Why is it unlikely for the carbon atom to have been taken into the birds bloodstream?

It is unlikely for the carbon atom to be taken into a bird's bloodstream because carbon in its elemental form is not readily absorbed by biological systems. Instead, carbon typically enters an organism through organic compounds, such as carbohydrates, fats, and proteins, which are metabolized. Additionally, carbon dioxide, a gaseous form of carbon, is absorbed during respiration but is not directly transferred as a single atom into the bloodstream. Therefore, carbon atoms usually come from complex organic molecules rather than existing freely in the environment.