All cavities are not equal

Come spring (late winter), the forests are bustling. Cavity-dwelling animals search for tree crevices and holes in which to lay their eggs and raise their offspring. Tree cavities provide a stable environment for successful nesting.

Wetland_Ecology_Group_University of Helsinki_tree cavity

Natural cavities are usually found in old wide trees, where the inner temperature of such cavities remains more stable than outside temperatures.

Only one problem remains. Cavities usually form in old or, at the least, decomposing trees, but forestry practices simplify forest cover composition. Fewer trees surpass forestry practice recommendation ages, so our forests have less large aging trees in which fungi can spread. More tree cavities are desperately needed. Nest boxes are our solution to this problem. The idea is simple: anyone can build a nest box and hang it on their own land (or somebody else’s with permission). This has helped boost the populations of certain cavity-nesters such as pied flycatchers (Ficedula hypoleuca) and great tits (Parus major).

It would be nice to think that we have solved the cavity problem, or that the problem will be solved if we raise the number of nest boxes to sufficient levels. But it’s not that simple. Several researchers have studied the functionality of nest boxes over the years. The microhabitats of tree cavities and nest boxes differ from each other in relation to temperature and moisture. Wroclaw University researchers were the most recent group to prove this distinction, but they also demonstrated that these functional differences drive the marsh tit (Poecile palustris) to choose natural cavities over nest boxes. Their study was conducted in two forests; the other had an unlimited number of tree cavities, while nest boxes were the only nesting option in the other forest. The marsh tits preferred natural cavities with thick walls buffering the holes from outside temperatures. And birds are not the only species that have been shown to prefer natural cavities, for example certain bats and the common brushtail possum (Trichosurus vulpecula) will settle in natural cavities due to their more stable microclimates.

Wetland-Ecology-Group_University of Helsinki_brushtail possum

The common brushtail possum is an Australian mammal that nests in tree cavities. Picture: Wikimedia Commons. https://upload.wikimedia.org/wikipedia/commons/b/b4/Brush_tail_possum_4-colour_corr.jpg. By user:benjamint444 modified by Tony Wills [GFDL]

Nest box temperatures in the Wroclaw study fluctuated significantly more than the inner temperatures of tree cavities. Nest box temperature also changed at the same rate as outside temperatures. Nest box temperatures can therefore rise to dangerous levels during the summer, to where chicks are at higher risk of dying from excessive heat compared to broods in tree cavities. During the winter, nest box temperatures drop to lower levels than cavity temperatures, decreasing the shelter effect that many small birds utilize to survive the harsh cold.

Nest boxes also average lower air moisture levels compared to natural cavities. This may hinder mold from growing in the nest boxes, but concurrently lower moisture may encourage wasps (Vespidae) and tree bumblebees (Bombus hypnorum) to settle in nest boxes, making them inaccessible for birds. Fleas (Siphonaptera) may also increase in dry and warm conditions, so the number of competitors and ectoparasites may increase.

Wetland_Ecology_Group_University of Helsinki_birch dead wood

Woodpeckers excavate cavities in decomposing trees and standing dead wood

To cap, nest boxes and natural cavities do not replace each other from a structural point of view and not all species will nest in boxes. The majority of nest boxes are so-called standard models, i.e. they are copies of each other in terms of dimensions and flight hole diameter. In real life, a standard model nest box is only accepted by a limited number of cavity dwellers. It is therefore imperative to conserve aging and decomposing trees, as their cavities are never of standard shape or size. If nothing else, decomposing trees in our yards should be conserved; trees can always be cut to a height that ensures they are of no danger to nearby buildings or people. Such standing dead wood is very rare in current heavily managed forests. With a standing birch dead wood tree it is even possible to attract the picky willow tit (Poecile montanus) to your yard.

The next best alternative is to ensure the structural heterogeneity of nest boxes, i.e. build boxes that are also suitable for species such as the common redstart (Phoenicurus phoenicurus), owls (Strigidae), treecreepers (Certhiasp.), and even certain mammals such as flying squirrels (Pteromys volans). This may require a little more trial and error, but it is the only way of maximizing the nesting alternatives in managed forests. Ideas for nest box designs abound online, Pinterest for example has a huge selection of box models. However, it is important to follow nest box construction instructions issued e.g. by the BTO and Audubon Society or these general safety instructions, to make sure that the boxes are as safe as possible for birds. Nest box positioning is also important; foliage has a protective effect, and the microhabitat of nest boxes positioned under foliage therefore remains more stable than in sun-exposed areas.

Wetland_Ecology_Group_Vehkaoja_Mia_blue tit and nest box

Blue tits often utilize nest boxes.

Adding insulating materials to nest boxes is one way of adding to the inventiveness of nest box construction. To mimic the microclimates of natural cavities, a team of Australian researchers recently compared nest boxes that had been fitted with three types of insulating or heat-reflecting materials. Nest box temperatures remained most stable around the clock in nest boxes insulated with polystyrene foam. The inner temperature of one polystyrene-fitted nest box was nearly six degrees Celsius less than outside temperatures. Nighttime inner temperatures were also higher in the polystyrene nest boxes compared to non-insulated boxes when a heat-producing pillow was placed in the insulated and non-insulated nest boxes, to mimic the effect of birds spending the night in the boxes. The Australian study showed insulation had a more significant effect on nest box temperatures than nest box placement in a shady or sunny location. However, for the environment and breathability, it is probably better to use some type of natural fiber insulation in nest boxes. Also, insulated nest boxes are not enough to fill the void created by the disappearance of natural tree cavities, as the study showed that the temperature fluctuation of even the polystyrene-fitted nest boxes was greater that of natural cavities.

P.S. It is currently trendy to set up cool or “beautiful” nest boxes without thinking about their safety at all. Not a good idea! For example, ceramic bird boxes are much worse insulators than wooden ones, and painted boxes should use lead-free paint. https://www.telegraph.co.uk/news/earth/wildlife/12165505/Novelty-nest-boxes-putting-garden-birds-at-risk-warns-RSPB.html

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