The Birdman cult involved an annual egg-collecting competition.

Evidence

The Birdman cult, as it has been called, focused on an annual competition to collect the first egg of the migrating sooty tern from a neighbouring islet. — Neil MacGregor, A History of the World in 100 Objects, 1091

Categorization

• corroborated: 2 independent passages support this claim — Neil MacGregor, A History of the World in 100 Objects, 1091