From ARC

In our last post, we looked at the incredible structure of owl feathers and how they allow these birds to achieve near-silent flight. One of the keys to that stealth is their velvety softness, which helps muffle sound and reduce air friction.

This velvet texture is created by structures called pennula, which are long, wisp-like filaments on the feather. Under a microscope, you can clearly see these long filaments on a tail feather from a great horned owl (first picture).

For comparison, the second image shows a microscopic view of a red-tailed hawk tail feather. This feather lacks those long, downy filaments. Instead, it features a tight, interlocking alignment of barbs and barbules, creating a smooth, streamlined surface built for aerodynamic efficiency instead of quiet flight.

Here's their original post. I think most of you have already heard me talk about this part a bunch, so I was originally going to skip it, but since they mentioned it, here it is.

Most owl species have incredibly soft feathers, which allow them to hunt almost silently and sneak up on unsuspecting prey in the dark.

Great-horned owl feathers, like Nonamé's in the first picture, are an excellent example. Great-horned owls have wide, oversized flight feathers that create a massive wing surface area relative to their body size. This low "wing loading" gives them tons of lift with minimal effort, meaning they barely have to flap to stay aloft. Less flapping automatically means a much quieter flight!

The feathers themselves are designed for stealth. Some of the primary flight feathers feature comb-like serrations on the leading edge (shown in the second picture) that break rushing air into tiny, quiet micro-currents. A velvety surface texture then absorbs any leftover sound, while a fringed trailing edge cleanly blends the air currents as they leave the wing (third picture).

A deeper dive:

From The feather's multi-functional structure across nano to macro scales inspires hierarchical design

Aerodynamic constraints drive microscopic functional morphology: variation in barbule shape can be examined between species, across a wing and even over a single feather. For example, some distal barbules bear lobes on their dorsal side (figure 6D) that interact with the ramus of other feathers, creating mechanical fastening zones between two adjacent overlapping feathers to form a continuous wing planform.

Termed 'directional Velcro', these distinctive underlapping three-dimensional hooking microstructures probabilistically fasten on to overlapping two-dimensionally hooked rami during wing extension (when the wing is spread out), and automatically unlock during wing flexion (when the wing is tucked in). This unique probabilistic mechanical fastener is secure, reversible and repeatable, but also makes a Velcro-like noise.

Consequently, silent fliers lack them. Instead, elongated pennula create a velvet-like feather surface that makes owl flight extremely quiet (figure 6E). These pennula reach their full length only in wing regions where flow separation occurs.