1. what would be a probable explanation for why there are many endemic plants and birds in new zealand, but no native mammals?

they were driven away by non-native mammals

plants and birds would make ocean crossings to the isolated new zealand island, mammals could not.

they were hunted to extinction by the moari

the island habitats were not conductive to mammalian life.

New Zealand is an island that is fairly far from other land masses, so all the life that ended there, would have to find a way to reach it.  Humans can build boats, but what about other species? 

Birds can fly, and while they fly, they can bring seeds, for example in their stomachs. 

So the birds that could fly to New Zealand found a safe enviroment there and thrived without many predators!

the answer is: Plants and birds would make ocean crossings to the isolated New Zealand island, mammals could not.