Of all the animals that man has caused to become extinct, the story of the passenger pigeon makes me saddest.

indefensible:

From Wikipedia:

The passenger pigeon (Ectopistes migratorius) or wild pigeon was a species of pigeon that was once the most common bird in North America. It lived in enormous flocks. During migration, flocks could be 1 mile (1.6 km) wide and 300 miles (500 km) long, taking several hours to pass and containing up to two billion birds.[1][2]

Perhaps you’re not familiar with the North American woolly mammoth. They were once abundant until the first humans arrived. Having never been exposed to Homo sapiens, they were peaceful and could easily be approached. This made them very easy to kill. They were so abundant, that there is evidence of nomadic tribes feasting on a woolly mammoth carcass for just a single day and moving on, leaving most of it behind uneaten. While you may miss a bunch of pigeons, of which there are many other varieties, I miss the chance they could have had to use woolly mammoths as work animals and, who knows, maybe miniaturized woolly mammoths as pets. The giant camel, which met the same fate in the Americas, would also have been nice to have around today.A great book that touches on these things is Jared Diamond’s “Guns, Germs and Steel”.

Source: indefensible