Brent geese are a much-loved winter visitor to our coastline.
Here Ranger Tony gives us a detailed overview of their behaviour, diet, habitat and migration patterns: all you’ll ever need to know about this intrepid goose.
The brent goose is a small, very dark goose – a little bigger in size than a mallard duck. In fact, it’s the smallest goose you’ll find on our shores.
There are three species of brent geese – the dark-bellied, the pale-bellied and the black. Here in the Solent it’s dark-bellied brent geese that regularly visit from their nesting grounds in Siberia. A handful of pale-bellied (and even one or two black) brents will be spotted here over the winter.
However you’ll need to travel to Northern Ireland or the North East to see significant numbers of light-bellied brents in the UK – they fly to this country from Canada and Svalbard, a group of remote Arctic islands.
Distinguishing features
Dark-bellied brent geese have a black head and chest with a black bill, and a distinctive white neck patch. You’ll often spot several juveniles among the flocks with black necks that have yet to develop the white neck patch.
A family of brent geese with the juvenile missing its lacy white neck band
If you keep an eye out, you can spot brents in small family groups – usually two adults and upwards of two youngsters. The brent’s upperparts are dark grey-brown whilst their underside is brown. They also have a bold white rear end.
They are very sociable birds, gathering in large flocks sometimes many hundreds strong. When brent geese pair up, they generally pair up for life.
Calls and sounds
Their call is a rhythmic, deep, throaty rronk, rronk. When numbers of brent geese flock together, they create a loud, murmuring chorus.
Listen to the call of dark-bellied brent geese
Diet
Brent geese have a vegetarian diet and feed mainly on eelgrass (zostera), sea lettuce and algae.
They are increasingly feeding on cereals and grass too, and since the 1970s, foraging on agricultural land and open public spaces, such as golf courses, in preference to their traditional intertidal food sources. This habit has led to some conflict with farmers who’ve sometimes blamed brents’ feeding behaviour on crop damage.
Brent geese feeding on grassland
Flocks of brent geese often feed on the water, upending like ducks to reach vegetation growing beneath the surface, as well as using the intertidal region – the area between the high and low tide marks.
As soon as brent geese chicks hatch and leave the nest, they’re led by their parents to suitable foraging grounds where they eat a similar grass-based diet to their parents of eelgrass and sea lettuce.
Brent goose feeding on vegetation in the water
Adults don’t feed their offspring, but the goslings learn to feed by foraging independently alongside their parents. A little bit of tough love!
Habitat and distribution
The main breeding range of the dark-bellied brent goose is North Siberia, and particularly the Taymyr Peninsula region situated within the Arctic circle. Some breeding also occurs as far west as the Kanin Peninsula.
Siberian summer breeding areas for dark-bellied brent geese
Over the winter months, they swap their Siberian home for warmer destinations.
At the most recent population estimate, the UK is home to more than 100,000 dark-bellied brent geese in the winter, mainly at coastal sites in southern and eastern England.
The Solent coast and its harbours are a particularly important area for brent geese. It is estimated that the Solent supports about 10-13% of the world population of dark-bellied brent geese and about 30% of the UK population.
Internationally important sites for brent geese include Portsmouth Harbour, Langstone and Chichester Harbours and the north-west Solent. Nationally important sites for brent geese in the Solent include Beaulieu Estuary, Southampton Water and Newtown Estuary on the Isle of Wight.
Brent geese start arriving along the Solent in October and by January there are around 25,000 of these amazing geese along our shoreline. They are also frequently spotted along the eastern coast of England, in such places as The Wash, the North Norfolk coastal marshes, estuaries in Essex, and the mouth of the Thames. Virtually the entire global population spends the winter in some part of north-western Europe.
In conservation terms, the species is of high international importance and is regarded as vulnerable because of its relatively small population size throughout the world and its highly variable breeding success rate.
Numbers have fluctuated over time. After decades of low numbers following a major population crash in the 1930s, numbers have steadily increased. However they are yet to reach the previous peaks recorded in 1993/94, possibly due to changing conditions in their breeding grounds in Siberia.
Brent geese traditionally spend the winter on coastal mudflats, where they feed on eelgrass, marine algae and sea lettuce. The availability of food will be dependent on whether the vegetation has died back in harsher winters and due to pollution. Availability of food is also dictated by tides which exposes the mudflats for varying periods of time each day.
Brent geese feed in daylight and the use of terrestrial feeding sites – grassy areas away from the coastal mudflats – is more common at high tide. In years with large numbers of juveniles (first winter birds), they’ll use these grassy areas more often.
Brent geese feeding on grassy areas
This is partly due to competition for food on the intertidal areas from older, more efficient feeders, and partly because grass is more nutritious. Although families may choose to graze nutrient-rich grassland for their young, there is a trade-off with the increased risks associated with exposure to predators and disturbance compared to feeding on the intertidal mudflats. Harsh winters also cause an increased use of terrestrial grass sites as eelgrass dies back.
Lifespan and lemmings
Brent geese are long-lived birds with one recorded as living for more than 28 years after ringing. Most don’t survive that long with an average lifespan of around 11 years.
The size of the brent goose population is affected by predation pressure during their breeding season in Siberia, which – perhaps surprisingly – is significantly impacted by the lemming population in the area.
In years when lemmings have had a good breeding season, predators such as Arctic foxes focus on lemmings as their main diet. This means brent young are predated upon less often and more goslings survive to the fledging stage.
Lemmings’ population size has an impact on brent geese breeding success
Arctic fox – a predator of brent goslings
However, in years where the lemming breeding season has been poor, predators are more likely to switch their diet to ground nesting birds, such as brent young, which can sometimes result in an almost complete breeding failure for the brent geese.
This trend follows an approximate 3-year cycle and is linked to the lemmings’ food source – moss. Essentially, moss regrows slowly, and so a horde of hungry lemmings can empty the larder very quickly. This leads to a crash in the lemming population due to food shortages. The moss then has a chance to regrow, supporting a growing lemming population, and so the cycle continues.
In recent years, this cyclic event has largely been absent leading to breeding success for brents. This also indicates that there are other factors, such a climate change, that are impacting the breeding season.
Nesting and breeding
Brent geese breed on the grassy tundra of Arctic Siberia. The female creates a hollow, bowl-shaped indentation on the ground among low-level vegetation encompassing good all-round views of the surrounding terrain.
Grass and feather down are used to create a nest lining which is reshaped every so often as incubation (keeping the eggs warm while they develop) progresses. The female generally incubates the eggs which she covers with plant matter when she needs to leave the nest.
Rocky islands, exposed gravel plains, and mudflats dotted with grass are commonly chosen nesting sites. These sites limit the chance of predation and offer some camouflage protection to their eggs.
Brent geese return to their breeding grounds between late May and early July, already paired. Laying begins within two weeks of the birds’ arrival and incubation, which is the sole responsibility of the female, last for 23 to 25 days. Brent females typically lay between 4 and 6 eggs, the eggs measuring 75mm by 47mm, which are plain white or creamy buff in colour. They only have one brood each season.
During the incubation period, the male brent guards the territory. Brent geese are highly territorial and aggressive and can see off most predation attempts, including those made by avian predators, such as gulls, skuas, and ravens but are unable to stand up to larger predators, such as the Arctic foxes. Chicks fledge six weeks later and then busily prepare themselves for their first migration as part of a family.
Migration
Once the breeding season is over and the surviving chicks have developed sufficiently to migrate, hormonal changes will start to take place within the population as the daylight hours shorten and the temperature drops significantly. The brents flock together in numbers, embarking on a feeding frenzy to build up fat reserves for the long journey ahead.
The precise date of migration varies with the cold. In a particularly chilly autumn, brent geese might leave earlier than expected. If the winter is mild, they might migrate as late as December or even January.
Brent geese use environmental cues such as diminishing daylight and food supplies to trigger their migration. Once their food sources ice over, it’s their cue to move further south.
But why do birds choose to breed in such cold, inhospitable regions in the first place?
The word ‘inhospitable’ is the clue – these challenging environments are not visited by many other animals and subsequently this means they are relatively safe from predators and other competition.
Once the young have been raised in the relative safety of the spring and summer, the severe cold of the Arctic winter is probably the reason why many birds have evolved to migrate.
So, the journey begins.
Brent geese don’t complete their migration in one go but tend to migrate in stages by using a network of staging grounds (much like motorway service stations) to rest and refuel.
These staging grounds are crucial as they provide vital food and hydration; the brent, unlike some bird species, are unable to accumulate enough fat reserves to perform the journey in one go.
Brent geese tend to migrate in family groups – sticking together year on year – with different family groups beginning their migration at different times. They fly in wavering lines, flocking in loose groups. They seldom fly in V-formation – that’s the Canada geese!
Different groups will stop at their own staging grounds, continually ‘leap-frogging’ each other as they go.
Their route will take them from the Arctic coast of Russia, along the Gulf of Finland, across to Denmark and Germany and then a hop across the English Channel/North Sea to Norfolk and North Kent before heading along the south coast to the Solent.
Migration may be either nocturnal or diurnal (during the day), and takes place almost exclusively over sea along coasts, although overland movements are occasionally recorded.
Each day, they search for marshland, coastal grassland or farmland to rest and feed on, before pushing on further. In places where the grazing is good, they may linger for up to a week at a time.
The White Sea is a critically important staging area for the Brent geese population, since it is likely that the majority of the birds and perhaps the entire population, stop here to feed during both the autumn (September) and spring migrations.
In late September and October, large concentrations occur in the Wadden Sea of Denmark, Germany and the Netherlands. By November, the population is concentrated in Britain and France, and in January and February these two countries frequently hold virtually the entire population, divided roughly equally between them.
The return migration begins in late February, and by May the majority of the population is again concentrated in the Wadden Sea.
Within Britain, numbers of dark-bellied brent geese build up from late September to a maximum in January, with most birds leaving for staging areas in the Wadden Sea in March. However, there is some regional variation in this pattern.
Numbers build up earliest on the outer Thames estuary, particularly on extensive eel-grass beds at Foulness and Leigh-on-Sea, and a large proportion of the wintering population passes through this area in October and November.
Smaller populations in Dorset and south Devon also peak relatively early, with an early December maximum and onward movement to other sites as intertidal food resources in these areas are depleted.
Most other regions broadly follow the national pattern, with peak numbers in the midwinter period from December to February, although significant numbers remain on the Wash and north Kent into May.
Although virtually the entire population leaves Britain in the spring, a few tens of birds commonly linger in parts of southeast Britain into the summer months.
Reasons for migration
One reason for migration is that birds do not enter true hibernation. Many mammals do hibernate, spending their entire winter torpid to survive months of cold conditions with little food around.
The preferred strategy for birds is to migrate to a more hospitable climate where food is plentiful.
But migration is a complex set of behaviours that must be executed carefully to successfully get from summer breeding grounds to overwintering sites.
- Before the bird sets out, it must prepare for the journey by storing fat as fuel.
- It must then choose the correct time of year and weather conditions in which to depart.
- In many instances, it must hold a compass course during flight (we’ll come onto that shortly).
- On long journeys, it may need to stop and refuel (as with the brent geese), using a site that it has never visited before (if it is a young bird), and which may hold food resources quite unlike those around its birthplace or departure point.
- It must decide when the journey is over, that is, when it has arrived at its final destination.
- Along the way, a migrant bird may be blown off course by adverse winds or storms, or its navigation system may fail altogether.
- It may run out of fuel when over the sea and plunge into the water or it may find a traditional refuelling site devoid of food. It may be attacked by a predator such as a bird of prey.
Migration is such a dangerous and perilous behavioural activity to undertake, so why do they do it?
The answers lie in consideration of the various benefits, as well as costs, of migration.
Natural selection favours individuals that adopt a lifestyle that leads to a higher ‘lifetime reproductive success’ (LRS) than that of other birds of the same species.
Lifetime reproductive success is measured by the number of offspring, carrying the genes of a male or female parent, that survives to breed themselves and so contribute to future generations.
Higher year-to-year adult survival, more young raised at each attempt, and higher survival rates of young to breeding age, all increase lifetime reproductive success.
Migration is therefore a behaviour that might boost the lifetime reproductive success rate, providing that the mortality risks of the flights are outweighed by the benefits of greater success in rearing young. For example, if migration increases a bird’s chances of overwinter survival allowing extra opportunities to breed.
Migration may occur, fundamentally, because survival would be low if birds remained in their breeding areas over the winter.
Benign conditions in southern wintering grounds might afford higher survival rates to migrants, but migration itself may also be hazardous and lead to high mortality, especially for young birds on their first outward and returned journeys.
So fundamentally, it’s about the survival of the species once all the pros and cons have been taken into consideration and migration therefore can be seen as an aid to overall species survival.
How do they do it?
Migration is clearly a complex set of behaviours but how is the amazing act accomplished?
Migrating birds can cover thousands of miles in their annual travels, often traveling the same course year after year with little deviation. First-year birds of some species often make their very first migration on their own. Somehow, they can find their winter home despite never having seen it before and return the following spring to where they were born.
The truth is the secrets of their amazing navigational skills aren’t fully understood, partly because birds combine several different types of senses when they navigate.
Birds can get compass information from the sun, the stars, and from landmarks such as river courses and mountain ranges seen during the day.
There’s even evidence that sense of smell plays a role, at least in a study done on homing pigeons, and even air pressure.
Scientist are beginning to think that birds can even sense the earth’s magnetic field.
Birds are known to possess crystals of magnetite (magnetic iron oxide) in front of their eyes and deep inside their nostrils, and it’s been suggested this gives them the means to sense Earth’s magnetic field.
The concept of avian magnetoception is not fully understood but scientists think that birds can effectively ‘see’ magnetic field lines.
Once an individual bird has successfully completed its first annual migration cycle, they often simply repeat the same pattern throughout their lives. If it works once, it’s likely to work again.
There is much research to be undertaken before scientists really understand the somewhat ‘mystical’ powers birds possess as they move from one part of the world to another, in search of food and survival.
Brent geese flying over the Solent
The sight – and sound – of a stream of brent geese ribboning across the sky is one of the glories of our coastline. And all the more awe-inspiring when you consider their extraordinary migratory journeys.
