Given the question: Read the following article and select the best answer. Article: Their beautiful call suggests the arrival of spring,but the routes taken by cuckoos during their annual migration have remained a mystery.Now,though,scientists from the British Trust for Omithology(BTO)have revealed the details of their remarkable 10,000-mile journey,with the help of tracking devices. Five male birds were caught in Norfolk last May and fitted with satellite-tagged "backpacks'' before being released.The scientists then monitored their progress over 12 months as they flew to Africa in the autumn,and returned in the spring. The project was carried out to discover why every year fewer of the birds return to Britain.Between 1995 and 2010,the population of cuckoos spending sumnler here fell by almost half,and the number is continuing to decline.Previously,the lack of information about the cuckoos'long migration has hamoered the understanding of how to help conserve the birds. Earlier this year,it was reported that cuckoos arrived in the UK much earlier than normal.Their call was recorded by ornithologists as early as February,a month ahead of their usual arrival. Like all migrating animals,cuckoos respond to the changing seasons.They depend on lush plants to provide the fruit and food for insects that they feed on.This reliance on seasonal patternsmeans that a changing climate could make an already challenging journey impossible for them. Question: Cuckoos arrived in the UK much earlier than normal probably because of _ . - 1acking food - changing climate - changing routes - cold spring in the UK
The answer is:
changing climate