A pin feather is a developing feather on a bird.[1] This feather can grow as a new feather during the bird's infancy, or grow to replace one from moulting.
The pin feather looks somewhat like a feather shaft. However, unlike a fully developed feather, the pin feather has a blood supply flowing through it; at this stage, it may also be called a blood feather. As such, if the pin feather is damaged, a bird can bleed heavily.[2][3][4][5]
As the pin feather grows longer, the blood supply is concentrated in only the base of the shaft, and the tip of the shaft encases the feather itself in a waxy coating. As moulting birds preen, they remove the waxy coating, and the feather unfurls.
When the blood has receded, the term "blood feather" is no longer synonymous with "pin feather" – it can only be referred to as a pin feather.