This is a condition caused by a mite called ‘Knemidocoptes Mutans’ which burrows under the skin on a bird’s legs to feed on the keratin. The tunnelling causes irritation with leg scales becoming thickened, as the keratin seeps from the leg tissue, and eventually becoming encrusted and lifting away from the leg. The legs will appear lumpy with large gaps visible between the individual scales.
This is generally not considered a painful condition, but can cause discomfort.