Researchers have found that migrating animals use a variety of inner compasses to help them navigate. Some (1)_____ by the position of the Sun. Others navigate by the stars. Some use the Sun as (2)_____ guide during the day, and then (3)_____ to star navigation by night. One study shows that the homing pigeon uses the Earth's magnetic fields as a guide (4)_____ finding its way home, and there are indications that various other animals, from insects to mollusks (软体动物), can also make (5)_____ of magnetic compasses. (6)_____ is of course very useful for a migrating bird to be able to switch to magnetic compass when clouds cover the sun (7)_____ it Would just have to land and wait for the Sun to come out again. (8)_____ with the Sun or stars to steer by, the problems of navigation are more complicated (9)_____ they might seem at first. For example, a worker honeybee (10)_____ has found a rich source of nectar and pollen flies rapidly home to the hive to (11)_____: A naturalist has discovered that the bee scout (12)_____ her report through complicated dance in the hive, (13)_____ she tells the other workers not only how far away the food is, but also what direction to fly in (14)_____ to the Sun. (15)_____ the Sun does not stay in one place all day. As the workers start (16)_____ to gather the food, the Sun may (17)_____ have changed its position in the sky somewhat. In later trips during the day, the Sun seems to move farther and farther toward the west. Yet the worker bees seem to have no (18)_____ at all in finding the food source. Their inner (19)_____ tell them just where the Sun will be, and they change their course (20)_____.