Unlike man, who has always dreamt of flying, some birds will eagerly shed this evolutionary advantage if it no longer suits them. To remain airborne it helps if you have hollow bones to keep the baggage limit down.
However, if you’re adapted to dive for fish, then having a denser structure helps you become less buoyant. Penguins use their flippers like wings, to guide and propel their torpedo-like bodies through the water to catch fish and evade predators – so they do fly in water, just not in air. Several families of birds have lost the power of flight, including many island forms. On small islands, being able to fly can be a disadvantage if you are likely to be blown out to sea by storms. Although regarded by some as flightless, the chicken and the jungle fowl – the chicken’s Asian ancestor – have limited abilities, which enables them to flutter away from predators.
Amy Lewis, communications officer for The Wildlife Trusts