Are long-term trends in Bewick's Swan Cygnus columbianus bewickii numbers driven by changes in winter food resources?

The north-west European population of Bewick's Swan Cygnus columbianus bewickii declined by 38% between 1995 and 2010 and is listed as 'Endangered' on the European Red List of birds. Here, we combined information on food resources within the landscape with long-term data on swan numbers, habitat use, behaviour and two complementary measures of body condition, to examine whether changes in food type and availability have influenced the Bewick's Swan's use of their main wintering site in the UK, the Ouse Washes and surrounding fens. Maximum number of Bewick's Swans rose from 620 in winter 1958/59 to a high of 7,491 in winter 2004/05, before falling to 1,073 birds in winter 2013/14. Between winters 1958/59 and 2014/15 the Ouse Washes supported between 0.5 and 37.9 % of the total population wintering in north-west Europe (mean +/- 95 % CI = 18.1 +/- 2.4 %). Swans fed on agricultural crops, shifting from post-harvest remains of root crops (e.g. sugar beet and potatoes) in November and December to winter-sown cereals (e.g. wheat) in January and February. Inter-annual variation in the area cultivated for these crops did not result in changes in the peak numbers of swans occurring on the Ouse Washes. Behavioural and body condition data indicated that food supplies on the Ouse Washes and surrounding fens remain adequate to allow the birds to gain and maintain good body condition throughout winter with no increase in foraging effort. Our findings suggest that the recent decline in numbers of Bewick's Swans at this internationally important site was not linked to inadequate food resources.