The Mute Swan

I recently purchased The Mute Swan, by Mike Birkhead and Christopher Perrins, after a visit to a local fishing lake where a pair of swans holds territory year round. It has been an interesting, informative read, especially the historical aspects, although I would have liked to read a little bit more on behaviour. Some aspects are dated, for example, lead weights have been banned since 1987, and the mute swans populations have recovered, probably also helped by milder winters (BirdTrends). Mute Swans are the heaviest British birds, with males over 10 kg in weight and 2.20 m wingspan.

Variation
There is an unusual colour variety called immutabilis or 'Polish swans' in which the young are white when downy and moult into white feathers, instead of the usual brownish feathers of the juvenile. The feet of the immutabilis form are pinkish grey instead of dark grey or blackish. Read more here.

A normal juvenile with its parents

An immutabilis juvenile (pink bill) with parents.

An immutabilis cygnet (left) with its normal sibling.

Calls
Mute Swans get their name from their relative lack of calls, especially in flight, and in stark contrast to their vocal relatives the Trumpeter, Whooper and Bewick. Although relatively quiet, Mute Swans are not mute. They have a repertoire of vocalisations, which, although weak and not carrying long, are quite obvious when at close quarters: they snort, a pig-like call; they hiss loudly like geese, a threat call to predators or people approaching too much, especially when young are about, and they have a short, quiet two note call reminding of rooks'.

A hissing swan

Their wingbeats, however, are loud and make a whistling noise that is audible, it is said, from over a mile. This sound might have replace the flight contact calls of the other swan species.

A pair of Mute Swans in flight.

Moulting
Non-breeders migrate to safe waters to moult in the summer, often congregating in large numbers. Moulting in breeding pairs is different. They moult when they are breeding: the female moults first, and then the male, once the female's feathers are fully grown. This way, there is always an adult with grown wings, which play an important role in the cygnets defence.

A moulting flock of non-breeding swans at Hornsea Mere

A semi-domestic bird?
At some point in history most if not all adult British swans, and most likely those from other European countries, were pinioned, and therefore, unable to fly. They were a heavily managed species, as it was regularly eaten, and it is likely that this management saved it from extinction from overhunting. The spectacle that is the sight of flying swans would have been much rarer, and restricted to movements involving young birds. In the early XX century, the custom of eating swans disappeared, and, management in most areas was much reduced. In a way, we can see swans as a once domestic, now feral species.

Food and the swan's neck
Swans have a record number of bones (vertebrae) in their necks for a bird: 25. Their long necks allow them to exploit aquatic plants and tubers much deep than other water birds. They use several feeding techniques, shallow feeding in which they keep their neck folded to up-ending, when they use their body and neck to reach deep levels, about a meter deep. They also use their feet in a paddling movement to disturb the bottom sediments and being able to filter out small particles or loosen roots and tubers. Swans might migrate some distances if their territories ice up in the winter.

These feeding swans attracted numbers of coots and various ducks, most likely benefiting of the swans disturbing and pulling deep aquatic plants.

A swan uses its long neck to feed from a hole in the ice

Courtship
I have never seen the courtship of the mute swan, which appears to have nothing to envy to the Great Crested Grebe's (watch a video here). It involves the pair facing each other with smoothed, dipped wings, and engaging in synchronised, repetitive head dipping and flank rubbing, swimming in circles and, after copulation, both partners paddle and raise their bodies together, with their necks arched and bills pointing down.

Territorial defence

A pair of breeding swans defend their territory, in a lake or river, fiercely: sometimes over the winter if enough food is available and there is no ice. They will chase other swans, with flapping wings being used as a threat, and use a threat display, busking, which I have covered at The Rattling Crow before, in which they swim with strong pushes of both feet together towards intruders, their secondary wing feathers kept high up, neck arched and neck feathers ruffled.

an approaching busking swan.

high intensity busking just before wing flapping and chasing of an intruding juvenile.

Both males and females participate in territory guarding and defence. This pair approached a juvenile busking, chased it away and then swam around the perimeter of their lake.

Nesting

Nests are often very visible as they are built from surrounding plants, creating a gap in reed beds or river margins. Swans lay 4-7 eggs that only the female incubates while the male keeps guard nearby.

Incubating female swan.

Parental behaviour
Like geese, which are the closest relatives of swans, both parents are involved in caring and defending the young. Adults will carry the young downy cygnets on their backs, often only the cygnet's heads showing over the parents wings, to suitable feeding areas. The male is more involved in territorial and brood defence, often chasing other birds like mallards or coots away. When little, the cygnets are unable to reach aquatic plants at the bottom of the lake, but parents will often disturb the water with their legs, or pull vegetation for the young.

Male swan (cob) chasing a mallard away.

Female with cygnets

Mute swans often swim with one leg, keeping the other dry out of water (cygnet on the left). The cygnet on the right is feeding by up-ending.

Up-ending adult pulling food for cygnet

Independence
Normally young disperse in September, once they have grown their first year plumage. Then they leave their parents territory and join a flock of non-breeding birds. However, some young stay with their parents until the spring. The local lake couple seem very tolerant of their young, and they often let them stay until the spring.

Last years first year swan moulting into adult plumage (23/1/15) still in its parents territory.

Non-breeding flocks
Mute Swans do not normally breed until they are 3-4 years old. Young swans unable to secure a breeding territory are more sociable and form non-breeding flocks.