The number of migratory birds visiting Bhitarkanika national park in Orissa's Kendrapara district has dropped by around 60 per cent this season.According to officials, in 2009, 1.20 lakh winged visitors had thronged the famous Ramsar wetland site but the figure dropped to 45,610 this year.Rajnagar Mangrove Forest Officer Manoj Kumar Mahapatra said, it is the unstable and erratic weather condition that has led to the drop in numbers. Formation of low pressure and depression on more than two occasions might have distracted the birds in their miles-long flight to Bhitarkanika.According to the survey, birds like the highly-endangered Indian Skimmer skipped their temporary sojourn to Bhitarkanikathis year.Forest officials said the drop in birds' census figure is a matter of concern and experts are ascertaining the exact cause.Several species, who trans-migrated from Siberia, Ladakh, Lake Mansarovar and the Himalayan region, were found to be fatigued after their long flight.Prominent species, who have made Bhitarkanika their winter home are brahmin duck, bar-headed geese, godwin, pintail, painted stork, seagulls, common teal, tawny eagle, osprey.