Preheat the oven to 450 degrees. In a small bowl, blend the butter with the juniper and season with salt and pepper.

In a large skillet, heat 1/4 inch of vegetable oil until shimmering. Season the squabs with salt and pepper. Add 4 of the squabs to the skillet, breast side down, and cook over moderately high heat, turning a few times, until richly browned all over, about 12 minutes. Repeat with the remaining squab, adding more oil to the skillet as needed.

Arrange the squabs breast side up on a large, rimmed baking sheet. Rub the squab cavities with the juniper butter. Arrange two bacon halves on each squab breast in a single layer. Scatter the grapes around the squab and roast in the upper third of the oven for about 15 minutes, until an instant-read thermometer inserted in the thickest part of the legs registers 125 degrees for medium-rare meat. Transfer the squab to a carving board and let rest for about 5 minutes.

Using a large knife, cut each squab in half, cutting through the breast bone. Transfer the squabs to plates. Spoon the grapes and roasting juices on top and serve.