It refers to a legend that the Ostrich eats iron:Huck wrote:
That's from the Schäufelein-deck ca. 1535. But it was a general motif, probably referring to a proverb.
In heraldry it is common to make the iron a horseshoe:
On a helm with a wreath Or and gules an ostrich argent, holding in its beak a horse shoe Or.
Motto: Ferré va Ferme
The horse shoes are a pun on the name Ferrar – since a farrier is one who shoes horses.
The crest shows an ostrich holding a horse shoe. Legend has it that the ostrich eats iron (in Shakespeare’s Henry VI Part II Jack Cade threatens to ‘make thee eat iron like an ostrich’), and in heraldry this is frequently in the form of a horse shoe.
The motto, a further pun on ‘Ferrar’ and on the horse shoes, is usually translated as ‘The shod horse goes surely’.
http://www.kershaw.org.uk/littlegidding ... indow.html
According to this medieval bestiary "A bird that can digest anything, even iron, but is careless of its eggs":
"During the 17th century, the ostrich became a popular symbol of gluttony and was often pictured eating iron":
There are extent images of the motif from the 12th century on, was particularly popular in French bestiaries.
Here is a marvellous ostrich egg goblet with a horseshoe eating ostrich:
"This piece is a typical object of the Kunst-und Wunderkammer (Art and Curiosities Chamber) because of the combination of natural material and noble gold work. Over the fragmented base is a moor leading a leashed ostrich carrying its own egg. Furthermore it carries a horseshoe in its beak, already in the 16th century considered a symbol for good luck. The motif of the iron eating ostrich comes from antiquity and symbolizes "strength through resistance because the bird has such strength that it can digest and nourish itself from stone and iron. The coral is appropriate because it supposedly heals "boiling of the intestines and the blood and protects against the evil eye and magic."
"Ostrich eggs, prized not only for their rarity, but also for their rich symbolism, were made into beautiful art objects. The exhibition includes three stunning examples of drinking vessels in the form of ostriches made by Elias Geyer in Leipzig, about 1589–95. The ostrich egg was a symbol of the Immaculate Conception and of the sol verus, the true sun, a metaphor for God. The Dresden ostriches hold horseshoes in their beaks, illustrating the belief that the giant birds were capable of digesting anything, even iron, as Pliny the Elder contended."
http://www.metmuseum.org/special/Prince ... n_more.htm