The caterpillars live on an empty space of land. The caterpillars are slowly getting eaten once the birds above have spotted them.
Now, more caterpillars are being born by the caterpillars that are able to survive. The ones that are not able to survive may be getting eaten by the birds that stop by.
Not all of the caterpillars are born green. Green will help them to blend in and stay alive longer. Now the caterpillars are trying to compete to stay alive by being hidden from the birds so they don't eat them.
The bright yellow and red catapilars stand out more than the green caterpillars. The green ones are hidding in the grass and it is easier for them to hide away from the birds so they don't get eaten, but the bright caterpillars keep on getting spotted by the birds.
Since the birds are focusing on the bright colored caterpillars the green ones are safer and they are eaten less because of their color. They often survive longer.
Only the caterpillars that survived which are the green ones are reproducing and they are reproducing stronger caterpillars.