Mass bird deaths rare - experts
Oslo - 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 UN Environment Programme 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, spokesperson of the 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.