injured Sandwich terns

We regularly see Sandwich Terns skittering around on the pier in Deal. The way they dart among the fishermen and tourists, sometimes up on the benches, other times along the pavement, is often very funny and cute. Yesterady we saw two or three with one foot quite badly damaged or possibly deformed. they seemed to be getting about OK, but clearly they aren't as fast as their two-footed competitors. Does anyone know why this is happening to them?