Squirrel baby carried down tree

I had seen the mother squirrel carrying nesting materials up the tree but what was it bringing down? In its mouth was a fluffy ball of baby squirrel. I watched it hop along and then up a tree covered with dense ivy, presumably providing greater protection from predators than the Scots Pine it was vacating.  What was the threat? Crows nest in the same tree but they’ve been neighbours for a long time. A buzzard was flying not far above the tree. Maybe it was a hungry pine marten?