When a mother bird sits on her eggs to incubate them until they hatch, this process is called brooding. Brooding is an essential part of the avian reproductive process that allows birds to regulate and transfer heat to their eggs. By sitting on the eggs, the mother bird keeps them at the proper temperature for embryonic development. She also protects the eggs from predators and environmental hazards. The amount of time spent brooding can vary significantly between different species, but it is a vital parenting behavior found in most birds that lay eggs.
What is brooding?
Brooding refers specifically to the act of a mother bird sitting on and warming her eggs in the nest. This provides the eggs with consistent warmth and humidity, which is essential for normal embryonic growth. The optimum temperature for incubation ranges from 99-105°F depending on the species. Without brooding, the embryo inside the egg would not survive.
Brooding begins as soon as the first egg is laid. The mother bird will sit for prolonged periods throughout the day, only taking short breaks to feed, drink water, and defecate. At night, she often remains on the nest constantly. This constant contact maintains the temperature and allows the embryos to develop.
The brooding parent regulates the temperature through their body. Areas called brood patches on their belly or breast lack feathers so heat can transfer directly from the skin to the eggs. The adult may shuffle the eggs around periodically to ensure even warmth distribution. They may also rotate or turn the eggs during brooding.
In some species, both the male and female parent will take turns brooding the eggs. They will exchange brooding duties, which allows each parent to take breaks to find food and replenish their energy. In most songbirds for example, the male and female share brooding equally. In other species, like owls and eagles, it is only the female that broods.
Why is brooding important?
Brooding serves several crucial functions:
Provides warmth
Bird eggs must be kept at the proper temperature for normal embryonic development. Most species require external heat sources, so the adult sitting on the eggs provides that warmth. The body heat transfers from the parent directly to the eggs. Without brooding, the eggs would quickly cool down and the embryos would die.
Controls humidity
In addition to warmth, proper humidity levels must be maintained during incubation. The adult’s body releases moisture that keeps the nest environment from becoming too dry. Too much moisture loss can stunt the embryo’s growth.
Protects from predators
A brooding parent also physically protects the vulnerable eggs from predators. Their presence helps deter predators from disturbing the nest. If a threat approaches, the parent may use alarm calls or even attack.
Turns the eggs
Turning the eggs during brooding ensures that embryos get proper oxygenation to all sides of the egg. It prevents them from sticking to the inside of the shell.
Allows embryo attachment
As the embryo develops, it must attach inside the egg to a membrane called the vitelline membrane. If the egg moves too much, this attachment may fail resulting in the embryo’s death. Brooding provides stable conditions so the embryo can successfully attach.
When does brooding begin?
Brooding starts as soon as the first egg has been laid in the clutch. It continues for the entire incubation period until the eggs begin to hatch.
In species where only the female incubates, she will often remain on the nest while laying. As eggs are laid daily, each one requires immediate brooding to start development. Leaving the eggs exposed for even short periods can jeopardize the clutch.
For species where both parents incubate, the male may begin brooding as soon as the first egg appears to give the female time to lay more. Then they will switch off duties.
Regular brooding continues 24 hours a day until hatching approaches. Some species may stop brooding briefly within the final days to allow the egg shells to harden just prior to hatching.
How long do birds brood for?
The total brooding time spans the entire incubation period from egg laying until hatching. This incubation period varies dramatically depending on the species:
- Small passerines like finches: 11-15 days
- Larger passerines like crows: 16-18 days
- Pigeons and doves: 14-18 days
- Chickens: 21 days
- Ducks: 24-28 days
- Peregrine falcons: 29-32 days
- Emperor penguins: 50-70 days
The brooding adult will need to remain attentive and on the nest for all of these days. Some may briefly leave the eggs exposed during the last 3-5 days of incubation as the shells harden and embryos prepare to hatch. But continuous brooding is required for the majority of development.
Exact total brooding times for some example species:
Species | Total Brooding Days |
---|---|
Hummingbirds | 14-22 days |
Barn owls | 32-34 days |
Mallard ducks | 28 days |
Chickens | 21 days |
Peregrine falcons | 29-32 days |
So brooding lasts for the full duration of egg development in the nest. Some birds may share brooding duties, but the eggs can never be left alone for extended periods. Continuous heat and humidity is needed for the embryos to mature.
How often do birds leave the nest while brooding?
The adult bird generally tries to minimize time off the eggs during brooding. However, they cannot remain on the nest 100% of the time and must take occasional short breaks.
Frequency and duration of nest departures varies by species. Small songbirds may take multiple brief absences daily totaling no more than 10-15 minutes away. Large birds like falcons may leave only once every few days for up to an hour. Penguin fathers may go weeks without a break during the brooding period.
Nest departures serve important needs for the brooding parent like:
- Finding food and water to replenish energy
- Defecating away from the nest to keep it clean
- Strengthening pair bonds with the mate
- Obtaining additional nesting material
However, every minute away puts the eggs at greater risk. Predators like crows or snakes may raid exposed eggs. Hot sun can overheat unshaded eggs in certain habitats. And eggs may cool excessively without the parent’s warmth.
To maximize efficiency, the brooding parent usually follows predictable patterns when departing. For example, they may take brief breaks at dawn and dusk when temperatures are mildest. Or coordinate a nest exchange with their partner. Strategic timing preserves safety during necessary recesses.
Does the mother stay on the nest all night?
Many brooding mothers remain on the nest continuously throughout the night. Others may make use of nocturnal recesses.
For small passerines, the female often develops an area of bare skin on her belly called a brood patch. Because these birds nest in dense shrubs that protect eggs from cooling, females with brood patches may be able to leave the nest for short 5-10 minute breaks overnight. This gives her time to forage under cover of darkness.
Larger bird species that nest on exposed cliffs cannot afford overnight recesses. This includes falcons, eagles, and owls. The female must incubate through the night when predators are active. If the male helps brood, he may take the overnight shift to give her a break.
In some cases, males perform all nighttime brooding. For example, Emperor penguins breeding in Antarctica experience brutal winter conditions. The male takes sole responsibility for incubating the egg throughout months of darkness while the female hunts at sea and replenishes herself. She returns to relieve him once the sun rises again.
Temperature regulation is the key factor determining if the mother can safely leave at night. Unprotected or exposed eggs require continuous 24/7 contact in harsher environments. But cavity or covered nests allow for some nocturnal respites.
Do both parents take turns brooding?
In around 90% of bird species, parents share brooding duties to some extent. But the level of male participation varies considerably between families.
Biparental brooding in songbirds
Most songbirds demonstrate a high level of biparental care. The male assists the female extensively with brooding after she lays the first egg. He may brood up to 50% of the time, allowing both mates to take feeding breaks. This gives the female energy to lay more eggs. Shared duties continue until hatching.
Some examples of songbirds with equal male brooding:
- Chickadees
- Larks
- Mockingbirds
- Cardinals
- Finches
Primarily female brooding in large birds
For birds of prey and seabirds, the female typically undertakes most or all brooding duties. The male provisions food but does not take shifts.
In these species, only the female develops brood patches ideal for heat transfer. Also, the male must hunt and fish constantly to feed both the female and hatchlings. Leaving the nest unattended is too risky.
Examples of females-only brooding species:
- Hawks
- Eagles
- Owls
- Albatrosses
- Penguins
- Flamingos
No brooding from the male parent
In some species, the female receives absolutely no brooding assistance from the male. These include most megapodes and coucals. The male may fertilize eggs but provides no parental care at all. The female is completely responsible for incubating the clutch.
In a few cases, this female-only system is swapped – the male brooding and female non-participatory. But solitary brooding remains the norm for these groups.
How do birds keep the eggs warm during brooding?
Birds have evolved specialized featherless areas on their bodies that allow direct transfer of heat to the eggs beneath them. This allows them to brood effectively.
Brood patches
In most smaller birds, heat is transferred through a brood patch. This is an area of bare skin on the breast or belly. As hormone levels change prior to egg-laying, feathers are shed from this region and blood supply increases. The swollen vascularized skin rests right up against the eggs for warmth.
Both male and female songbirds develop brood patches for shared brooding duties. In some species, males grow brood patches each breeding season then molt to replace feathers afterward. Females retain theirs year-round.
Bare skin on legs and feet
In larger birds that grasp eggs with their feet, bare skin on legs aids heat exchange. This can be seen in owls, eagles, grouse, and other raptors. The blood vessels swell to conduct warmth down to the unfeathered legs and feet pressed against eggs. Like brood patches, this bare skin is temporary.
Thick vascular leg skin in megapodes
Megapodes like the Australian brush turkey have a unique incubation method. They bury the eggs under mounds of rotting vegetation which generates heat. The male bird presses his lower legs into the mound to further warm the eggs. His legs have heavily vascularized skin ideal for heat transfer.
Bird Group | Body Area for Heat Transfer |
---|---|
Songbirds | Brood patch on belly |
Raptors | Bare skin on feet |
Megapodes | Thickened leg skin |
Penguins | Feet and legs |
So while specific body parts vary, all brooding parents have some region suitable for gently warming the eggs.
Interesting facts about brooding behavior
Beyond the basics, some fascinating facts about brooding in birds include:
- Penguin fathers huddle together in large groups while brooding to conserve heat in extreme cold.
- Fossil evidence shows dinosaurs like Citipati also had brooding patches and likely sat on nests.
- Male emperor penguins can go over 2 months without eating while brooding in winter.
- Some birds cool their brood patches with dropped wings to prevent overheating eggs.
- Both sexes of ostriches and emus brood the eggs laid into a communal nest by multiple females.
- Birds may add dark pigment to feathers on the brood patch to increase solar heat absorption.
- Some parasites like cowbirds sneak their eggs into other species’ nests relying on foster parents to brood.
Research continues to reveal fascinating aspects of how birds care for developing eggs. Their tireless brooding behavior ensures the next generation hatches safely into the world.
Conclusion
Brooding is a key element of avian reproduction across diverse species. By patiently sitting on eggs, parent birds provide the warmth, humidity, and protection needed for normal embryonic growth. While exact nesting habits vary, continuous contact for most of incubation is vital.
Mothers, fathers, or both partners may participate in brooding to differing degrees. But without their dedication, few chicks would survive to hatch. Brooding allows birds to successfully nurture and foster their young, hatch after hatch, season after season.