Limping wood pigeon in garden

This evening I saw an adult wood pigeon appearing to limp around our garden. He is one of two that have been visiting our garden regularly for nearly 2 years, and since he is usually very active and feisty this came as quite a shock.

Unfortunately I wasn't able to get a photograph or a closer look at him or his leg, but standing a few feet away, there didn't seem to be any obvious external injury to his leg. Although he ate some bread that I put out for him, and flew away to a nearby tree once he'd finished, I'm slightly concerned about his limping and was wondering what might have caused it, and if there is anything I can do to help him.