The migration of whales
Migration is the perilous seasonal journey undertaken by many whale species. Across the northern oceans it is prompted by the (17) ________ of food. Migrants are also (18) ________ programmed to respond to the changing temperature of the sea as autumn approaches. Nevertheless, near the equator, where there is little variation in the temperature of water, migration is still a surprisingly common (19) ________. Many whales will display considerable agitation before beginning their journeys. Their (20) ________ to the ocean’s magnetic field helps them navigate, but inexperienced whales may get things (21) ________ wrong and end up far from their intended route. In the past, the arrival dates could be predicted with great precision but climate change makes this harder. Although it is (22) ________ for whales to arrive earlier than their rivals so they can feed first, getting back too early could have incalculable consequences for their long-term survival. However, some whales are (23) ________ reducing the distances they migrate in response to a milder climate. Their adaptability in such a short period in (24) ________ terms has greatly surprised scientists.