The implication seems to be that the birds can see the dots in the daytime, although we can't:
> They also work during the daytime. But in sunlight, the human eye can only see green dots dancing across the berry-laden bushes.
> They also work during the daytime. But in sunlight, the human eye can only see green dots dancing across the berry-laden bushes.
The quote says we can.
Perhaps just the dots is enough to deter them.