A five-yearly international census of migratory swans is about to take place, amidst growing concern that climate change is influencing numbers and distributions.

In Ireland, around 250 people will survey a thousand sites this weekend for whooper and Bewick's swans.

According to the last census in 2010 the island of Ireland supports around 50% of the entire Icelandic-breeding whooper swan population, approximately 15,000 birds.

However, the number of Bewick's swans wintering here has dramatically declined over the past decade, with only 80 recorded in 2010 and fewer still expected to be recorded this year.

In Co Wexford, for example, only three Bewick's swans have been sighted so far this winter by staff from the National Parks and Wildlife Service.

It is thought many birds are staying put in Siberia because of warmer temperatures there.

"It is vital that we examine swan flocks very carefully so that we take account of those few Bewick's that do make the journey to our shores," said Helen Boland from BirdWatch Ireland.