RSPB Scotland said it believed flu was affecting greylag and pink-footed geese at its Highland reserve.
The conservation charity said the main signs of infection in birds included a swollen head, lethargy, loss of balance and watery eyes.
It said people should not go near dead or unwell birds at Insh Marshes, or at any other sites, and footwear should be cleaned thoroughly between visits to the reserve.
If no staff were available, it said visitors could report suspected cases to Defra on 03459 335577 or online, external.
Insh Marches’ senior site manager, Karen Birkby, said: “Bird flu is not limited to one reserve or one area, and this good practice applies in any place you visit this winter where wild birds gather.
“By following the guidance, everyone can help minimise the danger to dogs and humans and help to support wildlife through this winter.”
RSPB Scotland said more than 13,000 barnacle geese were killed across the country during the most recent large scale outbreak in 2021-22.
Populations have since bounced back but there has been concern that numbers are rising again.