More than 5, 000 species of birds manage annual round-trip migrations（迁徙）. These journeys can be thousands of miles, with many birds often returning to the exact same nesting and wintering place from year to year.

Migration is very important in the life cycle of birds, and without this annual journey many birds would not be able to raise their young. Birds migrate to find the richest and most abundant food that will provide adequate energy to raise young birds. If no birds migrated, competition for adequate food during breeding（繁殖）seasons would be fierce and many birds would starve.

Of course, not all birds migrate. Some birds can take advantage of different food sources（来源）as seasons change, allowing them to stay in one place all year round. Other birds are better adapted to cold climates with thicker fat reserves and better feathers, and they can survive long cold seasons while they forage for winter food. For more than half the world's birds, however, migration is necessary to stay alive.

When the timing is right for their migrating needs, birds will begin their journey. Food, weather, temperature and illness or injury are several minor aspects that may affect migration by a day or two, but most bird species follow precise migration "schedule". While migration is at its finest during spring and fall, birds migrate all throughout the year. Migration is actually an ongoing process and there are always birds at some stage of their journeys. The distance the birds must fly, the length of time it takes to mate and the amount of young birds all affect when any one species is migrating.

Migratory birds have several changes before the journey. Among them is hyperphagia, the process of migration-related weight gain. As daylight changes and migration times near, a bird's hormone levels will change and they will build a greater fat supply. Besides, old, ragged feathers create more wind drag and air resistance, which requires a bird to use more energy in flight, so many birds replace old feathers with new ones.

With a flock of birds, a group member can often be seen high in the tree acting as a lookout while the rest feed on the ground below. When a predator approaches, the lookout sounds the alarm.

To the casual observer, it may appear that the lookout is acting for the good of the greater group, but a new study suggests that the lookout – called a sentinel(哨兵) – may actually be working for itself.

While sentinel behavior among groups of birds has been observed, Dr. Roni Ostreiher and Professor Aviad Heifetz recently began researching the sentinel behavior of floaters –birds that have either been cast out or for other reasons decide to go it alone. What they found was that floaters also have similar lookout activities and will sound an alarm call if an enemy approaches.

This suggests that while the group benefits from having a lookout, the instinct to act as the lookout is motivated not by the group but by self-preservation. The sentinel may simply be trying to scare away its enemy instead of warning other birds down below.

It might be easy to understand that when a floater displays sentinel behavior, the bird is acting for itself. However, birds in groups may have less selfish motives since the birds appear to alternate as lookouts. “There are some adult group members who act as sentinels more often than others. As far as we understand, acting as a sentinel is not a duty. Those who sentinel less often are not punished,” Ostreiher said.

He also noted that the sentinel's primary purpose is to gather information about its surroundings – the presence of a neighboring group, changes in food sources and even weather – not keeping lookout for approaching enemies.

“We do not know what motivates them to stop eating and climb up to a tree branch and start to sentinel,” Ostreiher added. He and Heifetz are now focused on understanding what happens after an alarm call is sounded.

From July to October every year, about a quarter of the world’s blue whales feed in the Pacific Ocean off the coast of California. But the whales currently face a major threat in their favorite feeding area. Ships carrying cargo (货物) sail in the same area at the same time. All too often, the whales’ paths and the ships’ travel lines overlap (部分重叠), and a ship will hit a whale.

According to a new study, these ship strikes have become a serious threat to the overall population of the world’s blue whales. Only about 10,000 of the creatures still exist worldwide. Blue whales are the largest known animals ever to live on Earth. Even so, if hit by a container ship, a blue whale will likely die from its injuries.

In 2007 alone, large ships killed five blue whales in the waters of San Francisco and Los Angeles, California. The National Oceanic and Atmospheric Administration (NOAA) says that because there are so few whales already, losing three to five from the California whale population every year is a significant loss. “The estimated population of blue whales in this part of the Pacific is 2,500,” says Sean Hastings, an analyst of NOAA. “So every whale counts toward this species moving off the endangered-species list.”

Now, marine scientists must figure out how to protect the whales from the giant container ships. One very simple program is already underway in the Santa Barbara Channel, a waterway that separates mainland California from the nearby Channel Islands.

The Channel Islands National Marine Sanctuary has asked large container ships passing through the area to voluntarily slow down. Sailing slower will allow the ships’ crew (船员) more time to change course before hitting a whale.

Several of the world’s largest shipping lines are set to participate in the new program. For every ship that passes through the Santa Barbara Channel at or below the reduced speed of 12 knots, the company that owns the ship will be paid $2,500.