Nest box design for a changing climate: The value of improved insulation

Improving the science and practice of using artificial roosts for bats

Worldwide, artificial bat roosts (e.g., bat boxes, bark mimics, bat condos) are routinely deployed for conservation, mitigation, and community engagement. However, scant attention has been paid to developing best practices for the use of artificial roosts as conservation tools. Although bats readily occupy artificial roosts, occupancy and abundance data are misleading indicators of habitat quality. Lacking information on bat behavior, health, and fitness in artificial roosts, their conservation efficacy cannot be adequately validated. We considered the proximal and ultimate factors, such as evolutionarily reliable cues, that may prompt bats to preferentially use and show fidelity to suboptimal artificial roosts even when high-quality alternatives are available. Possible negative health and fitness consequences for artificial roost inhabitants include exposure to unstable and extreme microclimates in poorly designed roosts, and vulnerability to larger numbers of ectoparasites in longer lasting artificial roosts that house larger bat colonies than in natural roosts. Bats using artificial roosts may have lower survival rates if predators have easy access to roosts placed in conspicuous locations. Bats may be lured into occupying low-quality habitats if attractive artificial roosts are deployed on polluted urban and agricultural landscapes. To advance the science behind artificial bat roosts, we present testable research hypotheses and suggestions to improve the quality of artificial roosts for bats and decrease risks to occupants. Because continued loss of natural roosts may increase reliance on alternatives, such as artificial roosts, it is imperative that this conservation practice be improved.

Agriculture and hot temperatures interactively erode the nest success of habitat generalist birds across the United States

Habitat conversion and climate change are fundamental drivers of biodiversity loss worldwide but are often analyzed in isolation. We used a continental-scale, decades-long database of more than 150,000 bird nesting attempts to explore how extreme heat affects avian reproduction in forests, grasslands, and agricultural and developed areas across the US. We found that in forests, extreme heat increased nest success, but birds nesting in agricultural settings were much less likely to successfully fledge young when temperatures reached anomalously high levels. Species that build exposed cup nests and species of higher conservation concern were particularly vulnerable to maximum temperature anomalies in agricultural settings. Finally, future projections suggested that ongoing climate change may exacerbate the negative effects of habitat conversion on avian nesting success, thereby compromising conservation efforts in human-dominated landscapes.

The Importance of Nest Box Placement for Barn Owls (Tyto alba)

Nest boxes have been used for years to increase breeding bird numbers for conservation and also in biological pest control projects. Barn owls (Tyto alba) have been used as biological pest control agents for rodents for years, and since nest boxes are costly for growers there is a need to determine whether nest box placement can increase the occupation of nest boxes and breeding success. We studied whether barn owl breeding in agricultural areas varied in nest boxes located on trees, poles located in the shade, and poles in the sun. The occupation of nest boxes was highest in nest boxes located on trees, followed by poles in the shade, and finally poles in the sun. In comparison, the number of fledglings was highest for nest boxes on poles in the sun followed by poles in the shade in the first half of the breeding season, whereas more nestlings were fledged in nest boxes on trees in the second part of the breeding season, which is most likely due to the higher internal temperatures in the nest boxes located in the sun. Interestingly, all the nest boxes’ internal temperatures were lower than the ambient temperatures but were much lower on trees than those on poles, most likely due to the trees providing better protection from the heat. It is therefore important to not only consider the placement of nest boxes, but how occupation and breeding success may vary seasonally.

What makes a house a home? Nest box use by West European hedgehogs (Erinaceus europaeus) is influenced by nest box placement, resource provisioning and site-based factors

Artificial refuges provided by householders and/or conservation practitioners potentially represent one mechanism for mitigating declines in the availability of natural nest sites used for resting, breeding and hibernating in urban areas. The effectiveness of such refuges for different species is, however, not always known. In this study, we conducted a questionnaire survey of UK householders to identify factors associated with the use of ground-level nest boxes for West European hedgehogs (Erinaceus europaeus), a species of conservation concern. Overall, the percentage of boxes used at least once varied with season and type of use: summer day nesting (35.5-81.3%), breeding (7.2-28.2%), winter day nesting (20.1-66.5%) and hibernation (21.7-58.6%). The length of time the box had been deployed, the availability of artificial food and front garden to back garden access significantly increased the likelihood that a nest box had been used for all four nesting types, whereas other factors related to placement within the garden (e.g., in a sheltered location, on hardstanding such as paving, distance from the house) and resource provisioning (bedding) affected only some nesting behaviours. The factors most strongly associated with nest box use were the provisioning of food and bedding. These data suggest, therefore, that householders can adopt simple practices to increase the likelihood of their nest box being used. However, one significant limitation evident within these data is that, for welfare reasons, householders do not routinely monitor whether their box has been used. Consequently, future studies need to adopt strategies which enable householders to monitor their boxes continuously. Ultimately, such studies should compare the survival rates and reproductive success of hedgehogs within artificial refuges versus more natural nest sites, and whether these are affected by, for example, the impact of nest box design and placement on predation risk and internal microclimate.

Evaluating bat boxes: design and placement alter bioenergetic costs and overheating risk

Bat box microclimates vary spatially and temporally in temperature suitability. This heterogeneity subjects roosting bats to a variety of thermoregulatory challenges (e.g. heat and cold stress). Understanding how different bat box designs, landscape placements, weather and bat use affect temperature suitability and energy expenditure is critical to promote safe and beneficial artificial roosting habitat for species of conservation concern. From April to September 2019, we systematically deployed 480 temperature dataloggers among 40 rocket box style bat boxes of 5 designs and regularly monitored bat abundance. We used bioenergetic models to assess energy costs for endothermic and heterothermic bats and modelled the overheating risk for each box as a function of design, placement, bat abundance and weather. For endothermic bats, predicted daily energy expenditure was lower for solar-exposed placements, large group sizes and a box design with enhanced thermal mass. For heterothermic bats, shaded landscape placements were the most energetically beneficial and bat box design was not important, because all designs generally offered microclimates suitable for torpor use at some position within the box. Overheating risk was highest for solar-exposed landscape placements and for designs lacking modifications to buffer temperature, and with increasing bat abundance, increasing ambient temperature and slower wind speeds. The external water jacket design, with the greatest thermal mass, concomitantly decreased overheating risk and endothermic energy expenditure. By assessing bat box suitability from two physiological perspectives, we provide a robust method to assess the conservation value of bat box design and placement strategies. We recommend future studies examine how changing thermal mass and conductance can be used to diminish overheating risk while also enhancing the effects of social thermoregulation for bat box users.

Insulated nest boxes provide thermal refuges for wildlife in urban bushland during summer heatwaves

In urban bushland, the installation of nest boxes is widely used to compensate for the loss of natural tree hollows. However, current nest box designs may not provide thermal refuges for wildlife during summer heatwaves, particularly if internal temperatures exceed the upper critical temperatures of wildlife. We investigated whether the addition of roofing insulation to nest boxes deployed for sugar gliders (Petaurus breviceps) and squirrel gliders (Petaurus norfolcensis) in urban bushland would reduce internal nest box temperatures during summer heatwaves. We measured temperatures of 44 insulated and 47 uninsulated nest boxes during one of the hottest summers on record (2018–2019) in the Lake Macquarie region of NSW, Australia, a period during which several prolonged heatwaves occurred. Over the 90-day study, maximum temperatures were, on average, 3.1°C lower in insulated boxes than in uninsulated boxes. The addition of insulation significantly lowered nest box temperatures regardless of aspect (north or south facing) or day of measurement. Temperatures exceeded the upper critical temperature (35.1°C) of gliders more frequently in uninsulated nest boxes (28% of days) than in insulated nest boxes (8% days). Although the addition of insulation to nest boxes lowered their internal temperatures, during heatwaves spanning 23 days, nest box temperatures exceeded the upper critical temperatures of gliders on 58% and 23% of days in uninsulated and insulated nest boxes respectively. These findings underscore the importance of retaining natural hollows in urban bushland to provide thermally suitable refuges for wildlife during extreme heat events.

The effect of nest temperature on growth and survival in juvenile Great Tits Parus major

For birds, maintaining an optimal nest temperature is critical for early-life growth and development. Temperatures deviating from this optimum can affect nestling growth and fledging success with potential consequences on survival and lifetime reproductive success. It is therefore particularly important to understand these effects in relation to projected temperature changes associated with climate change.Targets set by the 2015 Paris Agreement aim to limit temperature increases to 2°C, and, with this in mind, we carried out an experiment in 2017 and 2018 where we applied a treatment that increased Great Tit Parus major nest temperature by approximately this magnitude (achieving an increase of 1.6°C, relative to the control) during the period from hatching to fledging to estimate how small temperature differences might affect nestling body size and weight at fledging and fledging success.We recorded hatching and fledging success and measured skeletal size (tarsus length) and body mass at days 5, 7, 10, and 15 posthatch in nestlings from two groups of nest boxes: control and heated (+1.6°C).Our results show that nestlings in heated nest boxes were 1.6% smaller in skeletal size at fledging than those in the cooler control nests, indicating lower growth rates in heated boxes, and that their weight was, in addition, 3.3% lower.These results suggest that even fairly small changes in temperature can influence phenotype and postfledging survival in cavity-nesting birds. This has the potential to affect the population dynamics of these birds in the face of ongoing climatic change, as individuals of reduced size in colder winters may suffer from decreased fitness.

Tolerance to high temperature by arboreal mammals using nest boxes in southern Australia

Nest boxes are used to manage populations of tree-cavity dependent birds and mammals. Concerns have been raised that due to their poor insulative properties nest boxes may cause heat stress and occasionally death during summers of extreme maximum temperatures. Our study investigated whether this nest box heat stress hypothesis applies to two small cavity-dependent mammals (brush-tailed phascogales and sugar gliders). Focusing on days when ambient temperature reached ≥40 °C, we recorded: i) temperatures within occupied nest boxes, ii) temperatures within nearby unoccupied tree cavities, iii) the duration of temperatures of ≥40 °C within nest boxes, iv) whether direct mortality was observed, and v) the relative abundance of these species in nest boxes before and after a very hot summer. When ambient temperature reached ≥40 °C, nest boxes were equivalent to ambient or 1-2 °C cooler, whereas tree cavities were 3-7 °C cooler than ambient. Exposure in nest boxes to temperatures of ≥40 °C lasted an average of 2-8 h. We observed no mortality over 65 records of phascogales and 31 records of gliders in nest boxes on days when ambient reached ≥40 °C. No decline in abundance was recorded after a summer with 11 days of temperatures ≥40 °C, with each species subsequently occupying >40 nest boxes. Our observations suggest these species are tolerant of the high temperatures that occurred. Nonetheless, provision of nest boxes designed to minimise summer heating is recommended.

Does temperature variation influence nest box use by the eastern pygmy-possum?

Cavity-using birds and mammals reliant on nest boxes may be negatively affected by the poor thermal buffering of nest boxes. I investigated whether nest box use by the eastern pygmy-possum (Cercartetus nanus) over a 4-year period was influenced by maximum ambient temperature, which ranged from 15.6 to 34.9°C during survey occasions. Occupancy modelling of 144 site detections over 30 survey occasions suggested that a model that included maximum temperature had little support whereas a model involving time-varying detection (i.e. detection differed across sample occasions) was the most plausible. I also investigated how temperatures in nest boxes and tree hollows varied over the four hottest days of summer, including one day when the temperature reached 40.6°C. Maximum temperatures were 3–4°C cooler in plywood nest boxes and 5–8°C cooler in tree hollows compared with ambient temperatures. Together, these results suggest that eastern pygmy-possums using nest boxes in coastal areas are unlikely to experience heat stress. Cavity-using species are a heterogeneous group such that empirical studies are required to identify those that may be vulnerable to heat stress if nest boxes are used to provide population support.

Temperature and breeding success for Cliff Swallows (Petrochelidon pyrrhonota) nesting on man-made structures: ecological traps?

When an environmental cue that previously signaled a suitable habitat leads an animal to use an unsuitable site, individual fitness can decrease, ultimately leading to population declines. Such “ecological traps” may be particularly likely for birds that use human infrastructure for nesting. Here we tested whether high nest temperatures and the physical properties of barns are associated with lower breeding success for a declining population of Cliff Swallows (Petrochelidon pyrrhonota (Vieillot, 1817)). We monitored nests under barn eaves below wood and metal roofs to determine nestling survival and mass, and recorded temperature under barn eaves, to relate ambient temperature to eave temperature. We found that eave temperature increased with ambient temperatures and was higher at high temperatures and lower at cool temperatures under metal roofs than wood roofs. Nestling survival was lower during periods with higher ambient temperatures, and both survival and mass were lower under metal roofs. Our findings suggest that barn eaves, especially those with metal roofs, may be an ecological trap for Cliff Swallows, where a seemingly suitable nesting site early in the breeding season results in low breeding success. Furthermore, warming temperatures may lead to ecological traps for other bird species, particularly those nesting in man-made structures.

Breeding birds actively modify the initial microclimate of occupied tree cavities

The microclimate of cavities used by endothermic animals may depend on dynamic relationships between a cavity's physical properties and the heating activity of cavity users, but the rudiments of these relationships are unclear. I compared the temperature and relative humidity of active tree cavities that were occupied by nesting marsh tits Poecile palustris with the conditions in vacant tree cavities previously used for breeding by this species. I tested how presence of active nests modified initial cavity microclimate, and if this modification changed with nest progression or cavity insulation. In 2013-2014, mean daily internal-ambient temperature differences averaged 1.5-4.1 °C higher and relative humidity 8-10% lower, in active cavities relative to vacant sites, with greatest differences in the late nestling period. Compared to vacant cavities and relative to respective ambient values, the greatest daily minimum temperature increase was in active cavities located in the thinnest trees, which insulated least efficiently. As daily minimum temperatures were elevated to a similar level relative to outside within all active cavities, birds appeared to compensate for heat loss from cavities by warming the air within in a homeostatic manner. Similar to vacant cavities, the differences between daily maximum internal and ambient temperatures decreased with tree girth in active cavities, indicating that daily temperature maxima were systematically moderated in the thickest trees. The study demonstrates the modifying effect of birds' breeding activity on tree-cavity microclimate and highlights the role of a cavity's thermal properties in reducing the energy expenditure and risk of overheating for cavity users.

Variation in Summer and Winter Microclimate in Multi-Chambered Bat Boxes in Eastern Australia: Potential Eco-Physiological Implications for Bats

Bat boxes are commonly used as a conservation tool. Detailed knowledge on the influence of box elements on microclimate is lacking, despite eco-physiological implications for bats. Summer and winter box temperature and relative humidity patterns were studied in narrow multi-chambered plywood and wood-cement boxes in eastern Australia. Box exteriors were black or white and plywood boxes comprised vents. Relative humidity was higher in white boxes than black boxes and box colour, construction material, chamber sequence and vents influenced temperatures. Maximum box temperature differences between designs varied by up to 9.0 °C in summer and 8.5 °C in winter. The black plywood box consistently recorded the warmest temperatures. This design comprised a temperature gradient between chambers and within the front chamber (influenced by vent). During the 32-day summer sampling period, the front chamber rarely recorded temperatures over 40.0 °C (postulated upper thermal tolerance limit of bats), while the third and fourth chamber never reached this threshold. At the study site, the tested black boxes are considered most thermally suitable for bats during average summer conditions. However, during temperature extremes black boxes likely become too hot. Wood-cement, a durable material not previously tested in Australia should be considered as an alternative construction material.