Why not four black blades but one?

[v0031]

Black wind-turbine blade cuts bird deaths
There are good things about wind farms. They provide green energy. However, there are also bad things. One is wind-turbine blades kill thousands of birds. They accidentally fly into the blades. Researchers have an answer. Painting one blade black helps birds see the blades. This cut bird deaths by 70 per cent. A researcher said bird deaths was "one of the main environmental concerns related to wind energy". The research is published in a journal.
Researchers looked at making wind-turbine blades safer. They painted one blade of four turbines black. The turbines with one black blade reduced bird deaths. A bird protection group was happy with the black blades. It said wind farms needed to be "in harmony with nature". It added: "Wind turbines are the right technology when we find the right places for them." It said wind farms should not be a risk to wildlife.

[True Federalist (진정한 연방 주의자)]

I can think of two possible reasons.

First, one might have been enough and saved the cost of repainting all four blades.

Second, given how persistence of vision works, it might well make it easier for the birds to see the blades as moving objects instead of a solid disk if the blades aren't all the same hue.

These are just hypotheses, I have no info on what the actual reason is, nor do I care enough to find out.

[KoopaDaQuick 🇵🇸]

read "blades" as "babes"