The question could be paraphrased as, "When do shrews become part of the airborne plankton we call dust?" This important question was answered by P. J. Darling in his paper "The origin of the fauna of the Greater Antilles,” Quart. Rev. Biology 13:274-300.
Here is the answer. Given a 5-gm shrew of 10 square cm in broadside cross-section, a 25 km/hr wind, which has a 0.5 g/square cm dynamic pressure, would bowl him over and over. A stronger wind, not unusual, would keep him airborne for some time. Well, the shrew should face into the wind to reduce his cross-section, reduce its (this isn’t a gender issue) exposure to the dynamic pressure of the wind, and so stand its ground. Or if just a bit smarter, the little fellow could dive for cover. CED readers [CED 1.2] might remember the little contribution on tornado transport of fish from lake to lake at North Temperate Lakes LTER. Getting life in the air is not all that hard but to understand it you need to know how the dynamic pressure increases with wind speed. It’s not linear.
Dynamic pressure is a bit complex. It is 0.5*(1+C)(density)(velocity squared). Dynamic pressure is the sum of that normal to the object on the windy side plus the suction pressure on the downwind or backside. C is 0 for a cylinder and 1 for a horizontal plate shaped obstacle. The broadside-shrew comes closer to the horizontal plate and so there must become suction on the downwind side that "pulls" the critter in the direction that the wind is pushing. Anyway, getting living stuff airborne is a velocity squared and sucking thing. Linear thinkers wouldn't understand.
I have yet to find evidence of shrews falling from the sky but did find mention of a turtle from above. [Source: Nature 125:728, 1930]. The location of the turtle fall is Borina. Borina is a small town just 8 miles east of Vicksburg, Mississippi. The turtle in question was a gopher turtle (Gopherus polypnemus). Vital statistics were 6 by 8 inches. It fell encased in ice during a hailstorm. There was a tornado on the day of the turtle fall and that might explain how this 747 version of aero-plankton got up to ice-making altitudes in the first place.