I can’t find any research about direct competition between mute swans and Canada geese. In the US mute swans and swan decoys are sometimes used to deter geese, but there is no scientific basis for the belief that this works and not much evidence either. Both species are social but become territorial during the breeding season (https://www.tandfonline.com/doi/full/10.1080/00063657.2013.776003). It is common to observe aggression between the species, and I personally have never seen the Canada geese coming out best in encounters. Mute swans weigh on average between 8 and 12 kg whereas the Canada goose weighs between 2.5 and 6.5kg. If you’ve ever seen a coot driving off a mute swan, you’ll see that size doesn’t always matter though!Canadas start breeding at abut 3 years of age, mute swans at about 2. The respective clutch sizes are 2-9 and 4-7 and they both have similar predation and propensity for lead poisoning (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5058547/). Canadas have a slightly wider range of food preferences which may confer slight advantage, but also means that competition with the swans for food resources is not acute.The British Trust for Ornithology state that mute swan populations had been fairly stable since the 1960s but increased progressively from the mid-1980s until they plateaued around 2000. This was attributed to the banning of lead weights used by anglers and a tendency for milder winters which increased the overwintering survival rate.The Canadas were first reported in the UK in 1665 (a difference sort of plague!) but the population increased hugely in the 1950s and ’60s. The increase was measured at 9.3% per annum between 1988 and 2000, but this has been attributed to them colonising areas that had previously low goose densities. I undertook research into the Barnacle Goose population with particular regard to the Solway Firth in Scotland. Numbers there have exploded, along with other geese species, since they were protected in 1953, to the extent that licensed culling needed to be introduced. Paradoxically, the population explosion has meant that breeding success has declined (harder for geese to find mates, stress because of the population density and pressure on resources leading to poorer health in general and a decline in fertility). It is likely the this will result in a population crash, something that is quite normal for most species. (Incidentally, I posit that the same is happening to humans). There is no reason to think that mute swans and Canada geese do not go through similar boom and bust cycles, including factors that relate to inter-species considerations.For the moment neither population is of concern, other than in areas where they may pose a local nuisance (and I’m mainly thinking about the Canada geese here!) There are heaps of swans in areas where I suspect that they are fed a lot by humans such as Windsor and the river Exe in central Exeter and where there is plenty of scope for them to establish and defend nests. Swans need a big "runway" for take-off too.
Charlotte Kasner ● 1886d