A Handler comes in each day with the birds, they are a deterent, but I was told that the younger pigeons are not afraid so once in a while they bring in an older bird that is happy to kill,
So its a mixture of regular daily visits with a couple of birds as a deterent and a less frequest visit with a "Killer" to educate any newcomers.
I was told its not just Trafalgar Square, 6 or more locations in Central London get crack of dawn visits,
They just happened to be later in the day on Sunday as the square was in lockdown for a concert happening at 11am, so they came in between 8 and 10 to give the pigeons an extra nudge!