If Air Force researchers have their way, the military’s next flying robots of doom will be tiny, and indistinguishable from the naked eye from small birds, bats or even insects. And they’ll take their first flight in a freaky “Micro-Aviary” in Ohio, where engineers make mini-machines modeled on those creatures of the sky.

Miniaturization is a major trend in drone tech. The Army’s new Switchblade drone is a semi-autonomous missile shot out of a mortar tube for kamikaze missions. Some robotic aircraft manufacturers, like the micro-machinists at AeroVironment, have even started experimenting with...

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