Clocks or ETS if the birds are not into the routine of entering the trap ASAP, the sprint races will be won or lost. Having used a clock for many years and now using ETS, I would never return to using rubbers and a clock. The advantages or disadvantages are far outweighed by the stressless clocking of your birds at all times. Having had the same pigeon come two or three times as my first bird with ETS they don't get themselves into a nervous or anxious state. My reason for having ETS is it is the best system for my BIRDS