Birds of prey were trained to return to the call of a whistle. These whistles were normally made of wood or bone, and very rarely do we find them in gold. Falconry was the sport of kings, and a familiar part of sporting life until relatively recent times. The fastest and most lethal birds were sought after like footballers are today, and exchanged hands for equivalent sums. The breeding and training of such birds was a deep science, understood and controlled from the moment the chick broke its shell. Frederick II von Hohenstaufen wrote a work on falconry in the 13th century that remains a classic to this day, with knowledge formed by his access to traditional Arab expertise on the subject, and his experience through continual practice. This Call is so delicate that it may have been made for a Lady of the Court, who practised the sport on equal terms with their menfolk.