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From tiny hummingbirds()to giant whooping cranes,roughly half of the world's more than10,000 bird species migrate().Longer wings and powerful flying muscles often help these birdscross vast expanses of air.But a study of nearly all bird species suggests many migrators share anotherunexpected flight aid:lighter-colored feathers.Being a bit more lightly colored than non-migrating birds may help these long-distance fliers staycool as they work hard under the hot sun to fly,researchers report December 6 in Current Biology.It's known that color can help birds hide from predators()by blending in,or attract matesby standing out.But color has subtle effects too,including regulating temperature by absorbing orreflecting light,says Kaspar Delhey,an expert at the Max Planck Institute for Omnithology in Germany.Forexamp,bird ggs laid or climates tend to be darker,which may help keep themwarm.Migrating birds push their bodies to the physiological limit,which creates much heat.Some species cope by rising to cooler air during daytime."If overheating is a problem in migratorybirds,another way of dealing with that would be to evolve lighter colors"that absorb less heat,Delheysays.Delhey and his colleagues analyzed over 20,000 illustrations of 10,618 bird species,rankingfeather lightness for each species and comparing that with how far the birds fly.On average,lightnessslightly increased with migratory distance,the team found.The longest-distance migrators were about 4percent lighter than non-migrators,an effect that wasn't explained by size,climate or habitat type fordifferent species."It's not a big difference,"Delhey says,noting that many migrators are darkly colored,perhaps forreasons unrelated to flight.But the trend was remarkably consistent."Very different groups with verydifferent biologies show this pattern,"Delhey says."That surprised us."28.Which of the following ignored previously help migratory birds fly?A.Longer wings.B.Vast expanses of air.C.Lighter-colored feathers.D.Powerful flying muscles.29.What does the underlined word"subtle"in paragraph 3 probably mean?A.Smart.B.Terrible.C.Ambiguous.D.Tender.30.How do migrating birds deal with overheating in them?A.By evolving lighter colors.B.By reaching bodies'limit.C.By flying higher to the sky.D.By absorbing more lights.31.What can be inferred from Delhey's study?A.10,618 bird species fly over a long distance altogether.B.Species of migrating birds'feathers vary significantly.C.Migrators with dark feathers are clearly related to flight.D.Migratory distance can be judged by feathers'lightness.一次梦按晒