When the pigeons eat the bait, they will emit distress signals which will frighten the rest of the flock and cause them to leave the area. 

Pigeon droppings carry a disease called Cryptococcosis. Many Pulmonary infections due to Cryptococcosis may go un-reported. Pest birds cause millions of dollars every year to homes and buildings, from nesting to feces accumulation. Bird nests in chimneys and ventilation systems not only spreading disease through the system but can block air flow which can have horrible consequences.