| redshank | (noun) a common Old World wading bird with long red legs | Synonyms: Tringa totanus |
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| redshift | (noun) (astronomy) a shift in the spectra of very distant galaxies toward longer wavelengths (toward the red end of the spectrum); generally interpreted as evidence that the universe is expanding | Synonyms: red shift |
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| redstart | (noun) European songbird with a reddish breast and tail; related to Old World robins | Synonyms: redtail |
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| (noun) flycatching warbler of eastern North America the male having bright orange on sides and wings and tail | Synonyms: American redstart, Setophaga ruticilla |