HUNDREDS of dead birds found washed ashore at Middleton Beach over the weekend were exhausted short-tailed shearwaters, which fly thousands of kilometres to breed across southern Australia.

More than 20 million shearwaters, also known as mutton birds, make the journey to southern Australia, including South Australia, from the north Pacific each year but many run out of reserves during the journey and die - often washing up on islands near Ceduna, or along the South Coast.

Department of Environment, Water and Natural Resources marine scientist Brad Page said the birds arrived within a day or two of each other.

"The islands off Ceduna are the hot spots for shearwaters in SA," he said.

"They travel enormous distances every year and unless they get enough to eat on the journey, they can easily die from starvation.

"Any food shortage along the way can have fatal effects, and birds often make it back to southern Australia with almost no energy reserves.

"It's not uncommon to see several thousand dead shearwaters washed up together on beaches near Ceduna and even as far along the coast as Victor Harbor during the migration time."

Between departing southern Australia in autumn and returning in spring, each bird travels more than 50,000km to reach feeding grounds off Alaska, after taking a detour to feed in the Antarctic on the way.

On Sunday, Magill resident Robert Kelly, who was visiting the popular beach town, said the birds spanned more than a kilometre of the beach from Chapman Rd, near where the Surf and Sun surf school runs.

'We headed to the beach for a quick walk at around 10am and as we walked along, we were dodging dead birds for about a kilometre," he said.

Councillor Alan Oliver, of the Port Elliot-Middleton ward for Alexandrina Council, said the problem of dead birds was not isolated to Middleton but had been a problem along many south coast beaches over the past week.

Shearwaters are usually dark brown to grey in colour, weigh about 500g and can have a wingspan of up to a metre.

They are the most common sea bird found in Australian waters, living for more than 30 years and nesting in burrows on offshore islands.

The pairs return to the same islands each year, raising a single chick each season.

Mr Page said the birds were highly synchronised in all their activities.

"One of the amazing things about shearwaters is that the flocks are so well coordinated,"

he said.

"They arrive at the same time, nest at the same time and leave at the same time."

He said about 10 million shearwaters would lay eggs in the first week of December, and the chicks would then make their first journey to Alaska in April.

"The mortality rate is very high for young shearwaters on their migration, with up to half of the birds not making the journey back."

Anyone finding a shearwater on the beach should leave the bird alone to give it the best chance of rejoining its flock.