“Q: I need you to tell me wether or not this is a good idea: replacing all mammals with birds”
You know what, instead of shoving my own very positive opinion on this topic down your throat, let me bring my good buddy NEW ZEALAND ENDEMIC FAUNA to the table to field this one!
For several million years, mammals were entirely extinct in NZ. Birds - and to a lesser extent reptiles, amphibians and insects - filled those niches that had been taken by mammals everywhere else in the world.
The kiwi bird, for example, is a case of convergent evolution with small omnivores like mice and other rodents; they’re nocturnal, have a highly developed sense of smell, and have lost the ability to fly due to spending their lives hunting for underground prey.
Source: San Diego Zoo
Moa were huge, flightless bird versions of deer - they were the primary large herbivore on the islands until humans arrived and hunted them to extinction in a turn of events that shocked absolutely no one. Haast’s Eagle was a huge bird of prey - filling a large predator niche like that of a wolf or a lion.
Source: Wikipedia
Flightless birds have (ironically) really taken off (ha, ha) in New Zealand; species like the kakapo, the takahē, the weka, etc. Without terrestrial predators to prey upon them, flightlessness wasn’t as big of a danger as it is anywhere else. The non-flightless birds are just as interesting - Nestor species are parrots that diverged earlier than any other extant psittacine, wattlebirds are found only on the islands, and the mohua are passerines of their own distinct family.
Of course, all good things must end. As is their wont, bats were the first to arrive as a sign of the mammalian apocalypse to come. Soon after them came humans - and with the humans came MORE MAMMALS. Rats, dogs, cats, rabbits, weasels - you name it, we ruined everything with it. These mammals have done their level best to eat every endemic species out of New Zealand, and they’re doing a bang-up job, in the worst kind of way.
Anyway, I got kind of sidetracked here, but I think the moral of this story is that mammals ruin everything and we should all be replaced by birds because at least birds are up-front about their affiliation with chaotic, destructive forces.