The main hypothesis relating to the factors causing the increase in this species concerns the use of lead as fishing weights (Rowell & Spray 2004, Ward et al. 2007). In the late 1970s lead poisoning was shown to be the largest single cause of death among Mute Swans in England, accounting for the deaths of 3,000-3,500 birds annually (Kirby et al. 1994). There is good evidence showing that lead contamination of Mute Swans in England caused local population declines during the late 1970s and 1980s (Blus 1994, Birkhead & Perrins 1985). The increase in the British Mute Swan population seen between the 1983 and 1990 censuses can thus be explained partly by the ban on the use of lead weights in fishing imposed by the Water Authorities in 1987 (Rowell & Spray 2004). There is no evidence to suggest that lead poisoning was ever a problem in Scotland (e.g. Brown & Brown 1984).
A second, not mutually exclusive, hypothesis is that warmer winter weather has benefited this species. Deaths during the winter due to poor weather are an important cause of mortality in many areas (Spray 1981, Perrins & Sears 1991) and a run of mild winters is likely to have reduced this (Rowell & Spray 2004). Mild winters are not only associated with low mortality but are also followed by high reproductive output (Delany et al. 1992) which has also contributed to the increase in the Mute Swan population. A study examining five years' data on breeding biology found that winter temperature was one of the factors significantly affecting the date of laying, which in turn was related to clutch size, which in itself was the most significant factor determining the number of cygnets fledged (Birkhead et al. 1983), hence demonstrating an effect on breeding performance. Esselink & Beekman (1991) have also shown that mild winters are not only associated with low mortality but are also followed by high reproductive output be enabling adults attain peak body condition. This may have been particularly important in Scotland.
Whilst the recovery of the British Mute Swan population may in large part be attributed to the reduced incidence of lead poisoning, locally other factors may have had an equal or more important contribution to the observed changes (Ward et al. 2007). Recent years have also seen an increase in the availability of suitable breeding habitats, in the form of the large numbers of gravel pits and ponds that have been created. Improvements to the water quality of rivers and canals, as a result of efforts to reduce pollution, may have also helped the species (Coleman et al. 2001, Rowell & Spray 2004). The number and activity of Swan Rescue Centres may also have an effect on the Mute Swan population size (Delany et al. 1992, Perrins & Martin 1999), although there is little documented evidence to support this. Other factors affecting local populations include increased protection of nesting birds; in an English Midlands study area, this was considered a key factor in the reversal of the 1960s and 1970s decline (Coleman et al. 2001).
In Scotland (and presumably elsewhere), the increased autumn sowing of cereals has improved the winter food supply for swans, enabling a higher proportion of birds to survive the winter (Delany et al. 1992, Ward et al. 2007), although there are no specific analyses to support this.
