Breeding Season Physiology of Birds

Successful reproduction is the driving force behind life on Earth, so the breeding season takes prime importance on the avian calendar. This is a crucial time of the year when birds undergo some incredible internal transformations and show off some fascinating behaviors, each evolved to ensure the survival of the next generation.

Timing is critical, of course, since most birds are pretty unlikely to succeed at raising their young in the dead of winter or during their annual migration! So, how do birds know when to reproduce?

Environmental cues like changing day length or the onset of the rainy season trigger a chain of reactions in birds. Hormonal changes translate into rapid development of the reproductive organs, but also visible plumage changes and many of the typical nesting behaviors we see each spring.

There’s much more to learn about bird breeding season physiology. Read along as we explore some of the remarkable processes and changes birds go through each year.

Environmental Cues and the Breeding Season

Photoperiod

Photoperiod, or day length, is the primary environmental cue that triggers physiological changes and seasonal breeding behaviors in birds. Rather than watching their wristwatches, birds have a built-in biological clock that is activated by photoreceptors in the brain and eyes. Increasing day length signals the start of the breeding season, while decreasing daylight triggers a reduction in reproductive organ size.

Rainfall, Temperature, and Food Availability

Successful nesting is hard work, and all that effort needs a steady food supply. Increasing day length typically coincides with brighter light and an increase in temperature, both of which stimulate plant growth. That’s great news for herbivorous birds, but also for other species since primary production kick-starts the whole ecosystem, resulting in increased insect abundance.

Even with longer days or warmer temperatures, a lack of rainfall can stall plant growth and insect emergence. Rather than relying on day length, some birds from dry and unpredictable climates begin breeding only after there’s been enough rain.

Hormonal changes translate into rapid development of the reproductive organs, but also visible plumage changes and many of the typical nesting behaviors we see each spring. Rose-breasted Grosbeak

Hormonal Control in the Breeding Season

Hormonal Triggers

Seasonal development of the reproductive organs is a chain reaction that involves various glands on the hypothalamic-pituitary-gonadal axis (HPG).

When the hypothalamus is activated by environmental cues like day length and temperature, it releases a gonadotropin-releasing hormone, causing the pituitary gland to release follicle-stimulating and luteinizing hormones. These hormones trigger the development of the gonads or reproductive glands.

Gonadal Development

Testes are the male gonads responsible for producing testosterone (a sex hormone) and sperm (a gamete). The female gonad, called an ovary, produces hormones called estrogen and progesterone and gametes called ova.

Gametes, which are also known as sex cells, are vital for sexual reproduction since they combine to create offspring with chromosomes from both parents.

Since birds have defined breeding seasons, there would be little sense in producing gametes throughout the year. After breeding, the female oviduct decreases in width and length, and the male’s testes may shrink to just a fraction of their breeding size.

Behavioral Changes During the Breeding Season

Courtship Displays

Increased hormone levels drive some interesting behavioral changes in birds at the start of the breeding season, and courtship displays are probably the most fascinating to observe.

Male birds really know how to put on a show to attract a partner, and typical displays include acrobatic flights, interesting vocalizations, and postures. Females also get in on the act, particularly when a pair has formed, and the two engage in ‘couples’ displays and rituals like mutual preening.

Territory Establishment

Some birds are territorial throughout the year, but many become territorial due to increased hormone levels in the breeding season. A territory may cover just a few feet of ‘prime real estate’ in a busy nesting colony or include an entire mountain or forest in larger species.

A territory excludes other pairs or members of the same species from foraging or nesting in the same area. This is especially important in habitats with scarce nesting sites or limited food sources.

Males advertise their territories by singing from prominent perches and by making conspicuous flights, but females may be highly territorial, too. Rivals that are brave enough to intrude may be chased or even physically attacked by the territory owners, and some birds even fight to the death!

Would you like to learn more about territorial bird behavior? Check out this detailed guide to discover much more.

Male birds really know how to put on a show to attract a partner, and typical displays include acrobatic flights, interesting vocalizations, and postures. Sunbittern

The Breeding Cycle of Female Birds

Egg Production

Female birds release ova from their ovary, which are then developed in the oviduct before laying. During their passage through the oviduct, ova are fertilized with stored sperm, provided with albumen to provide nutrients for the embryo, covered in membranes, and then enclosed in a protective shell.

Producing eggs may place significant nutritional and physiological demands on a female bird. For example, protein and calcium are required, which may be sourced from the female’s own body tissues or require increased food consumption. Carrying eggs could also increase the energy demands for locomotion and flight due to the weight of the egg and reproductive organs.

Laying and Incubation

Birds generally reproduce during a defined breeding season. Most species produce a single brood, although some birds may nest several times in succession when conditions allow it.

Depending on their species, female birds can lay anything from one to over a dozen eggs in a single clutch. Producing an egg takes a day or so, so birds usually lay about one egg each day.

Birds incubate their eggs for anything from ten days to more than two months, although the females of many species share incubation duty with their partner. Apart from transferring body heat to the clutch, incubating is not particularly energy intensive, although it does reduce a bird’s foraging time. Many monogamous birds will feed their partner during incubation for this reason.

Male Physiological Adaptations During Breeding

Sperm Quality and Quantity

Avian sperm cells are highly variable depending on species, but each must be motile (able to ‘swim’), have a healthy nucleus containing the male’s genes, and have an acrosome or tip with the correct proteins to penetrate and fertilize an ovum.

Sperm quantity varies among bird species, with highly competitive, polygamous species producing more than monogamous males. The quality of a bird’s sperm can be affected by the individual’s age, condition, and genetic fitness. External factors like nutrition and pollution also play a role.

Mate Attraction and Copulation

Male birds may go to extraordinary lengths to woo a potential partner, and many species engage in spectacular displays that require conspicuous breeding plumage. These behaviors have an energetic cost and may also increase the risk of predation, but they have evolved to signal the genetic and physical fitness of the male so that females can choose a partner with worthy genes.

The actual act of copulation is relatively brief, although some birds will mate frequently to transfer sufficient sperm. Northern Goshawks, for example, may mate hundreds of times for each clutch, and nesting Tree Swallows may copulate every few minutes!

The actual act of copulation is relatively brief, although some birds will mate frequently to transfer sufficient sperm. Pair of Tree Swallows

Nestling and Fledgling Stages

Parental Care

Most bird species are dedicated parents that provide prolonged periods of parental care. Raising chicks is a demanding task, involving frequent foraging trips to collect food and readiness to spring into action when a predator threatens their brood.

Brooding young chicks also limits the time an adult can spend foraging for itself and may force the bird to shelter its young from harsh conditions like heat and rain. Still, adults are driven by a powerful will to protect their own.

Weaning

Birds do not wean their young in the literal (mammalian) sense, although there comes a time when adult birds must stop feeding and caring for their young and encourage them to become fully independent.

This may occur within a few weeks of the young bird fledgling the nest or happen several months later, depending on the species involved. The adult may need to be quite aggressive in some cases and may even resort to chasing and pecking their young.

Post Breeding Recovery

Molt

Bird feathers are dead structures, and continual wear and tear means they must be molted (replaced) periodically. Many birds complete a molt after breeding when food is still abundant, but they are free of the responsibilities of raising their young.

Some male birds will lose their showy breeding/nuptial plumage at this time of the year in favor of a more subdued and camouflaged color scheme.

Would you like to learn how and why birds molt their feathers? Check out our comprehensive guide for loads of fascinating facts!

Energy Replenishment

Birds typically time their breeding seasons so the young birds will have an easy food supply after gaining independence. The adults must also take time to recover from the rigors of breeding, especially for species that undertake lengthy fall migrations to visit overwintering grounds.

Goldcrest feeding fledgling. Most bird species are dedicated parents that provide prolonged periods of parental care

Summary

Far from ‘business as usual,’ the avian breeding season is a time of dramatic behavioral and physiological changes. Without conscious thought, their glands and reproductive organs spring into action in preparation for gamete production and sexual reproduction, culminating in egg-laying, chick-rearing, and the first independent flights of the new generation.

The same chain of physiological events is seen across thousands of bird species in almost every habitat on earth and has kept bird populations going across diverse landscapes for millions of years.

However, the breeding season presents many challenges and dangers for our feathered friends, and they are particularly vulnerable to disturbance during these vital few months. You can help breeding birds by creating suitable nesting habitats in your community and supporting local and international conservation initiatives.