The Role of Hormones in Bird Behavior and Physiology

Have you ever wondered what drives birds to do what they do or how they maintain all the complex biological processes in their bodies? The secret is a system of chemical instructions and feedback mechanisms produced and coordinated by the endocrine system.

Glands located in various parts of the bird’s body produce numerous types of hormones, which are the chemical messengers that drive everyday behaviors like feeding and resting, but also highly seasonal and life-stage-specific behaviors like courtship, breeding, and dedicated parental care.

In this guide, we’ll explore the connection between hormones and bird behaviors to help you better understand this fascinating relationship!

Hormonal Regulation of Avian Behaviors

Territorial and Aggressive Behaviors

Testosterone is a hormone that drives aggressive and territorial behaviors. This hormone is made primarily in the testes, which are reproductive organs and glands found in male birds. However, heightened testosterone levels have some disadvantages during nesting and for general health and survival.

Some birds remain territorial throughout the year, but aggressive behaviors often spike in the early stages of the breeding season when birds are establishing nesting territories and aggressively defending them against intruders.

Mating and Courtship Rituals

Around the start of the breeding season, lengthening days trigger hormone production that stimulates gonad development in preparation for reproduction. However, birds must find a partner before they can breed, and this typically requires courtship rituals involving various displays, vocalizations, and often colorful seasonal plumage.

Testosterone is also involved in triggering song and courtship displays and has been shown to stimulate courtship behaviors when artificialy increased in wild birds.

Parental Behaviors

After finding a suitable partner and mating, birds’ priorities change to successfully rearing their offspring to maturity. Protective instincts and the drive to incubate eggs and feed their young are triggered by a hormone called prolactin.

Prolactin levels increase around the time of egg laying in female birds and remain high as long as eggs and chicks survive and rely on parental care. In males, prolactin increases as testosterone levels decrease.

Birds must find a partner before they can breed, and this typically requires courtship rituals involving various displays, vocalizations, and often colorful seasonal plumage. Pair of Crested Grebes

Hormones and Physiological Processes

Metabolic Regulation

Glucagon, which is produced in the pancreas, is considered the most important metabolic hormone, responsible for several roles, such as glucose generation and utilizing stored fat reserves.

Insulin is another pancreatic hormone involved in introducing and storing carbohydrates in muscles and organs like the brain and kidneys. Several other hormones are also involved in avian metabolism, including corticosterone, prolactin, and thyroid hormones.

Many birds show marked changes in energy use and storage across the seasons, and long-distance migrants are a notable example. These birds begin storing large fat reserves in anticipation of their strenuous journey and rely on fatty acid metabolism as their primary fuel source.

Stress Response

At times of acute or chronic stress, birds respond by releasing a hormone known as corticosterone from the adrenal glands. This stress hormone significantly alters a bird’s behavior to prioritize survival while suppressing non-essential physiological processes.

Long-term exposure to stress can be highly detrimental to a bird’s health, but interestingly, different individuals often respond to stress in different ways, depending on their personality.

Immune Function

Various hormones of the avian endocrine system are known to affect immune functioning, so seasonal fluctuation can have important effects on avian health. Molting, breeding, and migration are all taxing annual behaviors that coincide with hormonal changes.

Testosterone is an example of a hormone that may have an immune-suppressing effect, while at least in chickens, the pituitary hormones prolactin and growth hormone are thought to stimulate and maintain the immune system.

Hormones in Growth and Development

Growth Hormones

Growth hormone is responsible for the division of cells and resulting growth. This substance is produced in the anterior pituitary gland of adult birds, although developing embryos in the egg rely on growth hormones released from various body tissues.

Thyroid hormones also play an important role in bone growth and the differentiation of cells in a growing embryo or developing hatchling.

Feather Development

New feather development and molting typically happen on a cyclical basis, either after the breeding season or both before and after this crucial period. The thyroid gland and its hormones are important for feather growth and development in the follicle.

Other hormonal changes have also been described before and during new feather development. The stress hormone corticosterone is typically decreased during molting since elevated levels negatively impact on new feather growth.

New feather development and molting typically happen on a cyclical basis, either after the breeding season or both before and after this crucial period. American Goldfinch

Hormonal Cycles and Seasonal Changes

Molting

The endocrine system plays an important role in molting, which is the annual or biannual replacement of bird feathers. As new feathers form in the follicle, the old worn feather is pushed out, so molting and regrowing new feathers are part of the same process.

The partial molts of birds that develop seasonal breeding plumages are triggered by changing day length and coincide with reduced levels of corticosterone, a hormone produced by the adrenal glands. The prebasic molt, which occurs after breeding, coincides with a decrease in prolactin levels.

Migration

Bird migration is heavily influenced by the endocrine system, with various hormones regulating everything from preparatory fuel storage to initiating the journey and regulating metabolism along the way.

Thyroid hormones are thought to be involved in migration initiation and several related physiological and behavioral changes. These include migratory restlessness, fat storage, and muscle development. Other associated hormones include ghrelin, prolactin, and corticosterone.

Breeding Cycles

Most birds follow a predictable seasonal breeding cycle initiated by the endocrine system’s response to environmental cues like changing day length or temperatures. The primary hormones responsible for initiating and inhibiting reproductive behaviors are the gonadotropin-releasing and gonadotropin-inhibiting hormones produced in the hypothalamus.

Follicle-stimulating hormone (FSH) from the pituitary gland, triggers the production of sperm in male gonads and the development of eggs in the female ovary. A range of other hormones, including testosterone, progesterone, and estrogen, are involved in regulating the behaviors and physiological processes involved in the reproductive cycle.

Environmental Influences on Hormonal Regulation

Photoperiod and Climate

Birds time many of their essential life stages and behaviors (reproduction, migration, etc.) with favorable environmental conditions. Day length, or photoperiod, is the most important of these environmental influences as it accurately indicates seasonal progression and can trigger hormonal changes on a predictable cyclical basis.

Human Impact

Humans can impact the endocrine system of birds in various ways. Artificial light around urban and industrial areas can affect circadian and seasonal rhythms by altering melatonin production, while large-scale effects like climate change affect thermoregulation.

Pollutants and synthetic substances known as endocrine-disrupting chemicals (EDCs) released into the environment can also affect growth, survival, and reproduction.

Bird migration is heavily influenced by the endocrine system, with various hormones regulating everything from preparatory fuel storage to initiating the journey and regulating metabolism along the way. Flock of Bar-tailed Godwits

Summary

It’s pretty remarkable to think that chemical signals produced by the avian endocrine system can so effectively manage the complex processes and behaviors we see in birds. From migrating across the globe to finding a mate and raising a family, birds rely on a delicate balance of hormone production and feedback to inform their every move.

Studying the avian endocrine system is more than just academic. Developing a better understanding of this vital system may have important applications in conservation actions like captive breeding of endangered species, management of migratory birds, and monitoring the impacts of stress on bird health and welfare.