High mortality of Mute Swans at Whiteknights Lake
10 April 2007 | Peter Gipson
The Mute Swan Cygnus Olor has bred annually at Whiteknights Lake, which adorns the University of Reading campus, in the ten years since 1997. In the previous ten years, by contrast, breeding only took place in 1987 and 1992, with both females dying from botulism after hatching their young. To my knowledge the 4.5 hectares of lake have never supported more than one breeding pair. Outside the breeding season it is not unusual for two pairs to be present, and I have recorded up to 13 birds.
I have been watching Whiteknights Lake fairly intensively since 1987. An apparently high number of the adult Mute Swans there have died during this time, as have all of the young in the past two years. What follows is an exposition of this mortality, and the difficulties that attend its reduction.
It is on record that one quite exceptional Mute Swan kept in captivity lived for 26 years and 9 months, but how long do wild birds normally live in Britain? According to the Review of natural avian mortality rates by Balmer and Peach of the British Trust for Ornithology, published in 1997, adult Mute Swans have an annual survival rate of 0.795. Assuming a simple model of survival, a little mathematics indicates that adults alive on a given day have a mean expectation of further life of 4.4 years, with 50% of them staying alive for at least 3.0 more years. This last statistic provides a useful benchmark by which to compare the mortality of Mute Swans at Whiteknights Lake with the national average: if the proportion staying alive for at least three more years is much less than half, the mortality is well above average.
For several reasons, it is difficult to apply this method accurately to the available data for Whiteknights. Here are five: swans on the lake depart for varying amounts of time ranging from one day to many months; it is difficult to identify individual birds that are not ringed; a particular individual can lose its ring and be given a new ring with a different code; sick or injured birds may have been taken into captivity and died shortly after without my knowledge; and some corpses may not be found.
The following findings include birds that died after being taken into captivity and at least two that had to be put down. During the 20 years between 1987 and 2006, it appears that only three adults residing on Whiteknights Lake actually survived beyond three years. A strikingly higher number of adults died within three years: at least 15 cases of death have come to my attention. Although there is some uncertainty about what these figures should actually be, the difference between them is clear enough. Moreover, two of the three birds that defied the mortality odds and stayed alive for more than three years nearly did not do so! One contracted botulism twice and the other once, but were nursed back to health by the Swan Lifeline charity based at Eton. The causes of death have included suspected or confirmed botulism (11 cases), leg problems (two cases), lead poisoning (one case), and collision with a tree (one case). I am unaware of any adults or young being killed by a Fox Vulpes vulpes, even though this mammal can kill Mute Swans and is common on the campus. Whiteknights Lake is a bad place to be if you are a Mute Swan who wants to stay alive!
The young swans at Whiteknights have shown a more intriguing pattern of survival. From 1997 to 2004, a reasonable proportion of the cygnets hatched in spring survived through the summer months and grew up - typically four or five were raised each season. Unexpectedly, this figure dropped to zero in 2005 and 2006, and the circumstances surrounding several deaths were a mystery. In 2005, all five of the cygnets apparently died in succession after growing to an appreciable size, yet four of the bodies could not be found, despite careful searching. There were no signs of any kills by a Fox and each of the four had appeared to be healthy the day before it disappeared. The fifth died from lead poisoning. The following year two chicks died shortly after hatching and the third had passed five weeks old when it was attacked by a dog. Apparently not seriously injured, it survived for about three more days and vanished. The possibility of human involvement in some of these disappearances/deaths has to be contemplated.
At least two cygnets (as well as other bird species) have been killed by dogs, and an adult swan had to be put down when its leg failed to heal after being injured in an attack by a dog. Certainly since the 1980s it has been the policy of the University for dogs to be kept on leads, and recently several notices were put up advising this. Regrettably, many dog walkers are still flouting this rule, which has received little enforcement, and one notice has already been vandalised! Some years ago I happened to witness two small notices warning that dogs are to be kept on leads being illegitimately removed - the culprit was a female dog walker!
The recurrence of deaths due to botulism has prompted swan campaigners to put the blame on the excessive layer of silt in the lake, and to call upon the University to dredge it. It was last dredged on a major scale in the 1960s, and I have observed an increase in lake-bed material and a decrease in the clarity of the water since 1987 - the problem is exacerbated by the annual fall of leaves from the abundance of trees in the vicinity. The local media, including Meridian TV, have covered several stories of swans contracting botulism and ensuing pleas for Whiteknights Lake to be cleaned up. In 2001, after two adults perished from botulism and a third became sick, a swan lover blamed the deaths on the filthy condition of the lake, which he likened to a cesspit (Reading Evening Post, 16th August 2001). In rejoinders in the local press, the University argued that the saving of one or two swans a year did not justify dredging it, which would cost more than £200,000 and result in the temporary destruction of some plant and fish life, with the Head of Grounds stating that "The silt we dredge from the lake would have to go somewhere, and I'm not prepared to dump it on the university and destroy 20 acres of grassland." (Reading Chronicle, 15th August 2001).
In my view, the focus on just one bird species that is particularly common in the Reading area has done little justice to the case for dredging. A more cogent argument is that Whiteknights Lake is a Wildlife Heritage Site and historic amenity that must be preserved as a lake ecosystem before it clogs up so much as to become shallow, boggy pools or marshland. Admittedly, this potential fate is probably decades away, as most of the main lake was around one metre deep in 1990. But this is not the only consideration. The longer it is not dredged, the greater will be the amount of silt and debris that will have to be extracted at some time in the future and dumped somewhere nearby (or else transported away at enormous expense), and the greater will be the consequent loss of grassland or copse. If the University was "not prepared" to destroy 20 acres of grassland in 2001, will it be prepared to destroy even more land in the future? Or perhaps it is making prudent financial provision now for the future transportation of the silt away from the site? Anyone reading the rejoinders made by the University in 2001 could be forgiven for thinking that the lovely asset of Whiteknights Lake does not actually have a management plan! At present the issue of dredging is being reviewed by the University.
I doubt that as much as 20 acres of scenic campus will need to be lost. One cubic metre of wet silt eventually reduces to a much smaller volume of dried deposit. A practicable course of action may lie in the routine small-scale dredging of the end of the lake adjacent to Whiteknights Road every few years, with the silt being evaporated, deposited, and drained in the copse near Wessex Hall. The remaining silt should shift towards the gap left near Whiteknights Road, possibly assisted by the general flow of the water in this direction. It may also prove beneficial to undertake limited clearing of other parts of the lake.
One effective way to prevent Mute Swans from dying prematurely on the lake would be to remove them. However, it is very difficult to implement this apparently simple solution. It is illegal to relocate healthy Mute Swans without a licence from the Wildlife Licensing Unit of Natural England and their policy is to relocate this species at least 75 kilometres away, to minimize the risk of birds returning. The permission of the receiving site's landowner is required, and a Wildlife Advisor has to approve the suitability of the site to receive the swans, the need for their relocation, and the competence of the licence recipient to handle them. The relocation of just one swan would cost a large amount of time and petrol: get to the lake, catch the swan, transport it for over 50 miles, release the swan, return home. And sooner or later further swans would arrive at Whiteknights Lake! Who would want to apply for a licence to undertake this time-consuming work, and would it be reasonable to issue one?
If the swans cannot be removed, can they be discouraged from staying? The obvious way to attempt this would be to ban people feeding them. The surge in their frequency of breeding after 1997 coincided with a considerable increase in their being fed by one or two people, although it is not clear whether this played a role. I am not sure that banning the feeding of swans today would make much difference, and it would be difficult to enforce. Perhaps the high mortality of Mute Swans at Whiteknights Lake will simply continue.
© 2007 Peter Gipson