The wild birds known as ravens are thought to be very clever. In fact, they can solve some quite
(16).............. problems, especially when they're trying to get food. For example, one bird was
filmed taking a box of nuts from a bird table and (17) .............. it onto the ground, so that it
would break and the bird could eat the nuts inside!
Scientists also (18)
that the birds could actually use stones as tools to (19)
out simple tasks. So in an experiment, they taught five birds to use a tool to open a box with food
inside. The birds soon became very (20) .... . at doing this. So for the experiment, the birds
So
were given a small number of tools to choose from. They always picked the tool that scientists had
given them for the first task. And hours later, they could still remember which tool they'd used!