Breeding season is in full swing, and in a few weeks, there will be new birds everywhere. But have you ever wondered how long it takes for a bird to form an egg before laying it, and how long those eggs are kept warm? In today’s blog, we take a closer look at how the egg actually forms and how the incubation process, and therefore hatching strategy, varies significantly across bird species.
Another spring, another breeding season. All across woodlands, wetlands, and coastal cliffs, dedicated parents are tending patiently to their eggs, keeping them warm at night and turning them gently to ensure the developing chick is evenly heated. Eventually, each hatchling will break through the protective shell of its own private incubator, ready for life in the outside world. Some species, particularly songbirds, hatch into the world completely helpless and naked and need time in the nest first, like the Pygmy Nuthatch, and these are known as altricial birds. Others, such as ducks and shorebirds are called precocial birds, and emerge fluffy, open-eyed, and ready for life from the off.
But regardless of the chick’s level of independence, all birds go through the same extraordinary biological process to form an egg.
It all begins in the female’s ovary, with the formation of the yolk. A single, nutrient-rich cell built from proteins and lipids produced in the liver. This yolk gradually enlarges as more nutrients are deposited. When it’s ready, it’s released into the oviduct, where fertilisation may occur if sperm is present.
The yolk then passes through several chambers in the oviduct, each adding a key ingredient. First comes the albumen, or egg white, which cushions the yolk and contains twisted cords of dense protein called chalazae to keep it centred. Protective membranes are added, and finally, in the shell gland (part of the uterus), the egg spends around 18–24 hours as the shell is formed from calcium carbonate. Pigments may be added here too, creating those wonderful and familiar speckles or shades.
When the shell is fully formed, the egg is laid via the cloaca, usually blunt end first. For many small to medium-sized birds, like the American Robin, Blue Tit, and Rufous Hummingbird, this entire egg formation process takes around 24–26 hours, so they usually lay one egg per day, laying a clutch of several eggs over consecutive days.
Larger birds like the Brandt’s Cormorant, however, often need more time to form an egg, and typically lay an egg every 2–3 days. The American Flamingo may take 3–4 days to complete a single egg, while the Wandering Albatross takes even longer, laying only one egg per breeding season, sometimes after up to 10 days of formation.
Once the egg is laid, the real magic begins: incubation. Eggs need to be kept at a steady 37–38°C to allow the embryo to develop. But this is where species start to differ quite dramatically in their incubation strategy.
Some birds begin incubation immediately after the first egg is laid, which results in asynchronous hatching: each chick hatches on a different day, often creating a size and age gap between the siblings. This approach is seen in Peregrine Falcons, Horned Owls, African Penguins, and Herons. In these species, early hatched chicks often dominate the nest, which can be a brutal but effective survival strategy if food is limited: the strongest, oldest chick is more likely to thrive.
Other species wait until the entire clutch is laid before beginning incubation. This produces synchronous hatching, where all the chicks emerge within a day or two of each other. This is common among songbirds such as the Evening Grosbeak, American Robin, and hummingbirds like the Cuban Emerald pictured here, as well as in ground-nesting ducks and shorebirds like Mallards and Lapwings. Synchronous hatching helps reduce sibling rivalry and allows the entire brood to leave the nest together shortly after hatching, which is crucial for precocial species.
All of this variation comes down to basic survival. Birds that raise highly independent chicks or nest in places where predation is high, benefit from synchrony, as all their young are ready to go at once. Birds in harsher, less predictable environments often hedge their bets with asynchrony, ensuring that at least one chick has a head start if resources become scarce.
In the end, whether a bird lays one egg or a dozen, whether its chicks hatch in a tight sibling bundle or a staggered relay race, each species has evolved a strategy that gives its next generation the best shot at survival.
