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All vertebrates have a system of organs or glands, collectively called the endocrine system. Characterized by abundant blood vessels and the lack of ducts, endocrine organs secrete hormones -- chemical messengers -- into the blood stream. Because the hormones produced by endocrine glands must travel through the bloodstream, the endocrine system works much more slowly than the nervous system does.
External Factors, Internal Results
External stimuli -- including social factors, temperature, food availability and sunlight -- cause the endocrine system to release hormones. These hormones travel from an endocrine gland to a target cell, via the blood stream. These hormones affect many aspects of bird biology. For example, experimental results published in "Annals of the New York Academy of Science" have demonstrated that light levels affect the basal metabolic rate of chickens -- their metabolic rate drops when their light levels are reduced. Nevertheless, not all processes are susceptible to external stimuli: circadian rhythms appear to remain constant even when researchers manipulate the light levels and timing. Many hormones function as part of feedback loops, which have evolved to ensure homeostasis, and keep the organisms' chemical processes balanced. When these systems are disrupted, serious disease or death may result.
Endocrine Glands of the Brain
Endocrinologists often refer to the pituitary gland as the “master gland.” Located in the brain, this tiny organ produces a number of hormones that serve to start or stop the operation of other endocrine glands. The pituitary gland helps birds regulate reproductive events, such as yolk production and egg deposition, as well as growth. Additionally, the pituitary gland stimulates the thyroid gland, which has important roles in metabolism. The pineal gland -- sometimes called the pineal body -- produces melatonin. Melatonin is involved in the regulation of sleep cycles and behavior. The pancreas -- more specifically, small structures within the pancreas called the islets of Langerhans -- produce hormones that regulate blood sugar.
Thyroid and Parathyroid Glands
The thyroid gland is composed of two glands that lie on each side of a bird’s throat. The thyroid produces at least two hormones in birds: thyroxine is involved in carbohydrate metabolism, temperature control and growth rate, while triiodothyronine is important to the production of skin and feathers, and may control the molting process. Small glands located on the thyroid are called the parathyroid glands -- their primary functions involve regulating the level of calcium in the bird’s blood.
Below the Belt
Birds have two adrenal glands located on the anterior side of their kidneys. Adrenal glands have two parts -- the cortex and the medulla. Together, they help regulate the metabolism of fat and protein, as well as regulate blood pressure. Additionally, the adrenal cortex produces corticosterone, which is important for regulating bodily functions when birds are stressed or frightened. Gonads are the sex organs of birds: Males have testes, while females have ovaries. Both types of gonads produce the hormones oestrogen, testosterone and progesterone; however, the relative quantities produced differ between the sexes.
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