Why are some animals like birds and reptiles born from eggs while most mammals are not?

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Why are some animals like birds and reptiles born from eggs while most mammals are not?

In: Biology

Evolution, laying eggs was probably the best route for reptiles and birds. A few come to mind like birds can lay the eggs in tall trees away from predators and if they had to carry a fetus it would weigh them down. Mammals are warm blooded so keep the fetus at a good temperature for development, and it protects the fetus from predators/the environment