As Bewick’s swans depart the UK for their annual migration to Arctic Russia, researchers at the Wildfowl & Wetlands Trust have been observing the size of their behinds.
With Bewick’s swan numbers falling sharply, scientists are hoping to rule out shortage of suitable food at their UK wintering sites as a reason for the decline.
Trained observers have been undertaking the research known as ‘abdominal profiling’. By recording the size of the area between their legs and tail where they store fat they have built up over the winter, the researchers can determine whether the swans have been able to find enough food to survive the 2,500 mile journey back to breed.
Julia Newth, the WWT researcher said: “We need to eliminate the possibility that the swans are suffering while they winter in the UK. In a slim bird the bum will look slightly concave, whereas a well-fed bird will have a double bulge.
“The analysis has yet to be bottomed out but our observations in the field certainly suggest they are leaving for the journey with big healthy behinds. We need to do further work to see whether their body conditions have changed over the years, and, if so, whether this is connected with the decline in numbers seen in recent years.
“We know so much about Bewick’s swan behaviour, having followed the lives of many individuals visiting Slimbridge since 1964. It is really critical that we learn why their numbers are decreasing.”
The numbers of Bewick’s swans wintering in Europe have declined sharply since 1995 – dropping about 27 percent from 29,000 to about 21,000 in 2005.
It’s likely that a number of factors are affecting the swans’ survival and breeding success. It is suspected that the swans are being affected by habitat and weather changes at their breeding grounds.
Other known causes of death include collisions with power lines, lead poisoning and illegal shooting.
Julia Newth continues: “Results from this year’s observations will be compared with data collected previously during the 80’s, 90’s and last winter. It will help rule out that the reduction in overwintering swans is due to changes in habitat at UK wintering sites – and give an indication of how the swans are responding to environmental change.
“Researchers will be able to see if the population’s ability to gain condition over winter and their feeding behaviour has changed in recent years. This in turn will really help in making conservation management decisions for the future.”
The swans have been studied primarily at the WWT reserve in Slimbridge, but data has also been collected from other UK sites.
Every five years ornithologists across Europe complete a co-ordinated swan census to determine the total number of birds in each swan population. The census findings help us to keep track of what’s happening and are used to guide our conservation work with each species.
Later this year, we will have the results from the most recent swan census, and hopefully more insight into the causes of the decline which will help us reverse the fortunes of the Bewick’s swan.