Sometimes worms will cluster at the top of the bin, and on the underside of the lid if it is about to rain. This is a natural response to prevent them from drowning in the wild or migrating to fresh food when the ground is wet. They will return down into the surface layer when the rain has passed. If the conditions in the bin are unfavourable, the worms will also try to migrate. This is usually caused by overfeeding or if the food has become too acidic. However, if you keep the lid on as recommended, it is almost impossible for them to escape. Occasionally a worm may fall from the bin into the drip tray, especially if castings have recently been removed. If the food is too wet, the worms will look fat and pale. Try adding some dry leaves or shredded paper and gently use a fork to turn the top layer and create some drain holes on the surface. If heavy rain floods your bin, move it to a more sheltered location.