Author: Anne Goodall

Aims and Objectives

The study initially set out five major areas of investigation concerning respectively:

• the breeding population
• the non-breeding population
• feeding preferences
• individual marking studies
• collation of historical data

For the first three areas the aims are, broadly, to record the distribution, numbers and preferences, monitor long- and short-term changes and identify if possible the causes of such changes.  The marking studies aim to maintain a marked population in order to monitor health and life histories, including the extent of emigration and causes of death, and the last aims to compare present findings in at least the first two areas with the past situation.  Some progress has been made in each area, and this report summarises the progress and describes the ongoing studies.

Methods

The study area is centred on Lincoln’s Brayford Pool, which holds the major part of the non-breeding population for at least part of the year.  Upstream it follows the two watercourses which feed the Brayford: the River Witham as far as the River Brant infall and the Foss Dyke to the River Till infall and downstream, the River Witham as far as Southrey.  It also includes the tributaries to each of these watercourses and the lakes, pools, pits, ponds, fenland and washlands which lie between them. 

In December 2008 and in March, September and December 2009 as full a census as possible was carried out over this area, coinciding with the regular WeBS counts.  WeBS counts and casual observations from all sites within this area were also collected in all other months, and between April and August an attempt was made to locate all breeding pairs and follow their breeding attempts.  Volunteers were asked to record what the swans were feeding on wherever possible, and particularly for those away from waterbodies.

Between July and September 2008 and 2009 as many of the Brayford birds as possible were rounded up and ringed, or existing ring numbers are recorded.  At the same time they were weighed and examined for injuries or indications of ill-health, moult stage was recorded and if necessary food boluses trapped under the tongue were removed.  Throughout the year swans taken into care either locally by Lincoln Swan Care (formerly the Lincoln Swan Preservation Society) or the Weirfield Hospital, or removed by the RSPCA to East Winch Wildlife Centre, Kings Lynn, have been ringed once they are ready for release.

Meantime, the LBC database has provided records of mute swans in this area, including WeBS data, back to 1995, and further information is being collected from personal records and published papers.

The Breeding Population

Within the city and on the rivers, drains, pits etc to the south and west (including Whisby NP, North Hykeham Pits, Swanholme, Hartsholme, Boultham Park, Boultham Mere, Burton Pits and Burton Waters) 25 breeding pairs were identified.  Of these, half either lost their eggs before hatching, or had lost all of their young by the September count.  At this time the remaining 13 pairs has a total of 50 cygnets with them.  By the December count, this total had reduced to 24. It is likely however that further pairs nested in this area, both on the stretch of Witham which was not counted or on some of the private fishing ponds or small drains in the Lincoln area, since there were 31 adults and 11 young on the North Hykeham Pits, where none nested, on the September count.  Where observers indicated a likely predator of eggs and young, mink were most commonly mentioned.  Mink are regularly seen on the Fossdyke west of Lincoln, in the Swanpool area and at Burton Pits, for example.

Getting a full count of the breeding pairs along the Witham and delphs to the east of the city proved harder but the 12 successful pairs had reared 34 cygnets to September.  By December counts of young were confounded by the many more arriving in this area to winter.

Summering non-breeders

During the 2009 moulting period, approximately 70 birds in their 2nd calendar year or older remained on the Brayford Pool, moving between this area and the River Witham at Waterside.  At this time they are held in this area by the territorial pairs above and below the Brayford on the tributary rivers, which will attack, and on occasion kill, any swans which move into their territory.  No other summer non-breeding herds were identified in 2009, but it is possible they exist.

Wintering Population

Over the first winter of the study, wintering herds were found in Nocton Fen, Branston Fen and Heighington Fen in December 2008-March 2009 and in the Barlings floodplain at Barlings and Stainfield Fen in March 2009, with increased numbers scattered in smaller groups on the River Witham and North and South Delphs.  The Brayford herd decreased at this time, but this would not account for all of the wintering birds feeding out on the fields.  Almost all territorial pairs remained on their territories throughout. 

In late 2009 the wintering herd built up as usual on Nocton Fen in November but part of the group had moved west to Potterhanworth Fen in December, apparently as a result of deliberate disturbance (but see  below).  A second herd was present in Washingborough Fen, with a smaller group in the Barlings floodplain.  The autumn saw a build up in numbers on the Apex Pit at North Hykeham, which apparently didn’t occur in 2008/09, but was declining by December; however, in that month a feeding herd was located in the upper Witham flood plain, near Aubourn.  This herd has not previously been recorded, but may simply have been overlooked.  Later in the winter it apparently moved to Waddington Low Fields, although since the Aubourn site was not revisited, this has not been confirmed.

Population Summary

The totals for the four counts to the end of 2009 are shown in the table below.  As expected, the population peaks in later summer, when the maximum number of juveniles have entered the population, but the mid-winter totals are similar

Feeding Preferences

To date, all herds of swans feeding away from the river have been recorded on autumn-sown oil-seed rape, and this finding has been corroborated by recorders elsewhere in the county.  Rotation of this crop almost certainly accounts for the different locations used by the major wintering herds from year to year.  Pairs in spring may also feed in oilseed rape until it becomes too tall, but only if it is present within their territory.  Otherwise alternative young crops will be used.

On the rivers and waterbodies swans are most commonly recorded dabbling in the margins, taking a range of aquatic plants.  On a number of occasions the River Witham is subject to a bloom of duckweed or other floating waterplants (including the invasive alien water fern Azolla filiculoides) and the non-breeding Brayford Pool herd will concentrate on this.

Ringing Studies

In the two moult seasons 2008 and 2009, 99 swans have been ringed and a dozen or so others were found to be carrying rings known or assumed to have been put on at East Winch. Of these, one has moved to Grantham and around 8 are known to have died, either as a result of oiling or collision with bridges or overhead wires in Lincoln, or killed within the study area by territorial swans, or taken into care by the RSPCA and put down by them. 

Smaller numbers have been ringed at this site in the past, mainly by Mid-Lincs RG, but no swans more than 2 years old have been retrapped here as yet.  This is consistent with the observation that this is the main pre-breeding herd, with young birds leaving as they pair up and move off to find breeding territories.  It is believed that post-breeders may also ‘retire’ here, in which case continued ringing may eventually retrap some of these older birds, assuming that they have remained within the study area.

Collation of Past Data

Counts for the whole of the Lincoln waterways, i.e. Brayford Pool, the River Witham and Sincil Drain within the city limits and the drains joining them, are available for 8 of the 12 years 1995-2006.  Over this period the average monthly total has been 126, peaking in July and August at more than 150 birds and only 3 of the 96 counts are below 100.  The highest count over this period was of 210 in August 2002 and the average count for this year was 155 birds, but average numbers have steadily declined in every year since that time, to 102 in 2006.  This will have been exacerbated by the large-scale swan kill in 2004, when more than 100 birds died from an unexplained cause (believed to have been poisoning as a result of a pollution incident).

For some years, separate monthly counts are also available for Brayford Pool between April and September, and for these years on average half of the birds on the Lincoln waters, 63 of 130, were on the Brayford, with the rest scattered throughout the system.  In 2009, for comparison, numbers on the Brayford Pool averaged <50 birds, with the total for the system <80 in all months, a significant and continuing reduction.

Of the Lincoln waters counts are available for Boultham Mere in 6 years, Burton Pits in 10, Hartsholme Park in 6, Swanholme Lakes in 8, and Whisby NP in 9 years.  All of these counts confirm that over this time, as now, each separate water body has held a single pair, with breeding success variable but surviving cygnet numbers generally similar to those found in this study so far.  The pattern for North Hykeham Apex Pits is particularly interesting though.  Numbers there have been consistently lowest in mid-summer, consistent with no breeding in most years (if not all) but then building up, in some years only, to a September-October peak.  Peak autumn counts for 1999 and 2000 were 84 and 44 respectively, but the highest in any other year for which counts are available was 11. 

Counts for the River Witham and the fens downstream of Lincoln are more patchy, but tend to indicate that the currently used areas have held similar-sized wintering herds during the last 10 years at least, peaking in March.  One stretch of the River Witham and South Delph has also been counted in one full year and three further summers over this period, with counts similar to those made over this stretch in 2009.

Taken together these results appear to indicate that Lincoln has lost between a third and a half of its pre-breeding population since 2002, but that this reduction is not yet obvious in the breeding areas.  This is clearly consistent with the fact that these are long-lived birds, and that there is a fixed number of suitable breeding territories.

One unwelcome finding of this first year was the number of swans, up to 6 annually, dying under wires in the study area.  Most recently, a whooper swan has also died under the main culprit wires in Nocton Fen.  This area was particularly dangerous in winter 2009/10 since the fields under the wires were all planted with oilseed rape, and were thus particularly attractive to the herd.  It is possible that the flags put in the field under the wires prevented more deaths by moving the birds away from this risk.  Deaths are always logged, and since a clear pattern has emerged, an approach was made to the transmission company to put warning deflectors on the culprit stretches.  This was carried out in spring 2010, and the results will be monitored in future winters.

Acknowledgements

Grateful thanks for help with the counts and ringing are due to: Catherine Collop, Debbie and Lee Conlon and family, Chris Dobson, Janet Eastmead, Russell Hayes, Grahame Hopwood, Roger Ingoldmels, Katie Milburn, Chris Parker, Andy Sims, Nick Tribe and John Watt, with apologies to anyone accidentally missed off the list.  Thanks also to landowners, farmers and keepers who have been helpful over access and provided extra information on the swans using their land.