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11 points
Interesting perspective. I never really thought about that. For a mammal to carry and protect its young it’s not that much of an energy sink. They eat food digest it and provide milk. Birds have to scavenge food and deliver it via airmail.
6 points
There’s also a lot less energy spent upfront. A mammal (except that one) carries their child for a period inside of them while they grow. A bird lays an egg, which they have to care for but they aren’t as stuck with it and vulnerable, and usually for a shorter period. Essentially, a mammal’s child represents a much larger investment, so it makes sense to put more effort into ensuring it succeeds.