Variations on singing by corn buntings in the Western Isles is being studied by researchers led by Dr Rupert Marshall of Aberystwyth University.
He said the latest research suggests a fall in the number of dialects.
Around a reserve on North Uist there were up to five, while now there are two, he said. On South Uist, another dialect appears to have vanished.
Dr Marshall said of the South Uist case: "I looked at the fence post where I used to see one of the strongest singers there.
"I could see his droppings from the previous year and I knew he should have been there, but he wasn't and none of the other birds of that dialect were singing.
"It is like an old folk song but nobody knows it any more."
Numbers of corn buntings had a severe, long-term decline across Scotland in the 1970s to 2000s.
However, new research suggests numbers of the birds in Moray/Aberdeenshire may have stabilised.
A project recording the dialects of one of the UK's rarest birds has found some may have disappeared completely.