A new study is trying to answer the question that has divided biologists since a long time, which is the innermost digit of a bird's three-pronged wing is like a thumb or an index finger.

According to the new study published online by Nature on Sunday concludes that it is a bit of both. The study found that the stem cells that produce the first digit die off during early stages of embryonic development and cells responsible for developing an index finger develops a thumb-like appendage.

Birds, unlike all four-legged animals with backbones that have five digits per limb, have three in their wings and two, three or four digits on their feet. Biologists have been debating since a century as to whether the tripart scaffolding of a bird wing represents thumb, index and middle finger, or to the index, middle and ring fingers.

According to findings of the new research, the "two-three-four" scenario is the most likely. Researchers from Yale University led by Gunter Wagner used gene expression profiling technique to solve the mystery.

"We used a new technology called transcriptome sequencing. It has been around for a few years and we happened to be first to use it for this question," he said. "We want to find out how they acquired a unique identity."