Bird Reproductive Organs
Birds are a highly successful class of animals that have colonized every continent and evolved into the roughly ten thousand species we know today. Their advanced reproductive strategy is just one of the secrets to their success. Like us, birds reproduce by internal fertilization, although there are some interesting differences in their reproductive organs and strategies.
Seasonally developed reproductive organs and a strategy of laying fertilized eggs for external embryo development are key adaptations that allow birds to avoid carrying the weight of developed testes or multiple eggs, which would make flight difficult and put them at increased risk of predation.
There’s much more to learn about avian reproduction. Read along with us to learn more about the reproductive organs that make it all possible.
Male Reproductive Anatomy
Testes
Male birds have paired testes, which are responsible for sperm and sex hormone production. The testes are located near the kidneys and consist of various cell and tissue types, nerves, and blood vessels.
Since most birds have defined breeding seasons, males do not produce sperm throughout the year but rather begin production before each breeding season.
In some birds, the testes may become hundreds of times larger than during the non-breeding season, sometimes making up over three percent of their total body weight!
Vas Deferens and Cloacal Protuberance
Sperm produced in the testes is transferred to the female through a tube called the vas deferens/ ductus deferens and exits the body at the cloaca. During the breeding season, males may develop swollen structures called cloacal protuberances where sperm is stored near the vent.
Sperm Production
During the breeding season, male birds produce sperm within special cells of the testes called seminiferous tubules. Each sperm requires about two weeks to develop, and their shape varies greatly among different families and species.
Avian sperm are produced and stored within the relatively high temperatures of a bird’s body. After copulation, the transferred sperm can survive within sperm storage tubules in the female oviduct for several weeks or even months.
Male birds do not produce sperm throughout the year but rather begin production before each breeding season. A Sharp tailed Grouse dancing during mating display
Female Reproductive Anatomy
Ovaries
The avian ovary is responsible for ova storage and development. Most species have only one left ovary, while some birds, like Kiwis and hawks, have a second, non-functional ovary on the right. The loss or reduction of a second ovary reduces weight and energy expenditure for female birds.
Oviduct
The ova in the ovaries look nothing like the hard-shelled eggs we are familiar with. First, they must pass through an elongated structure called the oviduct, where a few vital processes occur. The oviduct includes the following sections:
- The infundibulum is the first section of the oviduct after the ovary. Sperm is stored, and ova are fertilized here.
- In the magnum, the fertilized egg is covered in albumen, which provides nutrients, water, and protection to the embryo.
- The isthmus is the part of the oviduct where the egg is covered in membranes.
- In the uterus, the membrane-covered egg receives a hard, calcareous shell and any coloring or patterns typical of the species.
- The vagina is the last section of the oviduct, where the egg is pushed into the cloaca before being laid.
Egg-laying
Birds lay eggs by contracting the muscular uterus and vagina and partially everting the inner walls of the vent. The process may take anything from a few minutes to mere seconds for some species, although it is understandably difficult to observe in wild birds!
A Hen's reproductive system showing the ovary and the various sections of the oviduct. Most species have only one left ovary, while some birds, like Kiwis and hawks, have a second, non-functional ovary on the right
Reproductive Strategies and Organ Function
Monogamy vs. Polygamy
Birds have evolved various mating strategies, including seasonal monogamy, long-term monogamy, polygyny, and polyandry. These mating strategies have some impact on the development and function of their reproductive organs.
For example, monogamous male birds have smaller testes than polygamous species since there is less competition. This benefits them since large testes increase body mass and require more energy for sperm production.
There are also some interesting adaptations seen in female birds. Several males may attempt to mate with a female duck even though she has selected a preferred partner. These birds have evolved ‘dead ends’ in their reproductive tracts to block sperm and prevent fertilization by unwanted males.
Seasonal Breeders vs. Year-round Breeders
Most birds breed at specific times of the year when their chances of successful reproduction are greatest. Food availability is vital since physiological processes within the body, courtship displays, nest building, and chick provisioning are energy-intensive activities. This strategy allows birds to ‘shrink’ their reproductive organs in the non-breeding season and focus on their own survival.
Some birds produce several broods of chicks in succession, and some species (e.g., Pigeons) may breed at just about any time of the year. In suitable climates, Pigeons can breed for longer than other birds because they produce crop/pigeon milk and, therefore, do not rely on warm-season invertebrates to feed their young.
Physiological Regulation of Reproductive Organs
Photoperiod Influence
Day length varies with the seasons, becoming longer as winter becomes spring and then summer, but decreases into fall and then winter. These changes stimulate the avian biological clock, prompting the release of hormones that signal the development of reproductive organs and behaviors.
Hormonal Control
Important hormones like luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are involved in regulating and developing the avian reproductive organs.
These hormones stimulate the growth of reproductive organs, which produce sex hormones like testosterone (primary male sex hormone) and estrogen (important female sex hormone).
Some birds produce several broods of chicks in succession, and some species (e.g., Pigeons) may breed at just about any time of the year. Common Wood Pigeon with chick
Development and Maturation
Juvenile Development
Gonad differentiation, and therefore sex determination, can begin within about a week of egg laying in chickens. At that age, structures associated with the testes of male birds begin to develop.
Female chicks hatch out of the egg, complete with all the ova they will require as adults. In fact, they typically have more than they will need since they cannot produce new ova as adults.
Age of Reproductive Maturity
Like humans, birds must go through puberty to reach sexual maturity. In songbirds, this typically occurs after less than a year so that yearlings have the potential to reproduce in the spring following their own hatching. However, long-lived species (e.g., Albatrosses) may require several years of growth before they reach sexual maturity.
Changing day length stimulates the development of reproductive organs in songbirds that breed after just one year, but other factors, like the presence of dominant sexually mature individuals in the population, may delay puberty in birds that reach sexual maturity after longer periods.
Health and Disorders of Reproductive Organs
Common Disorders
Birds are susceptible to various reproductive disorders, including the following common ailments:
- Obstruction of the oviduct or egg binding is a common condition where female birds are unable to expel eggs from their oviducts. Affected birds can be helped by trained individuals using medications and physical assistance.
- Female birds can develop neoplasia and cancer of the oviduct and ovary, which can spread to adjacent tissues.
- The oviducts are also vulnerable to rupture and various infections, such as the bacterium Escherichia. coli.
- Male birds may be susceptible to inflammation and infections caused by Salmonella bacteria.
Impact of Nutrition and Environment
Birds rely on a healthy diet to maintain all the physiological systems within their bodies, including the reproductive organs. Calcium and vitamin A deficiencies, for example, can result in reduced reproductive ability and poorly developed eggs.
Industrial and agricultural chemicals like pesticides, hydrocarbons, and heavy metals can have major impacts on avian reproductive health.
These environmental pollutants may affect adult bird physiology, fertility, and reproductive success. Embryo development can also be affected, and sex differentiation may be disrupted due to exposure to estrogen-like chemicals.
Long-lived species (e.g. Albatrosses) may require several years of growth before they reach sexual maturity. Juvenile Wandering Albatross
Summary
A bird’s reproductive organs are perfectly evolved for the task of continuing its species. These specialized structures show several adaptations and specializations for the avian reproductive strategy, including the remarkable ability to reduce the size and function of organs during non-breeding seasons to reduce weight and increase the chance of survival during migration and harsh winters.
Understanding bird reproduction is very important for birds and for ourselves. It allows better poultry management, effective treatment of companion and exotic birds, and the protection of wild birds, both in free-living populations and critical breeding programs for highly threatened species.