Why do birds lay eggs?

Question

All birds lay eggs. There are no avian species with a reproductive system similar to placental mammals, and there are no ovoviviparous species that retain the eggs internally until they hatch. Briefly describe how this phenomenon would be approached from a proximate vs. an evolutionary viewpoint. Help me with this! Proximate vs. evolutionary viewpoint???

Answer 1

(a-hem) Could you define in what context you mean proximate?

This is my understanding. Since the dinosaurs were ancestors to birds, and feathers are just altered forms of scales, evolutionarily speaking, birds lay eggs as they are more related to reptiles and have "retained" this reproductive system as part of their reptilian heritage.

The following is true for birds, but also for reptiles...without the "flying" part. First, birds cannot invest the energy required by an "internally hatching" (i.e. placental incubation). It would, evolutionarily speaking, make them "too vulnerable" to premature death. Think of this: the physical cost to the mother to carry a incubating egg and divert nutrients from its system during gestation (pregnancy) would make her too likely to die before the baby bird would be born, so that the species would die out extremely quickly.

Also, birds lay eggs because they do not "invest" as much in each "reproductive opportunity" (or pregnancy vs. egg-laying), which gives the parents AND the babies a higher rate of survival. Imagine it this way: if a momma bird had to be pregnant, might it not "weigh her down," making it more difficult to fly, making her more vulnerable to enemies and also making her need to eat more?

So if she needs to eat more AND its harder for her to fly AND she is more likely to be killed because she's flying lower, slower, AND less- than if she's not weighed down by a pregnancy, then WHY in God's name would she want to jeopardize herself AND her baby (and therefore all the other future babies she could have) by internally incubating a baby? Why not get rid of those risks by laying an egg and taking your chances with taking care of it outside instead of inside?

Okay, sure, she "knows" she'll lose more babies by laying eggs; but she figures she'll have more chances to lay more eggs than if she were to spend the time and energy pregnant. That means the chances of having babies--that go on to make more babies (that's the most important part)-- are higher with laying eggs outside the body.

I hope that isn't something you already know....

Answer 2

Why Do Birds Lay Eggs

Answer 3

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Yes, as you've already heard, all (female) birds lay eggs. I think I would be scared if any bird didn't... And, for the answer to your question... The dinosaurs came first. All birds evolved from dinosaurs/related reptiles.

Answer 4

if they were to keep them inside a womb until they are big enough to go out in the world like mammals do, the mummy bird will not be able to fly with the weight, and might fall to predators and so the baby and mother die.

so they just lay an egg and let it do the rest.

Answer 5

due to the body temperature,the young ones will have to face sudden change in the atmospheric temperature once they come out.
so they first grow in the eggs and adapt themselves to the temperature.

Answer 6

www.google.com - proximate vs evolutionary

Answer 7

thats how we know its not a mammal! btw mastermind has a better answer.