For writer, gardener and TV presenter Monty Don, swallows are as central to his garden as any plant. Their return to the garden in April brings a soaring familiarity of song, which when they depart in September leaves the skies above silent and empty, and for Monty a feeling of loss and longing for their return after the long winter months. Monty Don takes over the Tweet of the Day output this week with a selection of seasonally relevant episodes by Sir David Attenborough. Producer Andrew Dawes