Only mammals give live births <<<<
Whoever wrote that, is incorrect.
There are, in fact, many species of fish that give live births.
In my aquarium, I have guppies and platys, both of which have live young constantly.
I also have tetras, Cory doris, and gouramies. These all lay eggs.
Most fish are egg layers, but there are many species that do give live birth.
Large numbers of eggs (high fecundity) always correlate with low survivorship. It is a corner stone of Darwinism that species reproduce more offspring than can survive and "natural selection" weeds out the less fit. Of the ~30,000 species of bony fishes, there is tremendous variation in fecundity. Some fish (e.g., the Ocean Sunfish) lay >50 million eggs, while others (e.g., the common guppy) give live birth to <100 offspring. The key to the difference is parental care.
The answer to that is pretty simple. In the wild defenseless baby fish are easy targets for other fish. It's very easy for these baby fish to get swallowed whole and become a snack for someone else. Natures answer to that is for fish to have large broods of fry to increase the chance that some of them will survive.
No, birds lay eggs.
it lays eggs
Great Horned Owls and birds in general don't actually become pregnant the way most mammals do. Most mammals give birth to live young. Birds lay eggs instead of giving birth. Once the eggs are laid, incubation for Great Horned Owls is about 33 days.
Echidna and platypus.
Animals that lay eggs instead of giving birth to live young are known as oviparous.
they lay eggs
The answer I know is yes, she does give live birth instead of laying eggs. The internet lies about Lugia giving birth.
they are cold blooded, and lay eggs instead of giving live birth like mammals.
birds lay eggs
They don't. They are birds. They lay eggs. Eggs hatch. Baby birds.
Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals.
That is an evolutionary question. The simple answer is generally considered to be that eggs weight less than fully hatched chicks, and therefore it would make it difficult for a mother bird to fly if she were carrying five chicks instead of five eggs. As for flightless birds, if they evolved from birds of flight, then there is no evolutionary reason that they would have become viviparous.