Darwin’s finches show variation in their __________.

The correct answer is:

Beaks

• Darwin's finches, inhabiting the Galapagos archipelago and Cocos island, constitute an iconic model for studies of speciation and adaptive evolution. A team of scientists has now shed light on the evolutionary history of these birds and identified a gene that explains variation in beak shape within and among species.
• He noted that changes in size and form of the beaks have allowed different finch species to utilize food from various sources, such as seeds, insects and nectar, all of which are driven by natural selection.
• The thinnest beak belongs to the green warbler finch which uses it to probe for insects.
• So We can say Darwin’s finches show variation in their beaks  _