Birds falling out of the sky in the United States and Sweden are freak examples of the kind of mass animal deaths, from beached whales to deluges of frogs, that have unusual but not apocalyptic causes, experts say.
Storms, hail or lightning can kill birds while tornadoes or waterspouts may suck up small fish or frogs and drop them far away. Human causes, such as fireworks, power lines or a collision with a truck, may explain avian deaths.
The U.N. Environment Program (UNEP) urged more research into baffling deaths -- ranging from why whales sometimes make the fatal mistake of swimming onto beaches to recent bird deaths, dubbed the "Aflockalypse" by one newspaper.
"Science is struggling to explain these things. These are examples of the surprises that nature can still bring," said Nick Nuttall, spokesman of Nairobi-based UNEP. "More research is needed."
Modern threats such as pollution or climate change may be adding to background stresses on wildlife.
About 500 dead birds were discovered in Louisiana this week and 5,000 in Arkansas at New Year, many of them red-winged blackbirds. Swedish authorities have been investigating the deaths of 100 jackdaws found in a street in Falkoping.
"We made an autopsy on five of the birds yesterday and found internal bleeding but no external lesions," said Marianne Elvander of Sweden's National Veterinary Institute.
She said there was no sign that they had died from diseases such as bird flu -- the main worry in such cases. Among the theories was that a truck had collided with the flock.
In Beebe, Arkansas, one theory is that fireworks spooked the birds to fly into buildings or other objects. Such birds roost in vast numbers, fly fast and have poor night eyesight, Nuttall said.
The widely publicized deaths meant other incidents received far more attention than they otherwise would have. "This is a classic example of freak events coinciding," said Petter Boeckman, a zoologist at the Norwegian Natural History Museum.
Storms, hail or lightning can kill birds while tornadoes or waterspouts may suck up small fish or frogs and drop them far away. Human causes, such as fireworks, power lines or a collision with a truck, may explain avian deaths.
The U.N. Environment Program (UNEP) urged more research into baffling deaths -- ranging from why whales sometimes make the fatal mistake of swimming onto beaches to recent bird deaths, dubbed the "Aflockalypse" by one newspaper.
"Science is struggling to explain these things. These are examples of the surprises that nature can still bring," said Nick Nuttall, spokesman of Nairobi-based UNEP. "More research is needed."
Modern threats such as pollution or climate change may be adding to background stresses on wildlife.
About 500 dead birds were discovered in Louisiana this week and 5,000 in Arkansas at New Year, many of them red-winged blackbirds. Swedish authorities have been investigating the deaths of 100 jackdaws found in a street in Falkoping.
"We made an autopsy on five of the birds yesterday and found internal bleeding but no external lesions," said Marianne Elvander of Sweden's National Veterinary Institute.
She said there was no sign that they had died from diseases such as bird flu -- the main worry in such cases. Among the theories was that a truck had collided with the flock.
In Beebe, Arkansas, one theory is that fireworks spooked the birds to fly into buildings or other objects. Such birds roost in vast numbers, fly fast and have poor night eyesight, Nuttall said.
The widely publicized deaths meant other incidents received far more attention than they otherwise would have. "This is a classic example of freak events coinciding," said Petter Boeckman, a zoologist at the Norwegian Natural History Museum.
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