The birds were buffeted by unusually rough weather, wildlife groups and experts told reporters Thursday.

"It's a major disaster," Maria Ganoti of the wildlife protection group Anima told AFP, in a story published by The Guardian.

Ganoti said the birds had been found injured or dead in the last three days. They have been found on the streets and balconies of Athens, in the country's north, on islands in the Aegean and around a lake in the Peloponnese, AFP reported. The Hellenic Ornithological Society posted images of the carnage on social media.

The group wrote that the night of April fifth to sixth was especially perilous for the birds. A combination of cold weather, strong winds and few insects exhausted birds that had already crossed the Sahara and the Mediterranean. South-blowing winds pushed the birds to North Africa, where they then faced the north-blowing winds of the Aegean and flew towards eastern Greece for rest. The group described what happened next:

On April 6th, the morning light revealed a shocking sight: tens of thousands of birds, chilly and exhausted, were unable to move. Swallows and martins had to rest on the ground, on the blacktop, on balconies, on rooftops and any other surface available. Swifts –a purely airborne species that can not rest on the ground or on a wire– found a temporary shelter by grabbing a wall or by hiding in scuppers or air ducts. Flycatchers filled parks and gardens in Athens and other cities. All this happened throughout Eastern Greece, from southernmost Crete to northernmost Macedonia.

Greece is in the flight path of hundreds of thousands of migratory birds, AFP reported, and storms are a major danger for all migrating birds, according to the Royal Society for the Protection of Birds. Some deaths are sadly to be expected, and birds have evolved to weather harsh conditions to a certain degree, the Hellenic Ornithological Society explained.

"However, this specific weather combination was unprecedented and pushed entire populations to their limits," the group wrote. "If we take into account all the hardships that human activity brings about during their journey — habitat loss, desertification, illegal killing etc. — these losses could prove devastating."

They urged concerned bird lovers not to approach or feed the resting birds, to protect them from drivers and cats and to call a wildlife center if they encountered one who needed medical help.

Swallows, which were particularly impacted by the harsh weather, are Greece's most popular bird, according to Greek Reporter. Their arrival heralds spring. They also make an incredible journey.

"These birds fly up to 350 kilometers (217 miles) a day, with anticyclones favoring them on their journey as they lift them higher up in the sky," associate professor in the Department of Forestry and Natural Environment at Thessaloniki University Dimitris Bakaloudis explained to Greek Reporter.. "They spent the winter months in southern Sahara and as far as South Africa and start migrating in waves from early March on a route of more than 10,000 kilometers (6,213 miles)."