The starlings have been flying over the town centre during the early evening for a number of weeks.
Stuart Dobson, of Wiltshire Ornithological Society, said starlings gathered where there was "an extra few degrees of warmth".
Resident Kevin Grant said it was "more Hitchcock than Disney".
Mr Grant, who lives near Great Western Way, said: "Not one tile is un-pocked by this bird mess and every evening, about an hour before dusk, the first squadron explores and runs across.
"Then, within about a minute, every part of the sky is filled with these cutting and swirling birds.
"It's beautiful to watch although actually quite frightening."
Mr Dobson said that while "murmurations" of starlings were not a familiar site in Swindon they were common in some other cities.
"This could be a once in a lifetime, once in a decade, thing for people to see," he said.
"It will all be cleaned up when the birds return to Scandinavia in about a month's time.
"They gather in the evening because they're safer from predators in numbers.
"The flight is immensely well-co-ordinated. Each of those birds is keeping an eye on the newest five and then, in a flash of an instant, they come down to the ground to roost.
"The purpose of the roosting is for them to share warmth. Just to be a few degrees warmer than the surrounding area and to survive the night.
"I think this phenomena in Swindon this year is just because of one of those colder winters."
Ali Sari, of the newly-opened hand car wash on Corporation Street, said he was thankful for the birds' mess giving him extra business.
He said: "The last three months I couldn't pay my bill, but this month I [can] pay my bill because of the birds. Thank you for the birds."
The long cold winter is the most likely explanation for thousands of birds flocking over Swindon each evening, according to a bird expert.