Wesołowski T., Rowiński P. 2012. The breeding performance of Blue Tits Cyanistes caeruleus in relation to the attributes of natural holes in a primeval forest. Bird Study, on-line early; doi: 10.1080/00063657.2012.722189

Capsule Blue Tits nested in non-excavated holes with narrow openings, typically in live trees, which reduced both the risk of nest predation and of nest soaking.
Aims To describe patterns of nest-site utilization by Blue Tits in primeval conditions, to examine the influence of various hole attributes on nesting success, and to test the adaptive value of nest-site choice.
Methods Observations of birds living in undisturbed conditions in a strictly protected part of the Białowieża National Park (Poland) during 37 breeding seasons, coupled with measurements of hole attributes and observations of nests' fate.
Results Tree species used for breeding differed among habitats. Nest holes were situated mainly in trunks of living trees, were non-excavated, with narrow slit-like openings, and had relatively small bottoms. Nests were lost mostly to predation and soaking. Adults were frequently killed on nests. Broods were more successful in holes with a smaller entrance, smaller bottom, and in nests situated further from the hole entrance.
Conclusion The features of Blue Tit holes (strong walls, narrow entrances and nests moderately deep below the entrance) probably constitute adaptations to the need to evade predators while keeping the contents of the nest dry and adequately illuminated.