Word formation

For centuries, the autumn disappearance of swallows was one of nature's most 1. events; respected naturalists once argued that the birds spent the winter asleep in the mud at the bottom of ponds. Today the challenge is not mystery but detail. Tracking devices lighter than a paperclip now allow scientists to follow individual birds across entire continents, and the results have revealed feats of 2. that would have seemed fanciful a generation ago. One bar-tailed godwit tagged in Alaska was recorded making a 3. flight of more than 13,000 kilometres to Tasmania without once landing to feed or rest. Two research teams, working 4. on opposite sides of the Pacific, have reached strikingly similar conclusions about how such birds prepare: they almost double their body weight before departure and shrink their digestive organs, which are of little use in the air. Yet the data also carry a warning. As weather patterns grow more erratic and 5., birds that time their journeys by day length rather than temperature may arrive to find that the insects they depend on have already peaked. The long-term 6. of several species is now in doubt, with some populations shrinking 7. quickly, and a 8. of safe stopover sites along the coast is making each journey riskier still.