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@cuties-in-codices shares another interesting mirror (and another interesting manuscript). Below is the full folio from this particular 'edition' of this manuscript (Fol 88r):
The book is known as Le livre du roy Modus et de la royne Racio {The Book of King Modus and Queen Ratio}, and is usually attributed to a certain Henri de Ferrières. It is believed that the earliest manuscript was commissioned by the French king Charles V, also known as Charles the Wise, and produced in Paris in the late 1370s.
The book consists of two parts: Livre des deduis du roi Modus - which is essentially a long treatise on hunting, and includes both accounts of various animals (deer, bears, boar, and so on) and birds, including ways to hunt them; and the second, shorter part, called Le songe de Pestilence.
The hunting section apparently became very popular, and multiple copies were made for various patrons. Even today more than thirty manuscripts exist, and many more likely didn't make till now.
The most famous copy was commissioned by Philip the Good, Duke of Burgundy and made around 1455. It is now kept in the Royal Library of Belgium (KBR) and its illuminations (attributed to Master of Girart de Roussillon) are perhaps the best known of all the manuscripts.
Here is how the scene with the mirror-loving pheasant looks there:
The legend of birds being lured by mirrors is often traced back to Pliny, who wrote in his Natural History:
"Our eyes are so perfect a mirror that even a tiny pupil reflects the whole image of a man: this is why most birds that we hold in our hands try to peck at our eyes, because, seeing their image there, they are drawn to them as to the objects of their natural affection."
This illumination is from another manuscript, also in the BNF {BNF Français 12399 fol 90v}.
The treatise itself offers quite an elaborate explanation of why a pheasant can be reliably caught with this mirror trap:
"Here I shall tell you why the pheasant strikes at the mirror: pheasants are of such a nature that the male cannot tolerate in his company any other male pheasant; thus they clash with one another and rush at each other.
"The reasons are these: one is that, because of his beauty, he is envious of his likeness; the other is that a pheasant is never without a female, and for these reasons they do not at all like being in one another’s company.
And so he will not hesitate in the least to enter the cage if he sees his own form in the mirror, for he will strike at it very fiercely, believing he sees another pheasant; and thus the cage falls and he is caught.
And this is a certain and true thing."