Birdsong at Morning

The passing of time and the nature of solitude are heavy topics, but in the hands of acoustic music ensemble Birdsong at Morning, six meditations on their soon-to-be-released CD, "Vigil," see the light at the end of a dark tunnel. Alan Williams, Darleen Wilson, and Greg Porter make an appearance on Patty Larkin's new release, and at this gig, the trio welcomes a string quartet with ties to the New England Conservatory. The concert supports this third of four installments. -- June Wulff, Globe Staff