Hormonal Control of Reproduction in Birds
Every bird you see is the product of a complex and delicate reproductive process involving coordinated chemical secretions, dashing displays, and tender care and nurturing. Hormones produced by the avian endocrine system are the unsung heroes of bird breeding, responsible for regulating every step along the way!
Released into the bloodstream in a specific sequence and at varying levels, these crucial proteins and steroids cause targeted changes in avian reproductive organs and trigger important breeding behaviors. You’ve probably heard of well-known hormones like estrogen and testosterone, but many more are involved in successful bird reproduction.
In this guide, we’ll be introducing some of these crucial hormones and outlining their role during courtship, nesting, and parental care.
Overview of Reproductive Hormones in Birds
Gonadotropin-Releasing Hormone (GnRH)
Gonadotropin-releasing Hormone, or GnRH for short, is the master hormone that triggers the avian breeding cycle. This hormone is produced in the brain’s hypothalamus in response to environmental cues like increasing day length or rainfall events. The hypothalamus also produces gonadotropin-inhibiting hormone (GnIH) to reverse these effects after the breeding season ends.
Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH)
GnRH stimulates the production of another two hormones called follicle-stimulating hormone (FSH) and luteinizing hormone (LH), and both are produced by the anterior pituitary, another endocrine gland in the brain. FSH and LH stimulate gamete (sex cell) production or maturation in the gonads and the production of other hormones like testosterone.
Estrogens, Androgens, and Progestogens
Birds produce various other hormones responsible for reproduction and secondary sexual characteristics. Androgens like testosterone are primarily male hormones, which trigger sperm production and aggressive behaviors typical of the breeding season.
Estrogen and progesterone are female steroid hormones produced in the ovary due to the release of pituitary hormones. Estrogen triggers the development of the female oviduct, where ova are fertilized and formed into hard-shelled eggs, while progesterone triggers ovulation into the oviduct and fertilization within its first section.
Great-horned Owl with Owlets. Birds with slow-maturing young, like Great-horned Owls and Emperor Penguins, may begin breeding in the winter so that their young gain independence in the warmer months of spring and summer
The Hormonal Cycle and Breeding Readiness
Pre-breeding Season Changes
Birds undergo some significant changes before breeding, prompted by the release of various hormones. These changes occur both internally and in some pretty conspicuous ways.
Many male birds develop colorful and elaborate breeding plumage, and some male ducks even grow temporary external genitalia. Females exhibit some pretty dramatic changes, too.
A study on Zebra Finches showed that the oviduct may weigh over 14 times as much before egg laying than in the non-breeding season, while in other species, the oviduct becomes over 200 hundred times heavier in the breeding season than the rest of the year!
Synchronization with Environmental Cues
Birds synchronize these physical and behavioral changes with the onset of the breeding season, which occurs at the time of year that’s most favorable for nesting and rearing young. This usually coincides with spring and summer, when plants come out of winter dormancy and insects are most abundant.
Some species time their breeding cycle a little differently. Birds with slow-maturing young, like Great-horned Owls and Emperor Penguins, may begin breeding in the winter so that their young gain independence in the warmer months of spring and summer.
Hormonal Influence on Breeding Behaviors
Courtship and Mating
Hormones don’t only trigger physiological changes. Breeding requires many behaviors that birds just don’t perform during the rest of the year.
Courtship displays, for example, are vital for mate selection since birds want to know they are mating with a compatible and genetically fit individual. Hormonal cues also stimulate copulation and pair bonding for improved reproductive success.
Nesting and Territoriality
Birds become increasingly territorial in the breeding season due to testosterone production, and this has some important benefits. Defending a quality territory with rich food sources and safe nesting sites from others shows female birds that a male is dominant and fit. Maintaining a territory also limits competition for nest sites, building materials, and food sources while preventing infidelity.
Most birds exhibit advanced parental care, which starts with building a nest. A variety of hormones stimulate nesting, including prolactin, estradiol, and follicle-stimulating hormone (FSH).
One or both partners may be involved in nest building, although there’s a lot of variation in nesting behaviors. Female Hummingbirds, for example, will build a nest and rear their young without any assistance from a male. In contrast, male African Weaver birds construct the nest alone, and the quality of the build plays a big role in attracting a mate.
Female Anna's Hummingbird at nest with her young. Female Hummingbirds will build a nest and rear their young without any assistance from a male
Egg-production and Incubation
Hormonal Regulation of Ovulation
Female birds that lay large clutches may have several active follicles (hormone-secreting structures that contain an ovum) in the ovary, each in line to replace the last one after ovulation.
Follicles maturing in the ovary vary the type and amount of hormones they secrete. As they near maturity, follicles switch from estrogen to progesterone production. Finally, an increase in luteinizing hormone and testosterone triggers ovulation.
Broodiness
Broody birds stop laying eggs and begin the process of incubation. This behavior is triggered by the production of prolactin, an anterior pituitary gland hormone. Brooding also coincides with a decrease in hormones like LH, FSH, testosterone, progesterone, and estradiol.
The Role of Hormones in Parental Care
Parental instincts
Raising young is a long and challenging process for birds, involving nesting, egg-laying, incubation, brooding, and rearing young. The motivation to provide such care is stimulated by hormonal changes in both males and females.
Prolactin is the primary hormone for stimulating parental care instincts in birds, although LH, FSH, and mesotocin (comparable to mammalian oxytocin) are also important. Prolactin and mesotocin levels rise during the breeding season and reach a peak after egg laying.
Male vs. Female Hormonal Roles
Although there are exceptions, most bird species share parental duties. Interestingly, males may follow cues from hormonal changes in females without the same changes in their own endocrine systems. However, there is a decline in male testosterone levels after egg-laying.
Raising young is a long and challenging process for birds, involving nesting, egg-laying, incubation, brooding, and rearing young. Female Blue tit feeding her young
Hormonal Adaptations to Breeding Challenges
Stress Response
Birds produce hormones called glucocorticoids in the adrenal glands, which are released during times of stress. These stress hormones trigger a survival response in birds, where self-preservation is prioritized over reproducing. Stress has been shown to reduce reproductive success, at least in domestic bird species.
Multi-brooded Species
Birds undergo some pretty dramatic hormonal changes over the course of their breeding cycle, which may last just a few weeks in some species. However, some birds produce multiple broods each year, requiring cyclical changes in hormone production.
Male House Sparrows produce high testosterone levels at the start of the breeding season but then decrease these hormone levels after eggs are laid and while chicks are reared. Testosterone levels then rise before the next brood, a process which may be repeated several times in succession.
Male House Sparrows produce high testosterone levels at the start of the breeding season but then decrease these hormone levels after eggs are laid and while chicks are reared. House Sparrow perching on a tree stump
Summary
Successful reproduction requires a complex range of behaviors and physiological processes coordinated by various hormones of the avian endocrine system. Male and female reproductive hormones differ in type and quantity during each phase of the reproductive cycle since the sexes usually have very different biological and behavioral roles to play.
This carefully coordinated chemical sequence is tricky enough, but stress and environmental disturbances make reproduction even more challenging. So, next time you see a singing songbird advertising its territory, or a devoted parent carrying a beakful of insects back to the nest, take a moment to appreciate the complex hormonal processes that motivate these vital behaviors.