In the New York City area, bird's nest spruce (Picea abies 'Nidiformis'), unfortunately, can perform poorly with the hot, humid summers of USDA zone 7. It really prefers a cooler summer and should be planted in locations where there is good air circulation and well-drained soil to reduce humidity. In addition, the roots deteriorate in soil that isn't draining well so check if the soil around the sickly plants has become compacted.

Bird's nest spruce plants that become weakened for any reason (drought, hot summer conditions, poor drainage) are susceptible to some common diseases. Cytospora canker is one possibility that would produce the results you describe. You will need to examine the plants closely to look for disease markers and may want to take a magnifying glass with you. This article from University of Massachusetts has photos and a description of what to look for in the case of cytospora. Rhizospheara needle cast is another possibility, and you will find its photos and description here.  Otherwise, look for signs of insects, poor soil conditions, wood decay or spruce needle rust.

Courtesy of NYBG Plant Information Service