This can be difficult because the range on the green bird's scream which scares the pigs is not that large. Thus you have to use it very close to all six pigs. Start level 2-18 and use your first green bird to knock some of the stone blocks off the top of the largest, middle tower. They must fall in such a way that none of the pigs are killed. Fire your second green at the same location so that it arcs in between the middle tower and the farthest tower. The stones you knocked down with the first shot makes this possible. Immediately before it makes an impact and you are near the two groups of three pigs, make the bird scream.