Mute Swan

Mute Swan Cygnus olor (Gmelin, 1789)

Mute Swan Cygnus olor (Gmelin, 1789)

(Uncommon resident & SV. Breeds.)

Adult Mute Swan & cygnets Cygnus olor, Blagdon Lake © Nigel Milbourne, 2008

Cecil Smith (1869) wrote that the Mute Swan was introduced into this country during the reign of Richard I, who is said to have brought it from Cyprus. However, the British Ornithologists Union commented in the Records Committee’s Sixth Report (Ibis : 420-423) in 1971 that: ‘Mute Swan is in Category A because wild immigrants occur and because most of the breeding population is derived from semi-domesticated descendants of wild stock rather than artificially introduced’. Whatever their provenance, it is undeniably true that Mute Swans have long been domesticated and used for food, especially in the Middle Ages (Cramp & Simmons, 1978).

Smith also stated "hardly any large piece of water in the county is without a pair or more of these beautiful birds." So, it is reasonable to assume that they would have taken up residence as soon as the reservoir was flooded. However, the earliest record I have been able to find is from 'A Visit to Blagdon Lake, May 1925' a photocopy of bird notes made by Dr. L.P. Ashton during a bicycle visit with two fellow pupils at Clifton College. The following birds were seen by or on BL:- ...two swans. A swan's nest was seen, made completely of rushes was floating and was on about 2' 6" of water among the reeds.

H.H. Davis (1948) commented on their local abundance as being "frequent at the docks and reservoirs and not uncommon wherever there are suitable waters".

A.H. Davis (1980) analysed the wildfowl counts and concluded that regular and maximum numbers of Mute Swan had fallen during the period 1971-1980 cf. counts for the period 1948-1962 (Atkinson-Willes, 1963). He also concluded that there was some evidence to demonstrate that part of the Blagdon population had moved to Chew.

At the national level, numbers increased during the 1950's but declined in the 1960's due to the severe winters early in the decade and more rapidly in the 1970's and early 80's due to lead poisoning from ingested fishing weights (Snow & Perrins, 1998). It is interesting to note, however, that the Mute Swan population index in Cranswick et al (1997) doesn't demonstrate the dramatic changes claimed by Snow & Perrins for the 70's and 80's. Additionally, Bland & Tully (1991) inferred there had been little change in the status of Mute Swan in the county since the previous atlas compiled in 1968-72, whilst Hammacott (1983), stated that 33 pairs bred in Avon (seven at BL) in 1983 cf. 23 in 1978.

There is no doubt the British population has increased steadily in recent years but the number attempting to breed at the lake remains fairly constant, even if the moult flock has indeed transferred to Chew Valley Lake.

Looking at the monthly maxima, there is a build-up in numbers resulting in a small peak during March and April probably related to local population movement by birds seeking nesting sites. Based on spatial requirements and suitable habitat, it seems that seven nesting pairs is about the limit that the lake can support. This may be reduced by bankside disturbance, particularly at the end of March when the birds are looking for somewhere to build their nests and fishermen are lining the banks at the start of the trout-fishing season. Nests are usually made in emergent bankside vegetation, though not always so, and The Island, Rugmoor Bay, Top End, Home Bay and Pipe Bay are often chosen. Unusually, a pair nested in the open on Peg's Point in 19xx and continued to brood despite anglers standing beside the nest to fish. Presumably the birds did not wish to abandon the nest once they had made such a commitment. Those birds that stay to breed will usually be sitting on eggs during April.

For a period of 6 - 8 weeks in summer, adult Mute Swans become flightless (they moult their flight feathers simultaneously), so seek a safe refuge where there is sufficient food. During this, the complete post-breeding moult, they have to take in enough energy to facilitate the replacement of upwards of 25000 feathers (Sibley, 2000). Indeed, the daily food requirement of moulting adults is quoted as between 3.6 and 4 kg of wet vegetable matter (Cramp & Simmons, 1978). Presumably, large numbers coincide with abundant, obtainable weed to provide them through the moult period.

Monthly maxima over the last ten years indicate an annual peak in July, if a moult herd of non-breeders gathers, although this peak may occur in June in some years e.g. 1997. I suggest that birds arriving in June that don't find conditions to their liking move on and this would account for the June peak. If they stay to moult their flight feathers then the number would remain constant from June onwards as they would be unable to leave the lake. The key months, therefore, for a moult flock to gather appears to be July into August when birds freely associate. Analysis of the moult flock numbers ought to be made for the months of July - August as a late autumn peak may occur in September and October that could unduly affect calculated averages. I have re-worked the data and will present it in due course.

The average number recorded for each decade probably reflects a combination of the size of the Mute Swan population and the water level of the lake. Numbers appear to have changed very little (to be confirmed).

In those years when the water level drops considerably (e.g. 1996, 2001 and 2002) there may be an autumn peak in October as small numbers making local movements seek to join the new families and those of the moult herd that don't move off during late August and September. Shallow water and abundant weed growth may provide ideal conditions for feeding, but numbers and distribution of newcomers can be severely affected by the presence of territorial breeding pairs with young, who defend their own food source vigorously. Mute Swans may fight to the death in exceptional circumstances and on 22 June 1997 I witnessed blood drawn by two cobs fighting for over twelve minutes in Holt Bay during which time a pen and four cygnets circled the affray. One cob tried to drown the other and there was an attempted rape or drowning of the pen. If the cygnets got too close to the fight they were occasionally struck by one of the cobs as well. This was quite a harrowing experience for the onlooker, never mind the birds involved!

Up-ending Mute Swans feed to a depth of about one metre (Ogilvie & Pearson, 1994) so can utilise food over a wide area of the lake in low-water conditions especially in the bays and at Top End. As the food supply dwindles, the swans move away and there are many occasions when none are present during the winter months. It would appear today that Blagdon Lake is of little significance as a wintering site. No doubt artificial feeding has a bearing on the whereabouts of large wintering gatherings e.g. Bristol Docks, though large herds gather on the Avalon Marshes in Somerset to feed on fields beside drainage rhynes. One pair has taken to feeding on Holt Farm Farm fields on the south side of the lake during the winter in recent years rather than leave the lake altogether, perhaps to maintain their breeding territory.

There is no best location to view this species as they distribute themselves fairly evenly around the lake during the breeding season and tend to leave altogether in the winter during high water, which restricts access to aquatic vegetation, or freezing conditions.

Bibliography (sources of information)

  1. Atkinson-Willes G.L. (Ed.) 1963. Wildfowl in Great Britain. London.
  2. Bland R.L. & Tully J. 1992. Atlas of Breeding Birds in Avon, 1988-91. Gloucester.
  3. Cramp S. & Simmons K.E.L. (eds.) 1978. Handbook of the Birds of Europe, the Middle East and North Africa. The Birds of the Western Palearctic. Volume I. Oxford.
  4. Cranswick P.A., Waters R.J., Musgrove A.J. & Pollitt M.S. 1997. The Wetland Bird Survey 1995-96: Wildfowl and Wader Counts. Slimbridge.
  5. Davis A.H. 1980. A Review of some Waterfowl at Blagdon Lake, 1971-1980. Bristol Ornithology, 13, pp. 94-96.
  6. Davis H.H. 1948. A Revised List of the Birds of the Bristol District. Reprinted from the Proc. of the Bristol Naturalists' Society, vol. XXVII, part IV, pp. 225-268. 1947.
  7. Hammacott H.R. 1984. BTO Mute Swan Census 1983. Avon Bird Report 1983, p. 46.
  8. Ogilvie M. & Pearson B. 1994. Wildfowl. London.
  9. Sibley D.A. 2000. The North American Bird Guide. Sussex.
  10. Smith, Cecil 1869. The Birds of Somersetshire. London.
  11. Snow D.W. & Perrins C.M. 1998. The Birds of the Western Palearctic Concise Edition. Volume I. Oxford.

Updated 21 October, 2012