Birds are some of the most prolific egg layers in the animal kingdom. The ability to lay eggs is one of the defining characteristics of class Aves. However, there are a few exceptions to this rule.
If you’re short on time, here’s a quick answer to your question: Some birds such as the ostrich, emu, cassowary, kiwi, and rhea do not lay eggs.
In this comprehensive guide, we’ll discuss different types of flightless birds that do not lay eggs. We’ll look at why ratites like ostriches and emus don’t lay eggs. We’ll also explore some theories on the evolution of egg laying in birds.
Flightless Birds That Don’t Lay Eggs
Ostrich
The ostrich is the largest and heaviest living bird, standing up to 9 feet tall and weighing over 300 pounds. Though they have wings, ostriches cannot fly. Their wings are used mainly for balance, courtship displays, and shading their chicks.
Ostriches are flightless because of their massive body size and small wings relative to their body weight. Amazingly, ostriches are the fastest running birds, capable of sprinting up to 43 mph. These iconic African natives occupy open savannas and grasslands south of the Sahara.
The ostrich is farmed around the world for its feathers, hide, and meat. Ostriches lay the largest eggs of any living bird. A single ostrich egg can weigh up to 3 pounds! The eggs are incubated by the dominant female by day and the male by night.
Emu
The emu is the second largest living bird after the ostrich. These big, flightless birds are natives of Australia. Standing up to 6 feet tall and weighing nearly 100 pounds, emus are unable to fly due to their low wing-surface-area to weight ratio.
However, emus can run at speeds up to 30 mph using their strong legs. They use their wings primarily for balance during running. Emus inhabit grasslands, woodlands, and scrublands across Australia. These hardy birds can go for weeks without drinking water.
The emu is an important cultural icon in Australia, even appearing on the country’s coat of arms. The emu is farmed for its meat, oil, and leather. The eggs weigh around one pound and are dark green in color. The eggs are incubated by the male.
Cassowary
The cassowary is a large, flightless bird most closely related to emus that inhabits the tropical forests of New Guinea and northeastern Australia. It is the third-tallest and second-heaviest living bird after the ostrich and emu.
Cassowaries stand 4 to 5 feet tall and weigh between 60 and 130 pounds. Their wings are small and unusable for flight. However, cassowaries can run up to 31 mph through the dense forest. They use their wings for balance and making themselves appear more intimidating.
Cassowaries are solitary birds with glossy black plumage and a distinct bony casque on their head. Their claws are their most formidable weapon, capable of inflicting fatal wounds to predators and humans.
Cassowaries feed mainly on fruits that have fallen from trees and play an important role in dispersing the seeds of rainforest trees. The female cassowary lays 3 to 8 large green eggs which are incubated by the male.
Kiwi
The kiwi is a truly unique flightless bird found only in New Zealand. It is the smallest living ratite bird, standing just 20 to 25 inches tall and weighing 3 to 8 pounds. Structurally, the kiwi lacks several typical bird features.
Its small wings are hidden beneath shaggy, hair-like feathers and serve no purpose in flight. They do not even have a visible tail! But the kiwi makes up for its petite size with highly developed senses of smell, hearing, and touch.
In fact, kiwis have nostrils at the tips of their long beaks to help detect prey by smell as they probe the soil and rotting logs. Being flightless, kiwis face predation pressure from invasive mammal species in New Zealand. Sadly, kiwi populations are declining, with only around 68,000 remaining.
Females lay 1-2 eggs that weigh up to one pound, or 15% of the mother’s body weight! The eggs are incubated by the male.
Rhea
Rheas are large, flightless birds that inhabit savannas and grasslands of South America. They bear a strong resemblance to ostriches but are smaller in size. Rheas stand about 4 to 5 feet tall and weigh anywhere from 20 to 80 pounds. Their wings are vestigial, serving no purpose in flight.
Rheas use their sturdy legs and feet for running up to 40 mph. Their plumage varies in color from gray to brown to black depending on the species. Rheas utilize their beaks to graze on grasses, leaves, seeds, roots, and fruits.
They play an important role in seed dispersal across their grassland ecosystems. Rheas are social birds that live in small flocks, with a dominant male defending several females and their eggs. The male builds a nest where up to 60 eggs may be laid! The male incubates and raises the young alone.
Rhea meat and hides are economically important resources.
Why Ratites Don’t Lay Eggs
Evolutionary Adaptations for Flightlessness
Ratites are a group of large, flightless birds that include ostriches, emus, rheas, cassowaries, and kiwis. Over millions of years of evolution, ratites have adapted in incredible ways to a terrestrial lifestyle, losing the ability to fly.
One of the most amazing adaptations is that ratites do not lay hard-shelled eggs like other birds.
Hard shells evolved in birds as protection for developing chicks, especially in precarious nests high up in trees. But ratites built their nests on the ground, so the egg shell was no longer as crucial.
Additionally, a large, thick egg shell requires a lot of calcium to produce – calcium that ratites redirected to building their large bodies and powerful legs for running.
So over time, ratites evolved soft, pliable eggs with a leathery skin instead of a hard, calcified shell. These parchment-shelled eggs are better suited to incubation on the ground. The eggs can deform slightly to fit snugly into a nest scrape in the earth or vegetation, keeping them safe from predators.
The soft eggs also allow easier gas exchange for the developing chick inside. Truly an incredible evolutionary innovation!
Different Reproductive Strategies
Beyond their unique eggs, ratites have evolved varying reproductive strategies. For example, most ratites share parenting duties. Male and female ostriches take turns sitting on the nest and protecting the eggs.
Emus have a similar system, with males incubating the eggs and females defending the territory. Cassowaries, however, do no parental care at all – the male mates with multiple females who each lay 3-8 eggs in hidden nests that they abandon.
Another difference is the time it takes for ratite eggs to hatch. Ostrich eggs hatch in a speedy 35-45 days. But emu eggs take 50-60 days, and massive cassowary eggs can take over 80 days to hatch! The incubation periods match with the climate and habitat where each species lives.
While flightless, these powerful birds have adapted in amazing ways to thrive on land. Their evolutionary innovations like soft, pliable eggs and varied parenting roles allow ratites to successfully raise the next generation of chicks – all without ever leaving the ground.
Theories on the Evolution of Egg Laying
From Reptiles to Birds
Scientists theorize that the evolution of egg laying originated with ancient reptiles over 300 million years ago. As reptiles evolved into the first primitive birds, they retained the reproductive trait of producing shelled eggs fertilized internally and incubating them outside the body.
This likely provided advantages over species that gave live birth, enabling the success and proliferation of egg laying species.
Researchers posit that the hard eggshell offered increased embryo protection and helped prevent desiccation. The calcium-rich composition also gave a nutritional boost to developing avian young. As birds continued to evolve into diverse species spanning habitats worldwide, oviparity (egg laying) remained the dominant form of reproduction.
With over 10,000 living species today, from the flightless ostrich to hummingbirds that weigh less than a penny, egg laying persists as a key evolutionary strategy enabling the thriving diversity of birds across terrestrial and aquatic ecosystems.
Advantages of Internal Fertilization and Egg Laying
When compared to mammals that supply a live-born fetus with sustenance through an umbilical attachment, some key advantages emerge for egg-laying species. Relying on internal fertilization inside the female oviduct, the self-contained egg with its own food supply frees the parents from gestational constraints and energy expenditures.
This facilitates migration across vast distances, seasonal adaptations, and acquisition of ecological niches that may be inaccessible to pregnant animals and newborns.
Additionally, producing sometimes vast numbers of fertilized eggs maximizes reproductive success in everything from the tiny hummingbird to the enormous ostrich. Even if predation and elements diminish hatchling rates, the sheer fecundity of oviparous animals ensures continuation of genetic lines.
So while exceptions exist in the avian world of egg retention, incubation or live births in the mother’s body, the remarkable ubiquity of egg laying underscores millions of years of evolutionary fine tuning for thriving species proliferation from ancient primeval times to the age of flight.
Conclusion
In summary, ratites like ostriches, emus, cassowaries, kiwis and rheas are flightless birds that don’t lay eggs. Over millions of years, they evolved different reproductive strategies compared to most other birds.
While the origins of egg laying in ancestral birds continues to be debated, eggs provided advantages like internal fertilization and extra protection to bird embryos. Understanding egg laying diversity gives us a window into the evolutionary past of birds.
