Life Cycles of Birds
There are more than 9,000 species of birds that inhabit every region of the planet, from the burrowing owl of the American desert to the emperor penguin of Antarctica. Yet despite their incredible variety, all birds share the same basic life cycle events. All birds lay eggs, which hatch into nestlings; develop into fledglings; and mature into breeding adults. Migration in spring and fall is another important aspect of the life cycle for many bird species.
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The Egg
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An egg is a protective shell that protects a developing embryo. Unlike mammals, which carry their developing young inside their bodies, birds must keep their body weight light in order to fly. So after mating, the mother bird lays one egg each day until her clutch is complete. The mother then incubates the eggs, covering them with her body to keep them warm, until they are ready to hatch. The length of incubation ranges from 11 days for songbirds to 85 days for albatrosses.
Nestlings
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Birds that have hatched are called nestlings. Most nestlings are naked, blind and completely dependent on their parents. Exceptions include ducks, swans, geese and some seabirds, which can run and swim immediately after hatching.
Both parents may be involved in feeding, warming and protecting the hungry nestlings. An American robin leaves the nest at about 14 days, while a red-tailed hawk leaves at 45 days.
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Fledglings
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When the babies leave the nest, they are known as fledglings. Most fledglings remain dependent on their parents for food and protection. During this phase of life, which ranges from a few days to several weeks, the babies learn to fend for themselves. The parents eventually refuse to feed the fledglings and may even chase them away. In a few species that breed only once each summer, families remain together for the fall migration.
Fall Migration
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For birds that migrate, the shorter days of fall trigger the instinct to head south for the winter. During fall migration, the bird population is at its peak because of the new babies born in the summer breeding season. The journeys are often perilous; even hummingbirds may travel 500 miles across the Gulf of Mexico to reach their destination. Most breeding pairs do not stay together, although notable exceptions include swans, geese, pigeons and many hawks, some of which mate for life.
Spring Breeding Migration
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As the days lengthen, increased sunlight sets off hormonal responses that prepare the birds to breed. The spring migration is also called the breeding migration because the birds return to their summer breeding grounds. Males stake out nesting territory and compete to attract females. Some species engage in elaborate courtship display rituals. After mating, the birds build a nest, the female lays her eggs, and the cycle begins again.
Hazards
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The bird life cycle is fraught with danger from start to finish. Most birds do not survive their first year of life. Eggs and nestlings are a coveted protein source for predators, including other birds, raccoons and foxes. Immature birds that are not yet skilled at finding food and shelter fall prey to predators, starvation, exposure or disease. The risk of death decreases steeply after one year, although few wild birds do die of old age. In addition to natural hazards, more than a billion birds are killed each year by house cats, cars, television towers and poisoning.
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References
Resources
- Photo Credit Fledgling ducks and mother. Benimoto.Flickr.creativecommons.org
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bird cycies.
