How big was the world's largest bird's nest?

Answer 1

That depends on what you mean - sociable weavers live in colonies that build one structure, usually in a tree, within that structure are the nests of the individual pairs of birds. The whole structure can often weigh several tonnes. Very old eagle nests can also be vast as they can be in use for many tens of years, often by succeeding pairs in a territory. Each spring they might add to it so it grows and grows. How about mallee fowl? They bury their eggs in a nest that can weigh tonnes, but it is sand and debris in a heap.