Reputation: Swans' love for their partners is so deep they mate for life. They are creatures of myth that only sing when they are dying. Also, paradoxically for such paragons of elegance, they are vicious blighters that can break your arm with their wings.

Reality: Swans often do stay with their partners for life. But whatever feelings they may have for each other, this loyalty is a strategy for maximising the number of cygnets they can raise. They make plenty of noise, and yes, that does include mute swans. Finally, though they defend their young with extreme prejudice, they are probably too weak to break a human arm – although we don't recommend you try the experiment.

Swan courtship is both beautiful, and noisy.

Mute swans form the classic image of devotion, with their curved necks entwined in a perfect love heart. It's part of a courtship ritual, in which pairs face each other and, with a ruffle of feathers and lifted wings, bow gracefully.

Trumpeter swans go in for synchronised swimming, head bobbing and "singing"

But despite their name, mute swans are anything but silent. Their courtship "dance" is accompanied by a range of hissing and grunting sounds. The idea that swans only sing when they are dying, the so-called swan song, is a myth.

All six swan species perform this sort of mating dance, albeit with some variations. Australian black swans have special feathers for attracting a mate. The Eurasian Bewick's, whooper and whistling swans call softly to each other after they have mated.

Meanwhile, the North American trumpeter swans, as their name suggests, are much more boisterous. For one thing, thanks to a coiled, looping windpipe, they can honk. They also go in for synchronised swimming, head bobbing and "singing".

Once courtship is complete, male and female swans really are bonded for life, with few exceptions. This is unusual. Most other birds will raise their young as a pair for one season, but move on to new mates the next. What's more, many species also engage in "extra-pair mating" – "adultery" to you and me. Ducks are particularly keen on such additional copulations. By finding extra mates in these ways, each bird can produce more offspring.

However, for swans the benefits of "together forever" are greater than anything they could gain by sowing their wild oats. The bonded pair will raise clutch after clutch of cygnets in their lifetimes, learning from their successes and failures each time. This opportunity to learn may be one reason they don't break up.

Swan couples are also highly effective fighting teams. If they are separated, swans of either sex get threatened more frequently, and come off less well in aggressive encounters. Females also eat less when separated from their mates. By sticking together, they protect themselves.

Staying together can also save time and energy. Every year, Bewick's swans fly a long, hard race from breeding grounds in northern Russia to their European winter quarters. They only make brief pit stops, to refuel. So they have little time for flirting.

"Most Bewick's swans form pair-bonds during their breeding season in Arctic Russia," says Eileen Rees of the Wetlands and Wildfowl Trust (WWT). "It seems that the 2,500 km long migration and the shortness of the Arctic summer limits their ability to find a new mate and breed within a year."

Bewick's swans migrate the furthest of all swan species, which may be why they are the least likely to "divorce". In 50 years there have only been three cases at WWT Slimbridge, where 10,000 Bewick's swans have spent their winters since studies began in 1963.

So why do swans ever leave their long-term mate, who has become the perfect parent? Rather like humans, they sometimes get out when things go badly.

"Swans change mates sometimes after a bad breeding season or nest failure," says Radoslaw Włodarczyk of University of Łódź in Poland. "It takes 1-2 seasons for birds to match each other precisely and start to breed again." The new pair often raises fewer cygnets in their first year, but improve in later years, particularly for the female from the original pair. In Poland, the divorce rate in mute swans is about 4%.

As well as divorcing, swans do occasionally "cheat". But it's really only female Australian black swans that are regularly unfaithful. Around 1 in 7 eggs reared by a black swan male will not be his, almost always because the female has copulated with a different male just in case she cannot have offspring with her partner.

For the other five swan species, the risks involved in seeking an extra mate, such as contracting a sexually-transmitted disease, or simply failing to find a second mate, are just too high. So they stay loyal - and male swans are thoroughly modern husbands. Unlike most ducks and geese, they help incubate the eggs, allowing the female to feed more and rebuild the fat reserves she used up in laying them.

Male black swans actually spend more time on their nests than females. That may seem strange, because black swan females are the least loyal. But unlike other swans, they can lay multiple clutches a year. The more time a female spends sitting on her nest, the longer it will be before she is ready to lay another clutch. By giving her time off, the male ensures the next clutch comes along quickly. This should mean he has more offspring - so long as the cygnets are his.

Once the cygnets hatch, swan parents live with their young for six to nine months. For comparison, a male duck will spend about a month on parenting duties before, well, swanning off.

Like many parents, swans respond with extreme prejudice if their family is threatened. But could they really break your arm with their wing?

Woe betide anyone who accidentally disturbs a family of swans. Male swans of all species will ferociously guard their nests. As well as protecting the offspring from predators, this also prevents the female from mating with another male.

When it perceives a threat, a swan will rear up with dramatically flared wings and hiss, grunt, snort and flap. This display is called "busking". But this naked aggression is only for show: most of the time the swans are bluffing.

It's unlikely a swan could break an adult's bones, says Włodarczyk, although a child or older person might be unlucky as their bones are weaker. "They can hit hard with their carpal wing joint," he concedes, but they can only muster so much force.

That's because of the way their bones are built. All flying birds have a honeycomb-like bone structure to make them as light as possible. Swans are among the heaviest flying birds, so they need all the help they can get. The trade-off is that their bones are weaker than those of other animals. As a result, a swan would be more likely to break their own wing if they struck an adult human's arm. Rees confirms there have been no swan-caused broken bones at WWT sites.