More than 40,000 Svalbard barnacle geese arrive at RSPB Scotland's Mersehead reserve on the Solway Firth every year.
An opportunity has arisen to expand the reserve by 112 hectares (275 acres) to give them an "even bigger home".
RSPB Scotland needs to raise Â£285,000 by the end of October to buy the land.
It would allow two separate parts of the reserve, situated south of Dumfries, to be linked up.
In addition to the migrating geese, RSPB Scotland said the reserve was home to the only Scottish population of natterjack toads.
Other birds seen during the year include yellowhammers, linnets, lapwings, pintails, teals, widgeons, oystercatchers, golden plovers and curlews.
RSPB Scotland director Stuart Housden said the reserve was already an "outstanding place for wildlife" but there was now a chance to make it "even bigger and better".
"Throughout the year visitors to the reserve can experience the wonders of nature from the calls of skylarks in spring, and the tumbling display of breeding lapwings, to the iconic sight of the Svalbard barnacle geese arriving in huge numbers at this time of year," he said.
He said the site had been transformed since it became an RSPB Scotland reserve in 1994.
"We're immensely proud of what we have achieved here so far and any donations made to this appeal will help allow us to continue this transformational journey that began a generation ago," he said.
"Please do help us realise our vision of giving nature a far bigger, better home at Mersehead."
The new land would allow more access trails across the site and also help with reserve management, making the work more cost-effective.
One of Scotland's most important sites for migrating geese has started an urgent fundraising appeal to buy neighbouring land in order to expand.