Every 15 seconds, Skyfall's wave number ticks up, spawn rate increases, and — from wave 3 onward — the Phantom bird joins the flock. None of those numbers were arbitrary; they came out of a lot of playtesting against a simple question: how do you make a 60-second round feel like it's escalating without becoming unfair in the last 20 seconds?
We landed on 15-second waves because it splits a 60-second round into exactly four distinct phases — long enough that each wave has its own identity (you can feel wave 2 being different from wave 1), short enough that the escalation feels continuous rather than like discrete difficulty jumps. Spawn rate increases roughly 12–15% per wave rather than linearly stacking, because a flat per-wave increase compounds too aggressively by wave 4; a slightly sub-linear curve keeps the last 15 seconds hard without making it feel like the game is punishing you for surviving.
The Phantom bird's wave-3 entry point was one of the most-adjusted numbers in the whole game. Introduce it too early (wave 1) and new players never get a clean read on the base bird types before a fast, erratic, high-value target starts competing for their attention. Introduce it too late (wave 4, the final wave) and most players never see it at all, which makes a whole enemy type — and the highest non-boss score value in the game — invisible to the average session. Wave 3 is the sweet spot: players have had two full waves to learn crows, swifts, and the dove-avoid rule, and there's still a meaningful stretch of round left to actually engage with the new threat.
Boss birds are handled separately from the wave clock — they arrive periodically rather than on a fixed schedule, specifically so a boss encounter doesn't reliably line up with a wave transition and double-stack difficulty spikes.