If you have ever seen a bare patch on the bellies of birds then you may have wondered what this is for? Many think it is because of fighting or bad health, but in fact, it is all to do with the next generation of birds.
A brood patch is a bare patch of skin that can be found in the breeding season on the bellies of birds. The brood patch is used to warm up the eggs by covering them. This ensures that the eggs develop at the required temperature.
If you want to know more about this bald patch of feathers please read on.
How does the brood patch form?
The brood patch develops seasonally and can be found on the female, male, or both. This depends on which sex is responsible for the incubation. If it is the female, they will develop, the brood patch, and if it is the male they will develop the bald patch.
Hormones trigger different changes in the birds at the beginning of the bird season. These physiological changes can affect displays, behaviour, or calls to attract mates, growth of testes in the males for sperm productions, ovarian activity in females, and plumage alteration in both sexes.
As the plumage changes, feathers on patches of the belly may moult as the skin becomes vascularised with the increased blood supply. Some species will pluck their feathers to line their nests.
Why do birds have brood patches?
The brood patch is supplied with blood capillaries close to the surface. Because birds have a high body temperature of 38 to 40 degrees, the blood is perfect for the egg to keep warm and the embryo to develop.
Feathers are excellent at keeping in the heat and so the feathers need to be removed in order for them to be able to supply a constant body temperature to the egg. If the feathers were sitting between the bird and the egg, the embryo would not develop quickly enough.
The parents could not devote the length of time needed to grow an embryo at lower temperatures due to foraging and finding food. This could also lead to an increased risk of predation of the eggs.
Many species will have more than one brood patch but this generally depends on how many clutches of eggs they lay. As the incubation phase slows the feathers start to regrow.
Birds have developed to incubate and develop as quickly as possible, so a small patch of feathers come out until the hormones settle down again. The feathers will then start to grow again. The brood patch doesn’t last long because feathers are used for insulation and waterproofing.
Do all birds have a brood patch?
Because feathers are used for insulation and waterproofing some birds do not develop a brood patch, instead developing other ways to incubate their eggs.
Waterbirds such as cormorants, gannets, and darters need to keep themselves waterproof, warm and streamlined for when they dive into cold water. Instead of a brood patch, they keep their eggs warm in a different way.
All have fleshy, webbed feet which are filled with large blood supplies. They keep their eggs warm under their feet instead of their bellies so don’t need to lose feathers for a brood patch.
Others may push their eggs into the sunlight for periods of time to expose them to heat, although this can sometimes lead to the embryo not surviving.