The rhyme:

"Seagull, seagull sit on the sand. It's never good weather when you're on land."

The reason:

Seagulls are naturally at home in flight, and where they can, they sleep on the water. When the wind is too turbulent, seagulls prefer not to fly.

Usually a turbulent wind will also mean a choppy, unstable sea for them to rest on so this is an occasion when seagulls will move to land to rest. However, seagulls on the ground are not an accurate predictor of bad weather - they are a sign that the weather is already bad.