Birds have evolved a variety of strategies for incubating and hatching their eggs. Most birds sit on top of their eggs to provide warmth and protection, but some unique species actually hold their eggs between their legs or tucked under their bellies while they incubate them.
Ostriches and Emus
Two of the largest flightless birds, ostriches and emus, incubate their eggs in a very peculiar way. Rather than sitting on top of their eggs like most birds, they sit alongside their nest with their long, powerful legs on either side of the eggs. This allows direct contact between the eggs and the bird’s brood patch, a featherless region of skin on the belly that transfers warmth to the eggs. The ostrich typically lays 6-12 eggs in a simple ground nest or scrape, while the smaller emu lays 5-15 eggs in a nest of grasses, leaves and twigs. In both species, the male does most of the incubation during the day and all of it at night, for a total incubation time of around 40 days.
Ostrich Egg Incubation
The ostrich is the largest and heaviest living bird, making it well equipped to incubate a clutch of large eggs. Ostrich eggs can weigh up to 3 pounds each and are the largest of any living bird. During incubation, the male ostrich will lay his belly directly over the tops of the eggs while his legs cradle the sides. Using his long, thick toes, he turns the eggs periodically to ensure even heating and development. If threatened, he can also use his powerful kicks to ward off predators. Throughout the 6 week incubation period, the male will get up occasionally to eat, drink and turn the eggs, but never leaving them unattended for long. Once hatched, the precocial ostrich chicks are able to follow their father on foot shortly after hatching.
Emu Egg Incubation
While smaller than the ostrich, the emu is still a very large flightless bird native to Australia. They use the same incubation strategy, laying alongside the nest with their legs on either side of the eggs. The male emu balances his weight carefully not to crush the eggs, which can weigh around 1 pound each. Using his large feet, he regularly turns and repositions the eggs for even warming. During the 8 week incubation, he will take short breaks to eat and drink but is very protective of the nest otherwise. Emu chicks are also precocial, able to walk and feed independently soon after hatching while following their father for protection.
Emperor Penguins
Emperor penguins have adopted perhaps the most extreme egg incubation strategy to allow them to breed in the harsh Antarctic winter. After the female lays a single egg, she transfers it quickly to the male’s feet and returns to the sea to feed, sometimes traveling 50 miles or more. Balancing the egg on his feet, the male penguin then tucks it underneath a fold of skin over his belly to keep it warm and prevent it from freezing. He does this throughout the depth of winter, going without food for over 2 months as he huddles with other males for warmth. He incubates the egg and protects it this way for 64-67 days until it hatches. Once the female returns, he regurgitates food for the newly hatched chick before leaving to feed himself in the ocean.
Emperor Penguin Egg Incubation Adaptations
Several key adaptations allow the male emperor penguin to successfully incubate an egg in the extreme cold:
- A thick fat layer provides insulation and an energy reserve.
- Huddling together in large groups reduces heat loss.
- Rotating positions from the warmer interior to cooler exterior of the huddle.
- A featherless brood patch on the belly for egg contact.
- A fold of skin over the belly to tuck the egg underneath.
- The ability to withstand temperatures of -40°C and winds up to 200 km/hr.
Their remarkable adaptations allow emperor penguins to breed through the Antarctic winter when no other birds can.
Megapodes
Megapodes are a unique group of birds found in Australia, Pacific Islands and Southeast Asia that do not actually sit on their eggs to incubate them. Instead, they bury their eggs in mounds of decomposing vegetation, volcanic ash or other warm, loose substrate. The eggs incubate from the environmental heat in the mounds, which can reach temperatures over 30°C from microbial decomposition. Different megapode species choose specific types of incubation mounds depending on their habitat. The male megapode monitors the eggs and adjusts the mound as needed, but never directly contacts the eggs once buried.
Malleefowl
One well-known megapode is the malleefowl, which inhabits dry eucalyptus forests in Australia. Using its large feet, the male malleefowl constructs a large mound of sand, litter and earth up to 5 meters wide and 1 meter tall. In the center is a pit where the female lays 8-12 eggs in a circle around a dirt core. As the litter decomposes it can raise the core temperature to 33°C. The male monitors the temperature and opens or closes air vents to regulate it, and adds more litter when needed. The constant adjustments by the male continue for the 49-53 days until hatching.
Brushturkey
The Australian brushturkey, found in rainforests of eastern Australia, constructs incubation mounds from leaf litter up to 5 meters wide and 1 meter high. Multiple females will lay eggs in the male’s mound over several months. The male monitors the temperature and stirs the litter to optimize heating. The eggs incubate for around 50 days before hatching. The newly hatched chicks dig themselves out of the mound and leave the nest.
Surrogate Egg Incubation
Some birds that lay eggs in the nests of other species may not directly incubate their own eggs. These species rely on foster parents to provide full incubation duties.
Cuckoos
The most well known example is cuckoos, which lay eggs in the nests of other birds. Different cuckoo species target specific host species, laying eggs that closely mimic the host’s own eggs in color and size. When the hosts incubate what they believe is their own clutch, they unknowingly incubate the cuckoo egg too. The cuckoo chick then hatches earlier and ejects the other eggs from the nest so the parents raise it alone.
Honeyguides
Honeyguides in Africa have an intriguing relationship with certain bee-eating bird species. The honeyguide lays an egg in the host’s nest, and when the chick hatches it uses sharp hooks on its beak to kill the host’s own chicks. This ensures the hosts feed only the honeyguide chick. To better ensure the hosts will incubate the egg, honeyguides will actively lead them to bee nests to raid for food.
Conclusion
While sitting directly on top of their eggs is the most common incubation method for birds, unique species like ostriches, emus, emperor penguins and megapodes have adapted alternative strategies to successfully hatch their young. Relying on external heat sources or surrogate parents to incubate eggs allows certain birds to breed in challenging environments. The incredible diversity of avian reproductive strategies is a testament to the evolutionary adaptations that allow birds to thrive worldwide.
