We made a ladder trap for the shoot a few years back , we had loads of the blacks .A keeper pal told me never go near it during the day , always empty it at night which we did & it worked a treat ,then it stopped catching completely .We found out one of the guys on the farm had been "helping" by emptying it if he see any birds in it .they steered well clear of it then .We put it away then for a year before trying again but it was never as effective , they are one clever bird , know wonder the corvids do so well ..