The nests of most songbirds are small and hidden, made by one bird or a pair, and function only to hold, hide, and protect one clutch of eggs and young. Those of the sociable weaversof the Namib and Kalahari deserts of southern Africa do much more. They are the world’s largest and most populated tree houses, weighing up to a ton or more and ranging up to 20 feet wide and 10 feet tall. One of these communal homes contains a hundred or more nesting chambers, or apartments. They’re refurbished and reused, with residents adding new ones over successive generations, often for more than a century. Each generation inherits, builds on, and profits from the environment created by its predecessors.