1 . Unlike humans who enter a state of relatively complete unconsciousness while sleeping, birds can more carefully control their sleep. Birds often sleep with one eye open, only half their brain resting at the same time. The other half of the brain is awake, able to note danger if needed.
While the exact way in which birds control their sleep patterns has not been well studied, it has been shown that the safer a bird feels when sleeping, the more likely it is to sleep deeper. If the situation is more uncertain, however, the bird will sleep more lightly.
Sleeping together is another defensive method that many birds use. By living together, some species can create a nighttime place where thousands of individuals sleep. As a result, there are more birds to notice their enemies’ attack, giving each individual bird a greater chance of survival. In winter, many birds, particularly small birds, live together to share body heat and survive lower nighttime temperatures.
When birds sleep, they bury weak body parts in their feathers. A bird’s feathers create air pockets that help it keep warm, and by placing feet or the bill into the feathers, less body heat is lost. When a bird’s bill is buried deeply in its feathers, it is also able to breathe air warmed by its own body heat.
There is little data about how long birds sleep at a time, but studies show that birds may sleep longer during longer nights. This may be because birds are not able to search for food or take part in other activities without enough light and so sleeping is the next natural item on their to-do lists. Birds are able to sleep for a short time during the day, however, and can also sleep longer whenever they are in a safe place.
1. How can birds control their sleep?| A．By opening both of the eyes. | B．By ignoring danger frequently. |
| C．By entering a dream completely. | D．By keeping half of the brain awake. |
| A．Birds’ condition. | B．Birds weight. | C．Birds’ feelings. | D．Birds’ sleep patterns. |
| A．They can attack their enemies. | B．They can protect themselves well. |
| C．They can lower body temperatures. | D．They can share body heat in summer. |
| A．Into their feathers. | B．Into their feet. |
| C．Into their mouths. | D．Into their skin pockets. |
| A．Birds can search for food in weak light. |
| B．Birds should plan their activities more carefully. |
| C．Birds can usually sleep longer at night in winter. |
| D．Birds should place sleeping first on their to-do lists. |
