Some bird feet are covered by feathers (pictured) instead of scales and show greater similarity to wings than to legs at the molecular level.

Michael Shapiro at the University of Utah in Salt Lake City and his colleagues crossed breeds of domestic pigeon (Columba livia) that had feathery or scaly feet and analysed the offspring to identify genomic regions that contribute to this unusual trait. They also compared the genomes of 15 feather-footed and 28 scale-footed pigeons.

Credit: Bonzami Emmanuelle/Alamy Stock Photo

They found that the hind limbs of birds with feathery feet showed gene-expression patterns that resembled those of wings, or fore limbs. In pigeon and chicken embryos, a gene called Tbx5 that normally regulates forelimb outgrowth and identity was expressed in the developing legs of birds with feathery feet. Furthermore, the expression of Pitx1, a hindlimb gene, was lower in these birds than in those with scaly feet.

eLife 5, e12115 (2016)