What did crows do to researchers who once trapped them?

Remembered them, and told the others. Researchers at the University of Washington wore a distinctive mask while trapping and banding crows, then walked the campus in the same mask afterwards. Birds scolded and mobbed them - and so did birds that had never been trapped, including birds that had not been alive at the time.

The effect spread and persisted. The proportion of crows reacting to the mask increased over the years following the trapping rather than fading, which means the information was being transmitted from bird to bird and from parents to young, not merely remembered by the individuals who suffered. A neutral mask worn in the same places drew no reaction, so it was the specific face rather than the presence of a person.

It sits alongside a body of work that keeps embarrassing the phrase bird brain. New Caledonian crows manufacture hooked tools from twigs and keep favourites between jobs; one solved an eight-stage puzzle requiring tools to be used in sequence on the first attempt. Corvids cache food and will re-hide it if another bird watched them, which implies some model of what the other bird knows. All of it runs on a brain the size of a walnut, built to an entirely different plan from a mammal's.