For 25 years the U.S. Air Force Band has offered free "guest artist" concerts in February and March. The artist contributing his time Sunday was baritone Robert Merrill, who joined the U.S. Air Force Concert Band and Singing Sergeants in a well-received program at DAR Constitution Hall.

Although he is over 70 and his final high notes are sometimes clipped short, Merrill still has the tonal warmth and flexibility that brought him fame during his operatic career. He sang Bizet's "Toreador Song" with fluid phrasing, style and a charmingly soft croon on the "l'amour" passage. His wife, Mary, provided fine piano accompaniment for two songs he has performed many times: Figaro's "Largo al factotum" and Tevye's "If I Were a Rich Man." Merrill sang the first piece at breakneck speed and with excellent diction. For the second, he magically created "Fiddler on the Roof's" main character by singing, sighing, speaking and even imitating the geese squawking on the "rich man's" front yard.

Amy R. Mills led the band in a polished performance of Morton Gould's "American Salute," but the group's rendition of Bernstein's overture to "Candide" was less successful. The light, wispy touches in this piece were lost, partly due perhaps to the band's limited string section. The Singing Sergeants performed with energy and a nice blend, only occasionally covered by the band's volume. They coolly came to the rescue during the salute to the armed forces: The melody lines of the services' theme songs are scored for a tenor, and when Merrill had to drop out, the Sergeants increased their volume and kept up the music beautifully.