Nest-boxes are no substitute for the crucial role that tree cavities play in maintaining biodiversity

Dr Richard Broughton is an Ecologist and GIS Specialist at the Centre for Ecology & Hydrology. He explains how new research he contributed to reveals the benefits tree cavities provide to birds and other animals compared to manmade nest-boxes:

Marsh tit bird emerging from nest in tree cavityTree cavities are crucial to maintaining biodiversity by providing nesting and roosting sites for a variety of birds and other animals.

However, large, mature trees, which often contain the greatest diversity of cavities, are frequently removed in managed forests and urban areas and this can reduce the cavity abundance.

Nest-boxes can be seen as an off-set to this problem, particularly for nesting birds, but artificial sites can differ from natural ones in several ways.

These are the key findings from new research I contributed to along with colleagues Dr Marta Maziarz and Prof Tomasz Wesolowski, of Wroclaw University in Poland, published in Forest Ecology and Management.

We compared the microclimate (temperature and humidity) of tree cavities and nest-boxes used by a small forest bird, the Marsh Tit, at study sites in the UK and Poland.

We found that nest-boxes were significantly less humid and less insulated against outside temperatures than were tree holes, which was supported by our review of other studies.

Marsh tit nest with eggs in a nestbox Marsh tit chicks at 3 days old in a nest box Marsh tit chicks at 11 days old in a nest box
Above: Marsh tit nest in a nest-box showing eggs (left), chicks at three days old (centre) and chicks at eleven days old (right). Photos by Richard Broughton

"Nest-boxes were significantly less humid and less insulated against outside temperatures than were tree holes."

This may have consequences for the inhabitants of nest-boxes, as the drier air and more extreme temperatures may expose the chicks to dehydration or overheating.

These nest-box conditions are also more attractive to parasites, such as fleas, and even competitors, such as nesting Bumblebees or wasps, which can oust the birds.

Marsh tit bird bringing food to chicks in a tree cavity nestIn nest-boxes the old nest material can also become 'mummified' in the drier atmosphere, and build up over several years to make them unusable unless they are cleaned out and maintained by people.

In humid tree cavities such problems are rare, as natural decomposition of nests results in 'self-cleaning', ready for birds to use them in the following spring.

Our study, therefore, suggests that nest-boxes offer a very different environment from tree cavities, and, although nest-boxes can be useful in targeted situations, they should not be considered as a direct substitute for natural nest sites.

"Although nest-boxes can be useful in targeted situations, they should not be considered as a direct substitute for natural nest sites."

A more preferable conservation solution in the long term is the preservation of old, mature trees that provide a variety of natural cavities.

Richard Broughton

Additional information

Dr Broughton is a co-author to the paper "Microclimate in tree cavities and nest-boxes: Implications for hole-nesting birds", published in Forest Ecology and Management, 1 April 2017 and available online. DOI: 10.1016/j.foreco.2017.01.001

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