The term ‘bird’s nest’ covers many different structures – from a shallow depression in the ground or makeshift pile of vegetation to bowls, domes and complex dwellings with shelves and entrance tunnels.
There are over 9,000 documented species of birds and without exception they all lay eggs. Unlike mammals, birds need maximum mobility and can’t afford the weight penalty of carrying live young inside them.
Playing hard to get
In many species, the male bird uses nest building to signal their appeal to a prospective mate. The wren can build as many as twelve nests before the female makes it clear she’s happy with the result.
In good shape
The ovoid shape of a bird’s egg is no accident. Its convex surface resists extreme pressure, like an arched bridge. At the upper end of the scale it takes a hefty 50kg pressure to smash an ostrich egg.
The big hatch
Most birds start incubating once a complete clutch has been laid: that way the eggs will all hatch together. Incubation time for blue tits is around 14 days, the same as robins. Wrens take a couple of days more, while house sparrows range from 11 to 14 days.