The birds in the film aren't actually Tawny Pipits, they are Meadow Pipits. Tawny Pipits are very rare in the UK (even more so in wartime when this film was made) and it wasn't possible to find any to film. The rarity of the Tawny Pipit is a major thread to the story. It was decided to photograph a pair of ordinary meadow pipits and keep to shots which showed the back view only; the tawny has a plain breast and the meadow a speckled one, but their back plumage is very similar.