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Unlike their reptilian relatives, which sometimes give birth to their young, all bird species lay eggs. Although eggs look quite fragile, their oval shape offers great strength and they can withstand high pressures without breaking.
Because eggs are heavy and cumbersome to carry, females lay eggs as soon as they are fertilized, often in a nest built to protect the egg from predators and keep it warm during embryo development.
Different bird species lay different numbers of eggs - penguins usually lay a single egg, while European blue tit lay between 18 and 19 eggs.
Nest building is one of the great design and engineering achievements of the animal kingdom. Different species show extraordinary diversity in the construction of their nests. Some birds build tiny nests so well hidden that not even the most determined hunter can find them, but other species build huge, highly visible nests that they bravely defend against any approaching creature.
Swans often build nests several centimeters in diameter, while Scopus umbretta African builds dome-shaped nests that can weigh up to 50 pounds, taking several weeks to build. Birds use a wide variety of materials to build their nests.
Some species use only twigs and branches to build the types of nests commonly seen in gardens and hedges. Others use a little of everything: from leaves to feathers, to moss, and even man-made objects like laminated paper.
O Collocalia maxima from Southeast Asia it nests entirely from its own saliva, and builds them on cave ceilings. Not all birds build nests. The cuckoo, in particular, uses other birds' nest instead of building yours. The female flies quickly into an appropriate nest, removing one of the eggs from the "hostess" and laying her own egg, usually the same size and shape as the one she removed. The emperor penguin does not even use a nest: he lays his single egg directly on snow, and incubates it with his body temperature.
In their evolutionary path, the birds acquired several essential characteristics that allowed the flight to the animal. These include:
- Feather development
- Acquisition of pneumatic bones
- Loss, atrophy or fusion of bones and organs
- Acquisition of an air bag system.
- Egg laying
- Presence of keel, expansion of the sternum, in which the muscles that move the wings are attached
- Lack of urinary bladder
Feathers, considered as diagnostics of current birds, are present in other dinosaur groups, including Tyrannosaurus rex. Studies indicate that the origin of feathers came from modifications of the scales of reptiles, becoming increasingly differentiated, complex and, later, made possible the flights glided and beaten.
Feathers are believed to have been preserved in evolution for their adaptive value by aiding in the thermal control of dinosaurs - a hypothesis that points to the emergence of endothermia already in more basal Dinosauria groups (relative to birds) and in parallel with acquisition of the same characteristic by Sinapsida reptiles, which gave rise to mammals.
The pneumatic bones They are also found in other groups of reptiles. Although they are hollow (a better term would be "non-massive"), the bones of birds are very resilient, as they preserve a system of pyramidally arranged trabeculae inside.