
There are at least two different mutations that can cause a pigeon to hatch with webbed feet or fused toes.
The first is web foot. This is an autosomal recessive mutation, which means that for a pigeon to hatch with webbed feet both parents must carry the gene. Depending on the extent of the webbing, the bird may or may not have mobility issues, and some breeders actually find web foot to be useful on feather-footed breeds.
Another is web lethal. As the name indicates, this sex-linked gene is lethal when homozygous (or hemizygous in hens). Affected pigeons usually die before or soon after hatching.