Eggshell is essential for the reproduction of birds. The optical properties of their shells may have an impact on biological functions such as heat and UV protection, recognition by the parents or camouflage. Whereas ultraviolet reflection by bird eggshells has been superficially described in the scientific literature, the physical origin of this phenomenon remains poorly understood. In this article, reflectance peaks in the near UV range were observed by spectrophotometric measurements of hen eggshells. In addition, electron microscopy imaging revealed the presence of pores within the so-called "calcified shell"part (i.e., between ca. 20 μm and ca. 240 μm deep from the outer surface). The average radii of these pores range from 120 to 160 nm. Mercury intrusion porosimetry allowed to highlight a distribution of pore radii around 175 nm. Numerical and analytical predictions using scattering theory indicate that these pores are responsible for the optical response observed in the UV range.