Because of a soundproof jacket.

No.

Usually, when birds fly, they make sound.

This is because when they flap their wings, the wings create violent, unsteady movements in air.

This is called turbulence.

Turbulence produces sound.

However, an owl's wings have some special features that help it to fly silently.

What features?

The leading edge of an owl's wing has feathers that are serrated like a comb.

When an owl flies, these serrations break air into smaller streams.

Then, as the smaller streams of air move towards the trailing edge.

The soft fringe on the trailing edge breaks up the smaller streams even more.

This causes very less turbulence, thus creating very little sound.

However,

Even this little sound produced by turbulence is absorbed.

By the velvety feathers present on an owl's wings and legs.

Thus helping owls to fly silently.