Nestboxes are inadequate substitutes for natural tree cavities

A new study has suggested that, in many instances, nestboxes are not an adequate substitute for naturally occurring tree cavities.

The study, published in Forest Ecology and Management, found that microclimatic conditions varied between nestboxes and natural cavities, this in turn having an effect on their respective suitability for different tree-nesting species.

Measuring air temperature and relative humidity in vacant tree cavities previously used by Marsh Tits and in nestboxes provided specifically for the species, the researchers found that tree cavities were more efficient thermal insulators, with temperature extremes closer to the median values, while nestboxes produced a greater temperature range.

Marsh Tits show a preference for natural breeding sites, despite the provision of artificial imitations (Photo: John Caswell)

Mean daily relative humidity reached an average of 90 per cent in tree cavities, up to 24 per cent higher than in nestboxes at comparable ambient conditions (76–78 per cent relative humidity).

Co-author Marta Maziarz explained: "The main message from the study is that nestboxes cannot replicate tree cavities.

"The nestboxes do not insulate well but they are generally warmer and they are drier. They are not bad; they are just different. This has consequences for the birds."

The study, which was conducted at two woodland sites (one in the UK and one in Poland), found that different species showed different preferences for either natural or artificial nest sites. Some, such as Great Tit, favoured the provided nestboxes while others, particularly Marsh Tit, showed clear preferences for tree cavities — even when nestboxes were strategically positioned to mimic natural holes.

Dr Maziarz highlighted the diversity of natural cavities as a key factor when considering conservation measures for woodland. She explained that it is better to protect mature trees with a greater number and range of available cavities which in turn offer potential sites to a greater range of species — not just birds, but also to other animals, such as bats.

However, nestboxes are undoubtedly beneficial for the conservation of some species, with some observed population recoveries in Pied Flycatcher and Barn Owl largely thanks to the provision of artificial nest sites.