||Briskly simmer jam and vinegar in a small saucepan, uncovered, stirring occasionally, until glaze is reduced to about 1/2 cup, about 8 minutes. Cool to room temperature (glaze will thicken slightly as it cools).
Preheat oven to 450°F.
Cook bacon in batches in a large heavy skillet over moderate heat, turning occasionally, until some of fat is rendered but bacon is still translucent and pliable, 5 to 7 minutes. Transfer to paper towels to drain.
Cut out and discard backbone from each hen with kitchen shears, then halve each hen lengthwise. Pat hens dry and season with salt and pepper, then arrange, cut sides down, in a large roasting pan. Brush hens liberally with glaze, reserving remainder, and wrap 2 slices of bacon around each half hen, tucking ends under. Roast in middle of oven, brushing with pan juices and reserved glaze twice (every 10 minutes), until juices run clear when a thigh is pierced, 30 to 35 minutes.