Birds sit on their eggs before they hatch for several important reasons. The main reasons are to keep the eggs warm, protect the eggs from predators, and allow embryos to develop properly. Sitting on the eggs is an instinctive behavior in most bird species that ensures their young have the best chance of surviving after hatching.
Keeping Eggs Warm
One of the most vital functions of a bird sitting on its eggs is to keep them warm. Bird embryos need consistent, relatively high temperatures to develop correctly. The temperature under the brooding parent provides ideal conditions for embryos to grow.
Bird eggs have porous shells which allow oxygen in and carbon dioxide out. However, this porousness also means they are at risk of losing essential heat. Adult birds incubate by sitting on top of the eggs, transferring their own body warmth. Their feathers and surrounding nest material provide insulation.
If eggs become too cold, embryonic development slows or even stops. Cooler temperatures can cause abnormalities or death. Temperatures that are too high can also be problematic. Therefore, the parent bird maintains the optimal temperature range by adjusting their position on the nest.
Required Egg Temperatures
The exact temperature required depends on the species. For example:
- Chickens: 99-102°F
- Ducks: 98-100°F
- Robins: 104°F
- Penguins: 95°F
The parent birds instinctively know the temperature range needed by their young. By continually incubating, they ensure conditions are ideal for normal embryonic growth.
Protecting the Eggs
In addition to providing warmth, sitting on the eggs also protects them from threats. If left exposed, the eggs could face many dangers that reduce hatching success, including:
- Weather: Eggs must stay dry and not get overheated/chilled by sun, rain, snow etc. Adult birds shelter the eggs under their body.
- Predators: Eggs left unattended could get eaten by snakes, rats, cats, dogs or other predators. The brooding parent acts as a physical barrier.
- Damage: Unprotected, the thin shells could easily crack or break. The weight and movements of the adult bird prevent damage.
By staying on the nest almost constantly, parent birds can quickly react to and mitigate external risks. Their protective presence greatly improves the percentage of eggs that remain intact until hatching.
Allowing Proper Embryonic Development
Incubation does more than just provide warmth and safety. It also allows the chicks growing inside the eggs to develop correctly in preparation for hatching. When birds sit on their eggs, it:
- Maintains humidity levels optimal for the membrane and blood vessels.
- Allows the shell to “breathe” and exchange gases properly.
- Facilitates nutrient provision as the parents turn the eggs.
- Provides stimulation needed for bones and muscles to grow.
- Positions the embryo correctly for hatching.
Removing eggs from the nest, even briefly, can interfere with these processes and put the internal development of the chick at risk. Extended disturbances or incorrect incubation can be fatal to the embryo.
How Bird Embryos Develop
When a bird sits on the egg, it allows the chick inside to go through the full incubation period. This takes days or even weeks, depending on the species. Some key stages of development include:
- 0-3 days: Cleavage and blastulation occur as cells rapidly divide.
- 3-7 days: Organs, nervous system, eyes, and bones begin forming.
- 7-16 days: Feathers, beak, claws, and egg tooth develop.
- 16 days to hatch: Yolk sac retracts as the chick gets in position to pip out of the shell.
The embryo progresses through these changes in preparation for life outside the shell. Without the proper conditions created by incubation, development is impaired or halted, and the chick cannot hatch.
When do Birds Start Incubating?
Birds begin sitting on their eggs as soon as the last one is laid. For most songbirds, this is usually one egg per day until the clutch is complete. Larger birds like chickens may wait until the last few eggs are laid. Incubation generally begins when the last egg is laid, but may start a few days earlier in some species.
Once they start incubating, the parents are very dedicated. They will spend extended periods on the nest with few breaks. The highest risk periods for embryo mortality are the first and last few days of incubation. Therefore, the parents are most attentive and leave the eggs least often during these delicate times.
How Often do Birds Rotate or Turn the Eggs?
Although attentive, parent birds do still take occasional short breaks from incubating. However, they turn and reposition the eggs frequently before leaving the nest. Rotating the eggs helps ensure:
- The embryo remains properly positioned.
- Yolk nutrients are distributed evenly.
- Membranes stay strong and healthy.
- Muscle formation in the embryo is facilitated.
How often the eggs are turned depends on the species. Rough guidelines include:
- Small songbirds: 25-50 times per day
- Chickens: Around 5 times per day
- Penguins: Nearly 100 times per day!
Birds have an innate sense of how their eggs need to be turned. When taking breaks from direct incubation, they continue rotating at optimal intervals.
How Long do Birds Incubate?
Birds keep incubating non-stop until the eggs hatch. The full incubation period varies considerably by species, with songbirds on the low end and some raptors on the high end.
Here are example incubation times:
Species | Incubation Period |
---|---|
Hummingbird | 14-19 days |
Robin | 12-14 days |
Chicken | 21 days |
Duck | 28 days |
Goose | 28-32 days |
Bald eagle | 35 days |
When the chicks are ready to hatch, they will start to pip through the shell with their egg tooth. The parents often help the hatchlings fully emerge. Once hatched, the chicks may stay in the nest a while longer, where the parents continue caring for them.
Do Both Parents Incubate the Eggs?
In around 90% of bird species, both the male and female parent help incubate. They share the responsibility of sitting on the nest. However, the female usually puts in more time and incubates at night in most songbirds. The male primarily incubates during the day.
Sharing allows the parents to also take breaks to search for food. This is essential so they can maintain their own health and energy to care for the hatchlings. However, one parent stays behind at all times to continue incubating.
In some species, only the female incubates without help from the male. These include chickens, ducks, owls, and woodpeckers. In megapodes, neither parent incubates. The eggs are buried in composting vegetation which provides the needed heat.
Why do Birds Stop Incubating?
Parent birds rarely abandon incubation before the eggs hatch unless they have no other choice. However, they may be forced to leave the nest if:
- They are weak, injured, or in poor health.
- The other parent dies, leaving them unable to take breaks to eat.
- The nest is destroyed by weather, predators, or other causes.
- Resources become too scarce to provide for themselves and the eggs.
When this happens, the eggs will quickly get cold, fail to develop further, and die. The parents have no choice if they cannot adequately provide for their own needs.
Conclusion
Birds dutifully incubate their eggs by sitting on the nest because it is essential for the chicks’ survival. Brooding behaviors like consistent warming, protection, stimulation, and gas exchange provide what embryos need to grow until they are ready to hatch. Without attentive parents conducting these incubation activities, development would fail and the chicks inside the eggs would not survive.