Parasites, predators and nest boxes: facts and artefacts in nest box studies of birds?

Nest box studies of passerine birds have played a central role in the development of theories about avian population dynamics, life history evolution, quantitative genetics, sexual selection and several other fields. Many of these studies have actually had a large impact on these biological fields in general. I argue that many results based on nest box studies are afflicted by two kinds of experimental artefacts. First, nest boxes for birds, contrary to natural holes, are relatively safe from nest predators. Second, researchers as a matter of routine remove old nests, thereby reducing loads of detrimental ecto-parasites. Nest predation and parasites (and their interaction effect) may have profound effects on nest site choice, mate choice, reproductive success and nestling growth. Generalizations therefore should not be based solely on the basis of evidence from nest box studies, and research on avian populations breeding in natural holes (or better designed nest boxes) with abundant parasites and natural rates of nest predation is urgently required, not least to check the validity of all the nest box data available.