Which bird builds the largest nest?

Bald eagles build the biggest nests of any single pair. They return to the same one year after year and add to it every season, so the structure simply accumulates. The record, in Florida, was nearly three metres across, six deep, and weighed over two tonnes - enough that nest trees sometimes come down under the weight.

The largest structure built by birds at all belongs to the sociable weaver of southern Africa, which is communal. Hundreds of birds build one enormous thatched mass in a tree or on a telegraph pole, with dozens of separate chambers underneath, and it can be used continuously for a century. Inside, the temperature stays remarkably steady while the desert outside swings between extremes.

The mound-builders take a third approach entirely. A male malleefowl rakes together a compost heap several metres wide, and the eggs are incubated by the heat of rotting vegetation rather than by a sitting bird. He tests the temperature with his beak and adds or removes material to hold it near 33 degrees for months.