London, Aug 24 (IANS): Can a bird's nest bring down a huge tree? For Africa's incredible social weaver birds, pulling down a tree is a child's game.

About the size of sparrows, these birds come together in colonies of as many as 500 to build enormous nests that weigh over 900 kg, are 20-feet-long, 13-feet-wide and seven-feet-thick.

"The structures are so big they can collapse the trees they are built on and so well-constructed they can last for a century," Gavin Leighton, a biologist at University of Miami, said.

This is how they weave monstrous nests.

Social weaver birds line the insides of the chambers with grass and feathers and, occasionally, cotton balls taken from fields.

One chamber provides three or four birds a warm place to rest as winter settles in.

Social weavers build entrances to their nests at the bottom, making them more inaccessible to predators, wired.com reported.

This positioning also helps keep water out.

Chambers are never connected - each has a single entrance.

"From an evolutionary perspective, living in such big groups boosts an individual's chances of not getting picked off by a predator," Leighton added.