North American songbirds have been shrinking steadily in size over the past 40 years, according to scientists who measured tens of thousands of the feathered creatures from dozens of different species and attributed the changes to rising temperatures.

As the birds’ bodies got smaller, their wings gradually got longer, the scientists said in a paper published Wednesday in the journal Ecology Letters. The longer wings, the researchers said, may help offset the loss of body mass so the birds can fly efficiently on their long...