The harvest mouse is tiny enough to cling to a single blade of grass with all four feet. But being small hasn’t stopped it from becoming a remarkably capable builder.
Sources:
Mammal Society — Harvest mouse — probably the best single source for your script. It confirms the 4–6 g weight, 50–70 mm body size, prehensile tail, and—most importantly—the nest construction: leaves split lengthwise and woven together while still attached to the living plant.
Animal Diversity Web — Micromys minutus — good secondary zoological reference covering range, behavior, diet, reproduction, and the globular woven nests.
Mammal Society — Harvest mouse discovery on Isle of Mull — a newer 2026 source if you want something current showing the species is still actively surveyed and turning up in unexpected places.