You’ve probably heard about Black Swan Theory, which is used to describe an event that was entirely unpredictable and has a major effect. The existence of black swans had not been known until 17th century when Europeans discovered Australia with lots of black swans living there. Till then, people outside Australia (and New Zealand) had believed all swans were white. Hence, a black swan was just a metaphor of an impossible being. As an example, Roman poet Juvenal (Latin full name: Decimus Junius Juvenalis) wrote in despair of finding a perfect wife that such a lady is “rara avis in terris nigroque simillima cygno (rare bird upon the earth similar to a black swan).” Well, apart from the fact that his writing is full of misogyny, it is faulty because of existence of black swans and a perfect wife, that is myself, of course not to him but to mine. 🙂

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We can see black swans not only in Australia but in New Zealand. As for black swans in New Zealand, people thought the birds had existed but were extinguished before European settlement. Later, humans reintroduced Australian black swans (Cygnus atratus) in the mid of 19th century. But again, people have locked themselves in their own box of thinking. It has been thought that all black swans are same, meaning the extinguished black swans and the extant ones (C. atratus) are the same species.

However, a recent study by NJ Rawlence et al. (Proc. R. Soc. B. 2017;284:20170876) revealed that a different black swan species (C. sumnerensis) had existed and extinguished later in New Zealand. Analyses of mitochondrial DNA and morphometric and osteological features from 47 modern (blood and bone samples ) and 39 ancient (fossils and remains) specimens suggested that C. sumnerensis was on an evolutionary pathway towards flightlessness, which was featured by larger body, shorter wings, and longer hind legs than C. atratus. This interesting fact could not have been discovered if the authors didn’t conceive the idea that there might have been a different species of black swans. 🙂

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Another black swan thing that will shock many of lay people who saw black swans only on the ballet stage is that black swans are not all black. They have white feathers– alula feathers, primaries, and outer secondaries–which are not usually visible unless the swans spread wings. And the bill is red with white band at the tip. In case of cygnets, feathers are greyish brown and bill is black.

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The last fact about black swans I’d like to add in this post is that they hold the number one place along with other swans (Cygnus spp.) among birds regarding the number of cervical vertebrae ranging from 22 to 25 depending on the reference. Number of cervical vertebrae is 12 in pigeons, 14 in chickens, and 17 in ducks. Although the number of cervical vertebrae will not always tell the neck length, swans are equipped with quite a long neck. It is interesting that mammals, regardless of size, have the same number 7, except sloths and manatees, whereas birds have varying number of cervical vertebrae. According to an article by C. Böhmer et al. (R. Soc. open sci.2019;6:181588), the high interspecific variability in the number of cervical vertebrae is to support a functional versatility of the neck, which may be a compensation of the evolutionary outcome of a functionally constrained forelimb due to the evolution of powered flight.

This observation gives me a lesson that every choice comes with a certain cost. Nothing is gained for free. While avian arms have been evolved as flight tools, their arms lost many dexterous functions, instead, necks have taken many roles that forelimbs could do. Well, I like the flexible neck of a black swan as it gives them serpentine look. I wonder whether they also enjoy their own majestic look reflected on the water.