Male birds do have testicles, which are the reproductive organs responsible for producing sperm and male hormones. The anatomy of the avian male reproductive system differs significantly from that of mammals. This difference is largely due to the unique physiological demands of flight and the bird’s high body temperature. The location and dynamic nature of these organs reflect avian evolution.
The Avian Male Reproductive System
Male birds possess a pair of testes, situated internally within the body cavity. These organs are typically small, oval, or bean-shaped, located close to the backbone and nestled against the kidneys. Unlike most mammals, birds lack an external scrotum, which results in a streamlined body shape.
The function of these internal testes is to produce sperm within convoluted seminiferous tubules. Sperm travels through a rudimentary epididymis and then into the vas deferens. This duct transports the sperm toward the cloaca, the single posterior opening used for waste elimination and reproduction.
The Massive Seasonal Size Changes
The size of the avian testicles is not constant; they are highly plastic organs. In seasonally breeding species, the testes undergo extreme changes in volume between the non-breeding and breeding seasons. During non-breeding winter months, the testicles atrophy, shrinking to a minimal size.
When spring arrives, the organs undergo dramatic enlargement in preparation for reproduction. This increase can be massive, expanding the testicle volume by 200 to 500 times their resting size, with some reports noting an increase as high as 1000-fold. This cyclical growth is triggered by increasing photoperiod, or day length, which cues the onset of the breeding season.
The change in day length stimulates deep brain photoreceptors, initiating the hypothalamic-pituitary-gonadal (HPG) axis. The hypothalamus releases gonadotropin-releasing hormone (GnRH), prompting the pituitary gland to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones drive the rapid growth of the testes and the production of testosterone.
Why Birds Keep Their Reproductive Organs Internal
The internal position of the testes is related to the bird’s high, constant core body temperature, often ranging from 40 to 41 degrees Celsius (104 to 106 degrees Fahrenheit). In most mammals, this high temperature would render sperm non-viable, requiring external cooling provided by a scrotum.
Avian sperm, however, is adapted to withstand and develop at this elevated body temperature. This adaptation eliminates the need for external cooling, allowing the testicles to remain within the body cavity. The internal location is also an adaptation for flight and aerodynamics.
Keeping the reproductive organs tucked away eliminates external structures like a scrotum, contributing to a lighter, more streamlined body profile. The immense seasonal size change also supports this design, ensuring that the organs are only at their largest and heaviest when necessary for the short breeding season. This minimizes non-essential mass and bulk during the rest of the year, benefiting energy expenditure during migration and daily flight.