I started by recording multiple layers of my Roland Alpha Juno keyboard into the computer. The patch uses the filter going into self resonance with a high-speed LFO adjusting the cut-off frequency of the filter to get a moving squeaky sound. A bit bird like!
I played the notes in by hand un-quantised to get a bird like timing.
The rest of the sound-design is layers of effects in REAPER. There is a delay to fake the number of "birds" (eg. get more layers). Chorus to imitate more birds, and also to give it a classic sci-fi feeling (adding to the "impossible space"). Stuttery / glitch plugin is in the chain to give a robotic feeling. I imagine robot birds glitch and stutter. The final few effects are a few convolution reverbs, one using the impulse of a spring reverb: something a bit other worldly / not a realistic / not in the real world reverb for the "impossible space" part of the challenge.
Here it is over at my SoundCloud.