As the emperor of the Holy Roman Empire and king of both Sicily and Germany, Frederick II (1194–1250) was one of the most powerful figures of the High Middle Ages. He was a patron of the arts, a statesman, architect, scholar, poet and composer, as well as a speaker of Latin, Sicilian, German, French, Greek and Arabic. Alongside these, Frederick was also a scientist and animal enthusiast, particularly when it came to birds.
In the 1240s, Frederick II wrote an ornithological and falconry treatise entitled De arte venandi cum avibus, in which he referenced the work of earlier authors, whilst listing his own acute observations. In Book I of On the Art of Hunting with Birds, Frederick explored the localities, migration, flight formations, breeding, nesting and behaviour of all manner of birds. Then, he went into minute detail about avian anatomy.
He noted that the avian body is composed of cellular parts —that is, those made of similar elements such as bones, cartilage, nerves, blood vessels and fat—and organic elements (or organs) which were made from the different cellular parts. With their sensitive (or nerve-possessing) and insensitive organs, birds were able to perform those functions that were essential to their wellbeing, survival and existence. Each organ was made of material suitable to its function, even if, in some cases, this was to the detriment of other birds. For instance, Frederick told how predatory birds were built in a way and with the organs that allowed them to destroy non-rapacious species. It was simply the case that sometimes nature creates “one species for the annihilation of another” as “each species finds in another what is harmful to it.”
The emperor went on to describe the various organs in detail, both in terms of their structures and their purposes. His description of avian ears reads as follows,
The hearing apparatus lies within two apertures situated behind the eyes towards the back of the head. Two canals in the bone (twisted internally like a wine-press screw) are found, one on the right and the other on the left side. As a rule, they have externally no cartilage, commonly called the auricle, or lobe of the ear. This attachment is found only in eagle (or great horned) owls, the eared owls, and the long-eared (or screech) owl.
These birds have around the opening of the ear a membrane which, when drawn back, corresponds to the external lobe of the human ear. The circumference of the ears is in some birds clothed with a few feathers and in others with stiff hairs. The ears are used only for audition; from the sounds he hears, the bird decides whether they forbode good or evil.
It is not only the external parts of a bird which received Frederick’s thorough analysis. Indeed, he outlays the design and functions of the skeleton, trachea, oesophagus, lungs, stomach, liver, testes, ovaries and kidney, the latter of which he details thus,
Birds have two kidneys, one on the right, one on the left side. They lie close to the vertebrae under the iliac bones and extend towards the anus. The urine is excreted from the kidneys by way of the anus through the uriniferous tubules, which pass below and in close contact with them. As the urine is passed with the faeces, the bird does not require and does not, indeed, possess a urinary bladder.
Naturally, he also took time to elucidate the intricacies of the wings and feathers. He asserted that there are several joints and corresponding divisions of the wing, rather than a singular mass. Whether expanded or contracted, wings allow the bird to move in any direction it pleases, with each motion controlled by various muscles. Essential to this ability is the avian shoulder, the structure of which Frederick fastidiously outlines thus,
Three bones are interlocked and bound together; the furcula (arising from the top of the breast and bound to it for some distance by delicate cartilages, as well as by fine ligaments). The two furculae then branch out on either side of the neck and extend to the shoulders, girding the throat. At the point of each shoulder, they are connected by ligaments with two more bones that, in their turn, meet and are bound together at this point to form an angle. The coracoid, of which the terminal is angular but which in the centre is round, extends to the margin of the sternum, to which it is joined by means of cartilages and other connections. The third bone, the scapula, extends over the back and lies against the ribs near the dorsal vertebrae. Here it is bent, broad and thin, and has throughout no union by means of a ligament or other connection with any other bone at that part of the back.
Aside from mobility, the wings also protect the body from cold, though lengths and shapes vary amongst the different species. So too might the colour, size and shape of feathers. Even in the same species of bird, plumage could vary, and a bird might experience a change in its feather colours as it got older or during mating season. The same can be true of its organs and Frederick gives the example of a grey heron. During breeding and moulting seasons, the beak and feet turn red whilst the plumage also alters its colouration.
He also details how, regardless of season, the plumage and feet are kept healthy by the excretions from the oil gland which lies above the tail. From this gland, the bird squeezes out oil with its beak which it then conveys to its feathers, as well as its claws. This oil helps the bird to resist moisture—Frederick notes that “rain affects the oiled parts very little but runs off them completely and swiftly.”
Frederick II’s passion for all things avian was tied to his pursuit of falconry or “hawking” where birds of prey were used to hunt small game animals and other birds. In light of this, the other five chapters of De arte venandi cum avibus are aimed at perfecting the sport by learning best ways to capture, train and care for falcons, whilst using Book I’s knowledge of prey species’ habits.
To keep a falcon healthy and ready to hunt, diet was especially important. Frederick warned against feeding birds of prey aquatic animals such as eels and fish, as these are prone to make a bird heavy. Moreover, those birds that feed on fish usually end up with soft flesh and feathers, along with a poor blend of humours. Instead, the best food was meat from a healthy, middle-aged animal—this flesh yielding the best sustenance and being more easily digested. That said, the occasional meal of young animal meat acted as a good medicine to keep the bird’s stomach healthy.
Meat was best served fresh and as tender as possible, with the nerves, veins, arteries, cartilage, bones and fat removed. If there was no fresh meat available, the older flesh should be soaked in fresh warm water until it reached the temperature of a live animal. In the event that there was no meat at all, small pieces of fresh or cooked cheese would suffice. Another, and apparently more preferable, option was eggs and milk mixed and then poured back into the eggshell to be roasted over a charcoal fire.
It was crucial to give birds the right amount of food. Young falcons that were overfed could become weakened and exhausted. The stuffing of the stomach lessened their natural heat, resulting in poor nutrition, weakness of the limbs and hindered growth of the pinions. The underfed bird similarly suffered from a loss of natural warmth, and became thin, weak and out of shape, whilst its feathers turned ragged.
It was up to the falconer to determine precisely the right amount of food that their bird needed, as the levels of voracity and activity differed between creatures. Additionally, they needed to regulate the bird’s meals. If a falcon received a very large meal, the next meal should be delayed and decreased. Falconers could also determine whether or not food was completely digested by checking the bird’s breath, making sure they voided their mutes regularly and by examining the consistency of the excrement.
As Yannis Hadjinicolaou explains in his 2024 monograph on the art of medieval falconry, the sport held immense significance across various parts of life as it was intertwined with court culture, diplomacy and recreation. He argues that “the act of hunting with birds of prey encompassed more than a mere hunt; it was an artistic skill that had a profound aesthetic impact.” Hawking symbolised sovereignty and evoked ideas about the effective running of a state. Unlike other sports of the period, hawking was also open to men and women, albeit those of higher status and with a certain amount of wealth.
Frederick’s treatise was valued for centuries after its creation as an important textual and visual guide for those trying to master the art of falconry. It was copied numerous times, first in Latin, then in French, and there are around a dozen full or fragmentary manuscript versions that survive today. De arte venandi cum avibus was first fully translated into English by Casey A. Wood and F. Marjorie Fyfe in 1943.