Read the text. For questions 17 – 24, use the word on the right to form a word that fits in the gap. For each question, write your answer in the gap.
The migration of birds
Migration is the perilous seasonal journey undertaken by many bird species. In the northern hemisphere it is prompted by the17of food. Migrants are also18programmed to respond to the changing length of the day as autumn approaches. Nevertheless, in the tropics, where there is little variation in the amount of daylight, migration is still a surprisingly common19.
Many birds will display considerable restlessness before beginning their journeys. Their20to the earth's magnetic field helps them navigate, but inexperienced birds may get things21wrong and end up far from their intended destination.
In the past, the return dates could be predicted with great precision but climate change makes this harder. Although it is22for birds to return earlier than their rivals so they can establish territories, getting back too early could have incalculable consequences for their long-term survival. However, some birds are23reducing the distances they migrate in response to a milder climate. Their adaptability in such a short period in24terms has greatly surprised scientists.
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