugal is a researcher in the School of Biosciences at the University of Birmingham
Tamera Jones is a science writer at the Natural Environment Research Council.
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