What two disadvantages did the Dodo, Moas, and giants of Australia have as described in Guns, Germs, and Steel?

Diamond talks about the megafauna of
Australia in connection with the arrival of humans on the continent about 40,000 years ago. His
argument is that these creatures were hunted out of existence by early man, or died because of
man-made habitat destruction. Even though there is no direct evidence that early man hunted
these animals, the fact that they disappeared shortly after man appeared strongly suggests a
connection for Diamond.

The chief disadvantage these animals had was simply a
lack of fear of humans. Unlike animals in Africa, which had evolved together with humans for
thousands of years, animals in Australia were surprised by human hunters. The dodo is a case in
point: it was famously hunted to extinction by mariners because it had no natural fear of
humans. It was a simple matter for sailors to kill as many as they wanted. In the same way,
Diamond theorizes that these large animals were easy prey for the early inhabitants of Australia
and New Zealand.