Plumage morph influences survival probability.

Every year, a pair of Mute Swans (Cygnus olor) breeds on the campus of Wageningen University (the Netherlands). This year, they produced eight little swans (or cygnets). When strolling from my office to one of the education buildings, I regularly run into the local pair of campus-swans and their young. I noticed that two cygnets have white plumage whereas the other six are gray. Diving into the scientific literature, I found some interesting information about the genetic basis of this trait and the impact on individual survival. These juvenile plumage morphs matter more than I imagined.

A Single Sex-linked Gene

The genetic basis of the gray/white plumage morph has already been unraveled in 1968. Three ornithologists – Munro, Smith and Kupa – observed the distribution of these two morphs in 53 nests. The resulting patterns suggest that this trait is determined by a single gene that follows the Mendelian rules (see the table below). As you might remember from high school, one version of a gene (or an allele) can be dominant whereas the other version is recessive. In the case of Mute Swans, the gray allele is dominant and the white one is recessive.

Moreover, the researchers noticed that most white cygnets were female. This observation indicates that the gene is probably located on one of the sex-chromosomes. In birds, sex is determined by a ZW-system. Males have two Z-chromosomes whereas females have one Z-chromosome and one W-chromosome. Because the plumage-gene resides on Z-chromosome, females with a recessive allele will always be white. In contrast to males, females don’t have a second Z-chromosome that could carry a dominant allele.

In summary, the gray/white plumage morph is determined by a single gene on the Z-chromosome with the gray allele being dominant over the white allele.

Aggressive Experiments

Now we know the genetic basis of this trait. But does it really matter whether a cygnet is gray or white? Apparently, it does. In a study in the journal Behaviour, Douglas Norman wrote that “It appears that white cygnets are subjected to far more parental aggression than their brown coloured siblings [gray cygnets molt into brown subadult plumage].” This statement was largely based on personal observations by other ornithologists. So, Norman put this idea to the test with an experiment.

He presented brown or white swan models to adult Mute Swans in their territories. The white models were attacked more vigorously than the brown ones, supporting the idea that white cygnets experience more parental aggression. However, Norman noted that “the brown model might be less visible to the swans due to the lack of contrast between it and the marsh grass (predominately Spartina alterniflora) background.”

Therefore, he devised a second experiment in which both models were presented to the Mute Swans at the same time. Now, the adults consistently attacked the white model, even when they had to pass the brown model to reach it. Norman concluded that “this unequivocally demonstrates that the “whiteness” has the effect of eliciting aggression and attacks from these swans.”

Survival Rates

Now we have experimental evidence that parents are more aggressive towards white cygnets. But does this also influence the survival probability of these young birds? To answer this question, Michael Conover and his colleagues followed the fate of Mute Swans on the Chesapeake Bay in 1972–1980 and on Long Island Sound in 1982–1989. Their results – published in The American Naturalist – revealed that gray cygnets had higher survival rates from hatching to fledging than white ones. Moreover, when the white cygnets began to molt into their adult-like white plumage, their parents often attacked and drove them off. It seems that the parents regarded their white offspring as intruding adults. The gray cygnets – which molt into brown subadult plumage – from the same brood were allowed to remain on their territories for several more months. The gray plumage morph clearly has several benefits, but also one important downside.

Benefits include being allowed to remain longer on their parents’ territory and enjoying a greater tolerance from older conspecifics during their first 2 yr of life. The liability is that they must wait longer before they can reproduce.

However, breeding at an early age is not always an advantage. Less experienced parents might have a higher chance of brood failure. But this aspect remains to be studied in more detail. Perhaps we can follow the lives of the eight cygnets that are currently exploring the campus?

References

Conover, M. R., Reese, J. G., & Brown, A. D. (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.

Munro, R. E., Smith, L. T., & Kupa, J. J. (1968). The genetic basis of color differences observed in the Mute Swan (Cygnus olor). The Auk, 85(3), 504-505.

Norman, D. O. (1977). A role for plumage color in Mute Swan (Cygnus olor) parent-offspring interactions. Behaviour, 62(3-4), 314-320.

Featured image: The campus-swans with their cygnets © Bingxin Wang