6 - Fuel for the bird engine

A bird’s body needs a lot of food to function properly. That food provides energy for the bird’s activities and maintains its body temperature, which is several degrees higher than that of humans. Extra food is needed for laying eggs, feeding young, and building up a layer of fat for migration. The menu is varied. As you know, there are many different species of birds and they eat almost anything. There are typical plant and meat eaters. But there are also birds that eat everything. Most bird species live on animal food, taking advantage of the variety of insects available. In addition, many birds depend on fish. There are only a few bird species that use plants as a food source. Geese are true herbivores. Fruits and seeds certainly play an important role in the bird’s diet. This is because they are rich in sugars and fats. These fruits are easy to digest and are a great source of energy. Birds have a highly specialized digestive tract containing only a few enzymes. Because the digestion of each type of food requires different enzymes, most birds cannot digest food other than what they are accustomed to. In addition, their external structure, such as their beaks, is adapted to eating a certain type of food. The food of the young can differ greatly from that of the parents. Seed eaters feed their young mainly with insects. During the first days of life, they are fed only soft insects. These are easy to digest and, moreover, animal food provides ten times more energy than the same weight of plant material. In general, the energy requirement per gram of body weight increases as body weight decreases. Therefore, smaller birds need relatively more food. On colder days, for example, more food (energy) is needed to prevent a drop in body temperature. Migratory birds eat extra to build up reserves for long distances. Sometimes fat storage takes on extreme forms. In smaller birds, just before the start of migration, fat reserves can account for about fifty percent of total body weight. It has been calculated that a four-gram bird with 40% fat reserves can stay in the air for about 21 hours without eating. A bird weighing forty grams with forty percent fat reserves can fly for about thirty-seven hours. If we maintain an average speed of fifty kilometers per hour, the bird can cover nineteen hundred kilometers with such energy reserves. At twenty-five percent, this would be a thousand kilometers, and with fifty percent fat reserves, it would have been about three thousand kilometers. Stilts have been measured to fly distances of 1,500 to 2,500 kilometers without stopping to forage along the way.

In winter, the need for food increases because extra energy is needed to maintain body temperature. Greylag geese live on plants and eat more than twenty-five percent of their body weight per day. To consume this amount, they eat and digest throughout the day and defecate every two to three minutes. Wood pigeons that forage in fields in winter eat about forty-five grams of dry weight per day, which corresponds to about thirty-five thousand pecking movements. In the evening, their crop contains an average of fifty grams of food, which allows them to survive the long winter night thanks to this stored food. Carnivores also make an extra effort in winter. For example, Coal tits eat an average of two grams of dry weight every twenty-four hours. That amounts to one medium-sized insect every two and a half seconds. Small Goldcrests are also busy all day long looking for food. The bird weighs about five grams and needs about two grams of food. The larvae and insects are so light that collecting them is a full-time job. These small birds often die if their intestines are empty for a few hours. Their fat reserves are quickly depleted.

The need for water is directly proportional to body weight. The heavier the bird, the more water it needs. Some species never drink. They get enough moisture from their food. Larger birds need moisture regularly. Drinking is very simple: you take water into your beak, tilt your head back, and swallow. Pigeons can drink without standing up. They suck the water up. Swallows drink while flying by skimming the water with their open beaks. Birds can prevent dehydration by thickening their droppings. In addition, they do not perspire, which results in less fluid loss. However, this means that in hot weather they cannot cool their bodies through evaporation. This can be seen on very hot days, when birds sit with their beaks open, panting and trying to cool down. Seabirds face a different problem. Salt water acts like poison on their bodies. These birds have a special adaptation that allows them to excrete excess salt. This salt is removed via a special organ on the beak. It collects in a drop and is shaken off from time to time.