A wild goose census is being carried out at three Norfolk roosts this weekend amid fears of a sudden population decrease.

The results of a recent survey carried out by the Wildfowl and Wetlands Trust (WWT) has hinted the population of pink-footed geese may have dropped by around 100,000 in the last two years.

The majority of the world's pink-footed geese winter in the UK and will be arriving in big numbers at roosts in Snettisham, Holkham and Wells-next-the-sea this month from their breeding grounds in Iceland and Greenland.

Conservationists are heading to the Norfolk roosts on both Saturday and Sunday to count the birds to further assess the situation. A second count will also be carried out in November.

The population of the wild birds is still estimated to be at least 250,000 but the apparent drop of more than a quarter between 2009 and 2011 has led the WWT to believe it is an early warning of changing fortunes.

Richard Hearn, WWT's head of species monitoring, said: 'What has apparently happened to the pink-footed goose population in just two years goes to show how quickly fortunes may change, even for a bird whose numbers have been steadily increasing for decades.

'We are very fortunate in this country to have a large and dedicated network of volunteers who help us keep tabs on these birds.'

Mr Hearn said an indication of the possible fall in numbers is the proportion of young within the returning flocks which shows how successfully they have bred.

He said last winter the birds returned with only 8.5pc young – half the recent average – indicating that the 2011 breeding season was bad.

It is believed late snowfall in Iceland in the summer of 2011 may have forced geese to abandon their nests or it may have killed more goslings than usual.

Mr Hearn continued: 'When our surveys turn up something unexpected like this, we need to ask ourselves whether we think it is real or whether there's a glitch in our survey.

'We know the migration back from Iceland was late in 2011, so some birds may have been missed by the survey.

'However, two large declines in successive years suggest that numbers really have decreased significantly, even if it is not as great as the counts alone suggest.

'All this reiterates the importance of monitoring waterbird populations as frequently as possible and also highlights the value of additional data on breeding success as well as numbers.'

Anyone who would like to support waterbird conservation by volunteering their time and expertise to help with future monitoring should contact the WWT species monitoring unit monitoring@wwt.org.uk