A popular assumption is that the mass of a cell s 10 times the mass of a bacterium, which is about 10⁻¹² g.
Therefore, one cell of the humming bird has a mass of about 10⁻¹¹ g.
The number of cells in a hummingbird weighing 10⁻² g is
10⁻²/10⁻¹¹ = 10⁹
Answer: 10⁹ or a billion.
