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The Boid is a non-hostile bird-like creature seen on Xen. It is made of two wings and glide in flocks of three or more, flapping its sides and emitting soothing resonant calls.


Boids Xen

Standard Boids on Xen in Half-Life.

Standard Boid[]

The standard Boid's wings have several shades of blue, pink and yellow and its main body is grey. At the center and on each side of its body, the creature seems to have four red eyes and a mouth. It is the first variant seen in the series, during G-Man's speech at the end of Half-Life, where two flocks of are flying around islands in the third teleport location.

Two flocks of standard Boids are seen in Opposing Force as well, in an area of Xen accessed from the Biodome Complex with the Displacer Cannon.

Yellow Boid[]

Only appearing as three flocks when on Xen near the end of Blue Shift, this variant is completely yellow and has three red eyes on each side of its wings, but looking straight ahead, unlike to the standard Boid that has its eyes looking upwards and downwards.

Behind the scenes[]

Boids real

Screenshot of Craig Reynolds' original Boid simulation as a Java applet.

  • The term "Boid" comes from the games files, where it is used many times: as the name of the model used by the standard Boid ("boid.mdl", that of the yellow Boid being named "aflock.mdl"), as the name of its sound folder and sound files, and in subroutines dealing with its behavior in the file "aflock.cpp" featured in the Half-Life SDK and refers to what it is video game-wise:
  • A "Boid" is a virtual entity used in "Boids", an artificial life program simulating the behavior of birds or fish in their flocks or schools, respectively (with each other, as well as their environment), developed at Symbolics Inc. in 1986 by Craig Reynolds, an artificial life and computer graphics expert, and named "steering behavior".[1] The source code has been released a few years after 1986, allowing the creation of many variants since then, including Valve's creature (that even resembles the original Boids by Reynolds). It is also implemented in the free C++ library OpenSteer, designed to help construct steering behaviors for autonomous characters in games and animation.
  • The term "Boid" is the abbreviation of "birdoid" ("like a bird"), as the program rules applied equally to simulated flocking birds and schooling fish. It was also inspired by a scene from Mel Brooks' 1968 film The Producers where the playwright's landlord complained about his keeping pigeons on the roof, referring to them as "boids", or "bird" in a stereotypical New York accent. The term also linked up with the ellipsoid-based 3D modeling tool by Tom Duff at the Graphics Lab of New York Institute of Technology named "soids".[2] Therefore, in regard of the source of the creature's name, its proper in-universe name remains unknown.
Boid wounded

The Yellow Boid's "wounded" animation, having it flying with difficulty.

  • The yellow Boid featured in Blue Shift was to appear in Half-Life, as it is featured in the latter's files, and the SDK. It made its way into the canon through Blue Shift like the Chumtoad, also initially cut.
  • The Boid's behavior was originally to be more diverse. While the standard Boid has only one animation ("idle", showing it simply flying), the yellow Boid model features two other flying animations and several interaction animations such as "hit", "wounded" and "crash", showing that the player was originally to be able to kill it, added to unused "alert" sounds ("boid_alert" 1 and 2) found in the sound files. The file "aflock.cpp" found in the Half-Life SDK and again related to the yellow Boid also mentions that there was to be a leader forming a flock from surrounding Boids, and that the newly formed flock was to go idle after being formed and wait for a player to come along, then start reacting to its presence. It also mentions that after killing a leader the next Boid was to become the new one. While the yellow Boid was brought back for Blue Shift, it was without the cut features previously mentioned, having it only use the simple "idle" (flying) animation, as in Half-Life.


List of appearances[]


  1. Boids (Flocks, Herds, and Schools: a Distributed Behavioral Model) on Craig Reynolds' official website
  2. Roadmap-based methods for flocki - Appendix A on the University of Maryland - Department of Computer Science's official website

External links[]