Breeding safely is the biggest challenge for forest birds (reviewed by Wesołowski and Tomiałojć 2005). Predation is usually the main reason for nest loss and adult death (e.g. Nice 1957; Ricklefs 1969; Wesołowski and Tomiałojć 2005), but is better avoided by the birds occupying the safest nest sites. Tree cavities provide the most secure places for breeding and they are also usually superabundant in natural forests (review in Wesołowski 2007). However, numerous bird species fail to utilize this ecological opportunity. The question of why more bird species do not nest in tree cavities was posed by Lack (1954) and Nice (1957) more than half a century ago, and still awaits an adequate answer.

The existence of constraints that are impassable for some species could be an explanation (Wesołowski 2007). Successful breeding in tree cavities requires a cavity-nesting bird to deal with several problems that are nonexistent or of minor importance in other types of nest sites. The major issues of cavity nesting could include: how to keep the nest contents dry, how to provide sufficient air exchange, and how to tend broods in very dark places. Inability to tackle any of these problems would prevent the birds from taking advantage of the security provided by cavities. One can envisage that individual bird species have differed in their ability to cope with these challenges, and only a fraction of them have possessed the anatomical or physiological traits that were necessary for the evolution of the cavity-nesting habit (Wesołowski 2007).

There are reasons to expect that light conditions within tree cavities might constrain both the evolution of the cavity-nesting habit and the nest site choice within these species. Tree cavities are rather dark places in which the amount of light declines rapidly with increasing distance from the entrance (Wesołowski and Maziarz 2012). Therefore, to use such sites, a bird has to be able to operate in low light conditions, either by being able to see details at low illumination or by using other sensory modalities (reviews in Land and Nilsson 2002; Martin and Osorio 2008). The use of olfaction, touch and/or sound to orientate in darkness is observed in some birds (review in Martin 2011), but has not been demonstrated in tree-cavity nesters. Therefore, to guide behaviour within nest cavities, it seems highly probable that cavity-nesting birds must rely on vision. Changes in parental feeding behaviour in reaction to experimental manipulation of the chicks’ appearance seem to support this; for example in Great Tits Parus major (Heeb et al. 2003; Wiebe and Slagsvold 2009, 2012), Northern Flickers Colaptes auratus and Pied Flycatchers Ficedula hypoleuca (Wiebe and Slagsvold 2009, 2012).

If birds do rely on visual cues within the nest cavity (and no visual system can operate in complete darkness; Land and Nilsson 2002), there must be some limiting illumination level below which they cannot see, i.e. some nest cavities might be too dark to be usable. Additionally, a diurnally active adult bird, when feeding its young in a cavity, has to be able to rapidly adjust its sight to the light conditions at the nest by switching between the well-lit environment outside to the dim light inside the cavity and back again, often within a few seconds. The ability to cope with rapidly fluctuating light levels through, for example, the use of a pupil with a high dynamic range, may have evolved only in cavity nesters, but this supposition has not been tested (Wesołowski and Maziarz 2012).

There is limited data on illuminance in nest cavities (summarized in Wesołowski and Maziarz 2012). Our previous study of illumination in the nest cavities of Marsh Tits Poecile palustris and Great Tits breeding in the primeval forest of Białowieża National Park (Poland) showed that the birds were able to feed young in dim light when illuminance fell within the mesopic–scotopic range of vertebrate eyes, equivalent to moonlight–twilight in open habitats (Wesołowski and Maziarz 2012). This work also suggested that the patterns of nest-site use could be affected by the requirement for sufficient illumination within the cavity. However, we do not know how important the problem of “having enough light” is for other cavity-nesting birds, or how the birds deal with it within specific cavities.

To check how general this constraint could be, we measured illumination in cavities of another obligatory cavity nester, the Collared Flycatcher Ficedula albicollis (Old World Flycatchers, Muscicapidae), breeding in the same forest. We chose this species for two reasons: (1) the cavity-nesting habit of flycatchers has evolved independently from that of the tits (von Haartman 1957), and (2) Collared Flycatchers usually use tree cavities of different dimensions than the tits in BNP (Wesołowski 1996; Walankiewicz et al. 2007; Wesołowski and Maziarz 2012). We believe that examining the illumination within flycatcher nest cavities would not only enhance our understanding of the flycatchers’ behaviour but also, by comparison with the tits, provide a greater insight into the evolution of cavity nesting in general.

In the primeval forest habitat of Białowieża National Park (Eastern Poland), Collared Flycatchers have a wide spectrum of tree cavities to choose from (Tomiałojć et al. 1984; Walankiewicz and Mitrus 1997; Czeszczewik et al. 1999, 2012; Wesołowski 2003). In such conditions, they breed mostly in nonexcavated cavities in living trees (Walankiewicz 1991; Walankiewicz et al. 2007). The position and dimensions of the cavities used by this species overlap partially with those used by Pied Flycatchers, Marsh Tits, or Great Tits (Walankiewicz 1991; Wesołowski 1989, 2007; Czeszczewik and Walankiewicz 2003), but, as the cavities are superabundant (see above), Collared Flycatchers are not constrained by interspecific competition for nest sites. By building bulky nests in cavities, the birds may better prevent soaking of the nest contents by water accumulating within the cavity (Wesołowski et al. 2002), and adjust the distance of the nest from the cavity entrance to avoid predation (Wesołowski and Maziarz 2012).

We took advantage of intensive studies of Collared Flycatchers carried out in Białowieża National Park (Walankiewicz et al. 2007; Czeszczewik et al. 2012) to measure illuminance inside nest cavities used by this species. Here, we present the results and compare them with the values obtained in cavities utilized by tits (Wesołowski and Maziarz 2012). We discuss the implications of the findings for understanding adaptations to cavity nesting in birds.