Birds (鳥) appear as one of the stage enemies featured in the NES Strider.
A flock of birds also appears briefly in the Strider Hiryu manga. Native to the Amazon forest, these birds are startled as Hiryu and Kain move across the forest trees. They then observe as one gets split in half mid-flight, having inadvertently fly into one of the ultra-sharp wires setup by Strider Arana, revealing the existence of his almost-invisible wire cage trap to them.
Information
Appears in Africa
The dense jungle areas of Africa are inhabited by small, brown-feathered birds that serve as one obstacle during Hiryu's visit of the area. These birds have a very simple pattern: they come into view from either the right or left and slowly fly forward while waving up and down all the way across the screen. They often spawn at the same height Hiryu is located, which makes traveling through the upper vines and avoiding falling into the spikes below a bit more difficult.
These birds are a bit annoying, but otherwise easy to deal with as they can only harm Hiryu via direct contact, and a single strike of the Cypher eliminates them.
Trivia

- The original Arcade Strider has 4 unused sprites found in its code, for a little flying bird with green feathers and a set of red ones on its head. While it is unknown what this bird's function or purpose was, based on its exotic appearance it's possible it was intended to appear as a harmless ambient object for the Amazon stage, similar to the Piranha.