1 . In habitats across the planet, animals periodically drop everything to walk, fly or swim to a new place. Wildlife such as whales and geese learn migration paths by following their parents. Others, including small songbirds, gain the distance and direction of their migration within their genetic code. And some animals use a combination of genetics and culture to guide their migration.

Another group of migrators does not quite fit either model, and researchers have only recently started to figure out how they find their way. Take the Cory’s shearwater, an oceangoing sea bird that migrates over the Atlantic every year. The young do not migrate with their parents, so culture cannot explain their journeys. And the exact paths vary wildly from individual to individual, making genetics equally unlikely.

Cory’s shearwaters are long-lived, rarely producing young successfully before age nine. This leaves an opening for learning and practice to develop their migration patterns. Researchers call this the “exploration-refinement”, and until now it has been hypothetical（假设的）because of difficulties in tracking migratory animals’ movements.

But a team of researchers has done that by attaching small geolocators to more than 150 of the birds aged four to nine. They found that younger birds traveled longer distances, for longer periods, and had more diverse paths than older birds.“We finally have evidence of the ‘exploration-refinement’ for migratory birds,” says Letizia Campioni, who led the study. Younger Cory’s shearwaters are able to fly just as fast as, the adults — but they do not, suggesting that the young do more exploring, which gradually fades as they mature and settle into a preferred course.

Although it may seem less efficient than other strategies, “exploration-refinement could be beneficial to birds and other organisms in a rapidly changing world due to unpredictable man-made changes,” says Barbara Frei. “It might be safer to repeat a behavior that was recently successful than to rely on patterns that were perfected long ago but might no longer be safe.”

As you have noticed, you and a fish are different. Eyelids (眼睑), for example. Even sleep is a difference between you     1     a fish.

As far as scientists can tell, even though it’s hard     2    (study) the brains of fish, fish do sleep in     3     way. They at least rest at times, with a slowdown of their body “machinery”. And that is much like sleep.

For fish brains to relax, eyelids may not     4    (need). Some fish lie near the bottom or top of the sea or lake at night. Others hide     5    (them) in the sand or produce a covering as     6    (protect) against predators (捕食者) while their defense (防御者) are sleeping. And sharks—even the kinds     7     continually swim—still have periods of rest in between periods of activity.

Fish don’t have eyelids or need them. Eyelids help us     8    (human) by keeping our eyes from     9    (get) dry, among other things. But dry eyes are     10    (probable) not a problem for animals living in water.