An iconic bird in the rivers and lakes of the northern hemisphere this week we are looking at the mute swan. Although not actually mute, they are impressive birds and almost every swan in England and Wales are technically under the protection of the ruling monarch of the UK.
Taxonomy and Evolution
Mute swans, Cygnus olor, are one of the six species of true swan, all of which belong to the genus Cygnus. Despite their size they are not the largest species of swan with the trumpeter swan just exceeding them in size. Another interesting thing to note is that mute swans are strangely more closely related to the Australian swans than their fellow Eurasian swans, like the whooper swan. Swans first appear in the fossil record during the end of the Miocene between 10 and 5 million years ago, with the oldest fossils coming from Europe. From there they spread across the northern hemisphere and down into South America and Oceania. We have subfossils belonging to mute swans dating from around 6,000 years ago in what is now England, although subfossils from that age can be found in Italy, Malta, and as far east as Azerbaijan. This might indicate that mute swans evolved in the Caucasus before spreading into the rest of Europe and Asia.
Biology and Behaviour
Mute swans are some of the heaviest flying birds in the world, only being exceeded in weight by trumpeter swans and wandering albatross. They range in weight from 9.5 to 12 kilograms (21-27 pounds) so mute swans are far from small birds. With wingspans exceeding 2 metres (6.56 feet) seeing them extend their wings in person is quite intimidating. Even with their wings tucked in mute swans can reach 1.5 metres in length from the tip of their bill to the tip of their tail. I have grown up seeing mute swans and they always blow me away with their size. Mute swans are white except for their bill which is orange with a black knob at the top of the bill. Often, swans rest their necks in a s-shape very rarely extending that neck. Baby swans, cygnets, are instead grey. You will mostly see them on the surface of the water or on the land, but, as mentioned, they do fly. However, due to their weight and as they are built for the water they need to take a run while flapping their wings to get enough thrust. Thanks to the power of their wings they are even capable of doing this on the water. Their large, webbed feet are used as break-pads when landing in the water.
Swans are well known for their devotion to one another. Most swans mate for life, ‘divorces’ are rare, and go into an intense grief period if the partner dies. For this reason swans can take a long time before forming a bond, sometimes waiting for ten years. Mute swans are incredibly territorial and will jealousy guard their nests which are large mounds made of twigs, foliage, and mud. Both parents take care of the 6 to 10 eggs which hatch after 37 days. Almost instantly the chicks can go into the water, although the dad will often rest the cygnets on his back or under his wing. The parents are fiercely defensive of their chicks who stay with them for just over a month before they get their white feathers and can fly. There have been recorded cases of swans adopting orphaned cygnets treating them as their own, but this is rare. Eventually the cygnets will leave and after 3 years will be able to mate. If lucky mute swans can live to be 30.
As mentioned, mute swans are incredibly territorial and are powerful birds. While tolerant of other cygnets any other birds or mammals that come near their cygnets will be aggressively scared off or attacked. Normally they are silent, hence the name ‘mute’ swan, unless they feel themselves or their cygnets are threatened. Then they will make a loud hiss which I can attest to is terrifying. Mute swans are known to drown potentially threats if the hisses do not work. When flying they do make a noise but this is from their wings which creates a ‘throbbing’ noise. Swans occasionally migrate but most often they congregate together to keep warm during winter months.
Distribution and Habitat
Mute swans are naturally found across Eurasia ranging from the British Isles all the way to Russia’s far eastern territory. Some of the largest populations are found in the British Isles, where from a Medieval law still in effect the ruling monarch owns all ‘unclaimed’ swans in England and Wales except for those on the Thames, and the Volga river. They prefer more temperate climates but are known from warmer climates, such as southern France, Greece, and the Middle East. Mute swans have been introduced to Canada, the United States, Australia, and New Zealand, and have become naturalised, but this is not without controversy. They are found in rivers, marshes, and lakes, except for the winter when they are found more along coasts and other marine environments as it is warmer.
Diet and Predators
Mute swans are primarily vegetarian with their diet consisting of aquatic plants and algae, but they will also happily eat aquatic snails, worms, and other invertebrates. Swans do not dive, instead using their long necks to plunge into the water get food that way. This helps other waterfowl as it brings to the surface hard to reach food sources for others to access. Due to their large size and aggressiveness adult swans have very few natural predators, although smaller swans and cygnets can be prey. Wild cats, mink, stoats, weasels, bears, and otters can go after the cygnets and eggs. A potentially large pike could bring down a small cygnet as well. Bears, eagles, and very rarely eagle owls are capable of taking down an adult swan, but this is rare. Humans have historically used swans as a source of food – one of the reasons why English and Welsh swans belong to the monarch is because their size made them a fit meal for the royals.
Conservation and Threats
The IUCN Red List currently classifies mute swans as ‘Least Concern’ meaning that they are not in danger of going extinct in the wild. However, some populations are in danger of vanishing, especially those whose river homes are at risk of pollution like those on the Volga (which would be even more damaging as it is also home to one of the largest populations). This was not always the case. Overexploitation, river pollution, and lead poisoning, from pellets during fishing and hunting, from the 1960s caused a sharp decline in mute swan numbers, but environmental policies and removal of lead from pellets helped swans bounce back. There is an issue still. Mute swans are an invasive species and directly compete with other swan species in North America and Oceania. Luckily the other swan species are not endangered, but as we continue to poison rivers this could put introduced mute swans into increased competition with native swans.
Bibliography:
- Peter Frances, (ed.), Bird: The Definitive Visual Guide, (London: DK, 2007)
- Don Wilson, (ed.), Wildlife of the World, (New York, NY: DK, 2015)
- Tiffany Ki, Deborah Pain, Jennifer Gill, and Rhys Green, ‘The relationship between Mute Swan Cygnus olor population trends in Great Britain and environmental change’, Bird Study, 70:3, (2023), 99-112
- Alicia Ivory, ‘Mute swan’, Animal Diversity Web, (2002), [Accessed 30/03/2024]
- James MacDonald, ‘Are Mute Swans a Harmful Invasive Species?’, JSTOR Daily, (24/12/2015), [Accessed 30/03/2024]
- E. Marjorie Northcote, ‘Size differences between limb bones of recent and subfossil mute swans Cygnus olor‘, Journal of Archaeological Science, 8:1, (1981), 89-98
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