Why do some birds' heads not rock forwards and backwards when they walk?

The long-running series in which readers answer other readers' questions on subjects ranging from trivial flights of fancy to profound scientific and philosophical conceptsWhen most birds walk, their heads snap backwards and forwards - pigeons being probably the most extreme example. Ducks' heads don't seem to do this. Why do ducks not and those others do? And which other birds' heads don't move as they walk? David Cockayne, CheshirePost your answers (and new questions) below or send them to nq@theguardian.com. A selection will be published on Sunday. Continue reading...