Sentence:  Naturally , Mr. Deller and the other singers in his troupe <WSD>are</WSD> most charming and elegant when they are squarely in their tradition and singing music by their countrymen : William Byrd , Thomas Morley and Thomas Tomkins . 
Meaning of the word marked with <WSD> tags:  have the quality of being; (copula, used with an adjective or a predicate noun)