Anxiety and fear gripped Gurgaon after 47 migratory birds were found dead in Sultanpur National Park on Saturday.
None of the birds exhibited any apparent sign of avian influenza.
The sanctuary, about fiften km from Gurgaon, has been temporarily shut for the public.
The carcasses of the birds have been sent to the National Institute of High Security Animal Diseases in Bhopal for testing, The Times of India reported.
All of the birds were found dead at the same spot. The dead birds included Eurasian coots (36), common moorhen (9), spot-billed duck (1) and jungle babbler (1).
The exact reasons behind the birds' death are yet to be known. Initial reports from authorities showed that the birds were killed after eating crops sprayed with pesticides.
"Prima facie, the deaths seem to have happened after the birds ate pesticide-infested crops at a nearby field. They move in flocks, which is why the entire group of birds died at once. In case of bird flu, birds develop red spots in their legs. None of the birds that died had these spots," District forest officer (wildlife) K S Khetkar , told TOI.
The sanctuary, which is located about 35 km away from Indira Gandhi International Airport, New Delhi, is home to more than 250 species of birds. During winter, more than 100 species of migratory birds, including Siberian Cranes and Greater Flamingo, reach the sanctuary.
The incident follows death of 37 ducks and 10 crows in a village in Fazilka district of Punjab on Friday. Swab samples have been sent for testing.
The country has witnessed two episodes of bird flu outbreak in the past two months. Kerala culled about 2.76 lakh ducks in November to contain the H5N1 outbreak which killed about 20,000 ducks in Kuttanad.
Similarly, authorities had to cull more than hundred water birds after one of the six ducks that died at the Sukhna Lake in Chandigarh, tested positive for the H5N1 virus in December.