Bearded Parrotbills and winter

Bearded Tits change their diet in autumn and winter and ingest grit to aid seed digestion as shown in this quote from BWPi below:

From BWPi --- Marked seasonal differences in diet and associated gut morphology, also in feeding methods. In winter, for digesting seeds, stomach develops hard plates, increasing its weight to 0·88–1·21 g, and is filled (from September) with small stones: in November–January, 12 stomachs contained on average 609 (420–850) stones of very uniform size (each 0·18–0·40 mg), total weight 0·100–0·298 g.

Clearly taking on 600 small stones must take a bit of time but where do they get them from? well this pair were busily engaged in stocking up with winter stones on a convenient pathway -- there are presumably photos of this behaviour but a quick search on the net failed to locate any --