The new wood is made up of thin walled cells which have a large central cavity. This creates the light coloured rings. During the summer and autumn, when there is less rain the tree produces cells with thicker walls and a smaller central cavity. This creates the dark coloured rings. Early in the morning, the ground is still cool and damp, and this encourages the worms to crawl up.
Birds have sensitive feet and they can feel the vibrations of the earthworms moving underground.
The birds then use their hearing to zero in on the worms. Finally by stabbing the soil, the bird grabs the worm and pulls it out.