Plovers eat the wind.

Turtle-doves eat juniper seeds and sweet-smelling herbs.

Larks eat only pebbles and sand.

From Le Ménagier De Paris (1393), cited in the Oddities section of the excellent Medieval Cookery website.

NOTE : Though let us not forget that, as Christopher Smart reminds us in Jubilate Agno, “For mustard is the proper food of birds”.