Per the Farmer's Almanac, the leaves of some deciduous trees do indeed curl upwards before storms, reacting to atmospheric changes.
But does seeing the undersides of leaves really mean rain is on the
way? In this case, our forebears were definitely onto something. The
leaves of deciduous trees, like maples and poplars, do often to turn
upward before heavy rain. The leaves are actually reacting to the
sudden increase in humidity that usually precedes a storm. Leaves with
soft stems can become limp in response to abrupt changes in humidity,
allowing the wind to flip them over.
FarmersAlmanac.com - Can Leaves Predict a Storm?
From below they would appear to whiten before the storm or as the old adage puts it,
“When leaves show their undersides, be very sure rain betides.”
As to the latter point, this is a rook (in this case an American Rook). No special symbolism required.