Does darkness limit the use of tree cavities for nesting by birds?

Tree cavities are usually superabundant, and offer the safest nest sites. However, not all bird species utilize them. One of the reasons could be insufficient illumination in such places. As no visual system can operate in complete darkness, some nest cavities may be too dark to be usable even by cavity nesters, and some species may not have sufficient visual sensitivity to make use of dark cavities. So, presumably, light conditions could constrain both the evolution of the cavity-nesting habit and the selection of the nest site by the cavity-nesting birds. However, due to a paucity of data on light conditions in cavities, these suggestions have not been investigated to date. To provide more information on illumination inside tree cavities, we took measurements inside the nest cavities of Collared Flycatchers Ficedula albicollis breeding in a primeval forest (BiaowieA1/4a National Park, Poland). Illuminance inside cavities was recorded at daybreak, just after the parents commenced feeding nestlings. We found that only ca. 4 % of incoming light reached the nest level, and birds could adjust light conditions inside cavities by placing their nests closer to smaller entrances. Illuminance at nests (median = 0.05 lx) fell within the mesopic-scotopic range of vertebrate eyes. Measurements performed on a sunny, cloudless midday in a model cavity showed that illuminance strongly decreased with distance from the entrance. Our results indicate cavities are always dark places where colour vision is usually impaired or absent and the "lighting" requirements of cavity nesters can affect their use of tree cavities. We discuss the implications of the findings for understanding the adaptations of cavity nesting in birds.