Isidore of Seville [7th century CE] (Etymologies, Book 12, 7.54): The mergus has its name from its continuous diving (mergere). Often, with its head lowered under the waves into the depths it gathers signs of winds and, foreseeing a storm at sea, turns noisily to shore. There must be a very strong storm at sea when the mergi flee to the shore. - [Barney, Lewis, et. al. translation]

Saint Ambrose [4th century CE] (Hexameron, Book 5, 13.43): I shall not overlook the diving gulls. They have acquired that name from their frequent diving operations. They are always able by their diving to gather signs of the approach of a wind storm. When they see a threatening tempest, they quickly 'fly back from mid-ocean' and withdraw 'while their screams rebound on the shore to safety ! What shall I say of the waterfowl a bird that finds delight in the depths of the sea? Soon he sports in the shallows, after taking refuge from the sea's upheaval which he foresaw. - [Savage translation, 1961]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Birds 5.90): As Isidore says, it is a diving bird, swimming in rivers and ponds. It hunts fish. It is so called from 'immersing' [mergendo], because it immerses itself in water. However, it is cannot stay long under the water, because it recovers his breath in the upper air. Liber rerum: As soon as the eggs are hatched, the young of these are so vigorous that if the mother should happen to destroy them, they live by their own strength. Severus Sulpitius: There is a certain species of them which are called 'horned' [cornutos] from the fact that they have red feathers on their heads like horns. Contrary to all kinds of birds, they have their feet near the tail, so that when they stand on the ground, they are raised like a man by the gesture of its chest. By divine power the blessed Martinus, bishop of Turones, compelled these birds to abandon the waters contrary to their nature, and to stay in the dry and desert places. Of these Ambrose relates that, sinking into the waves, they collect the signs of the winds; foreseeing the approaching storm, they fly back from middle of the water, and with a cry struggle for the safety of the shores. The mergus, when it is attacked, sinks into the water; and it signifies those who excuse themselves when they are accused in confession. As the Experimentator says, they are fatter in the winter because of their lack of movement. For every animal delights in the clear air, and wanders in it more than in a turbulent one. - [Badke translation/paraphrase]

Albertus Magnus [ca. 1200-1280 CE] (De animalibus, Book 23, 82): Mergus is not so much a species as a genus of birds that has many species. However, the birds that are most often referred to as divers are varicolored like magpies but have the body shape of ducks, i.e., size, beak and feet. They capture their aquatic prey by taking a deep breath of air and diving under the water. Obviously their ability to stay below water is limited by the length of time they can hold their breath. The chicks of this bird are so robust they can feed themselves without their mother’s aid as soon as they hatch from the shell. According to popular belief, at the time of an approaching storm these birds leave the calm waters of the sea and betake themselves to the safety of the shore, thus forecasting the onset of bad weather. - [Scanlan]