Climate change can strike at each stage of their annual trek, from breeding ground to rest stops to their final destination.
Studies cited by the convention say arctic permafrost and tundra where many species breed are melting. Even moderate rises in sea levels can swamp wetlands where birds stop to feed. Deserts are expanding, lengthening the distance between rests.
Hepworth recalls watching great V-formations of Bewick's swans arriving in his native Britain from northern Russia for the summer. Fewer are seen now.
The extraordinary travels of the red knot are another example. The medium-sized shore bird breeds in Siberia and migrates to southern Africa, shedding half its body weight under the strain of a flight that reaches survival limits. The expansion of the north Africa deserts could push them over the edge.
The convention's scientific council says 84 percent of the 235 species listed its annexes could be affected by changes in water availability, mismatched foods supplies, more frequent storms and competition with alien species intruding into their habitat.
The convention came into force in 1983 and is signed by 101 countries that pledged to help preserve the habitat of wild animals.
