Mute swan colour polymorphism

 After weeks of waiting for the cygnets to hatch, we were rewarded today by the mute swan family approaching us in a local lake. I noticed the two cygnets were different, one had grey-brown down and black beak and legs.

The other, had creamy white down, with pinkish-grey beak and legs.

It was quite noticeable when they were together.

 Next up a close up of the male, I even used the macro setting, that close we were. He was very relaxed around us, but he reacted by opening his wings, raising his neck feathers and hissing loudly when a dog approached the family. Both parents were incredibly protective of their cygnets, chasing away ducks, moorhens and even fluffy ducklings!

 The swan family, on the right, the female, which has pale feet.

The white cygnet illustrates the fact that Mute Swans (Cygnus olor) have a colour form called 'immutabilis', or unchanging, and also known as Polish swan. The mutation is sex-linked and recessive, and occurs in a gene located in the Z sex chromosome. Unlike most mammals, female birds have two different sex chromosomes (ZW), while males have two of the same kind (ZZ). If the female has the mutation, she will be white as a cygnet and will moult into an adult white plumage directly. A male needs to have two mutated forms of the gene to be a Polish swan. 'Immutabilis' adults are hard to tell apart when they are in the water, as the only trait that is different is the colour of the feet, which is paler. The immutabilis form is rarer in populations than the normal form, and it is more common in female swans.

 Other than the 'cute factor' when cygnets, the immutabilis form carries advantages and disadvantages for swans. Swans are very territorial, and they react aggressively toward white.  As they start moulting into their white feathers while being in their parents territory, the adults may start behaving aggressively towards the immutabilis cygnets. This means that young immutabilis flegdlings could be expelled from the territory earlier and may suffer highest mortality, as they need to venture and find a territory of their own when too young. Swans do not usually breed until their 4th year of life, so moulting into a brown subadult plumage pattern advertises to other swans, including their parents, that they are not ready to breed. On the other hand, if population densities are low, it could be advantageous to moult straight into an adult plumage and breed earlier.

Conover and coworkers studied the feral swan population in the East of the USA for nine years to establish the survival of cygnets of both morphs and the age at which they first reproduced. They ringed and neck banded young swans in two populations and follow their fate by thousands of resightings. Their results were striking: immutabilis cygnets (which they called AP, or adult plumage) had significantly lower survival rates from hatching to fledgling (73%) than grey cygnets (which they called SAP, or subadult plumage) (87%). This effect was mostly due to the higher mortality of male immutabilis cygnets, and mortality seemed to occur between August and fledgling, when cygnets grow their flight feathers. They stated:

We watched the parents of eight broods, which contained both SAP and AP phenotypes (four in the Chesapeake Bay and four in Long Island Sound), attack and drive out of their territories their AP offspring during August or September, while continuing to care for their SAP cygnets for several more months. We were able to keep track of eight of these ostracized AP cygnets; four of them perished within a month.

But all is not lost for the immutabilis swans. Although males suffered a higher mortality, the survivors were able to breed earlier than SAP males: by age 3, all of the surviving immutabilis males had bred, while just over 30% of the SAP males had done so. In the studied populations, the frequency of the immutabilis form does not seem to be changing, so the persistence of the immutabilis form could be reflecting a balance between the costs of being expelled early from the parental territory and the benefits of early breeding.

More information

Conover MR, Reese JG, & Brown AD (2000). Costs and Benefits of Subadult Plumage in Mute Swans: Testing Hypotheses for the Evolution of Delayed Plumage Maturation. The American Naturalist, 156 (2), 193-200 PMID: 10856201

Munro, R. E.,, L. T. Smith,, & J. J. Kupa (1968). The genetic basis of color differences observed in the Mute Swan (Cygnus olor) Auk, 85, 504-505

UPDATE 23/0/2022. I'm reading 'The Swans' edited by Sir Peter Scott, and he discusses the possible force behind this colour polymorphism, which appears to incur in the cost for cygnets that they suffer more aggression from parents and are expelled from home territories at an early age: hunting pressure. Young swans were preferred as food as they were more tender, and therefore the immutabilis variety were hunbted less, as they were thought as adults.