I'm not sure that a speaker saying "nigger nigger nigger" or "jews rape kids" would be productive, but one can come up with a suitable message, such as "vaccine deaths are at acceptable levels".
You'd want a device that can power itself, either off solar or off the mechanical motion of the bird, or both. You'd want the battery to charge up to, say, 20% before turning the speaker on.
A practical obstacle is finding or making such a device, and then catching a pigeon so that it can be attached.
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