Introduction

Nest cavities can provide shelter from precipitation, strong winds and buffer again extreme temperature fluctuations. In contrast, poor insulation is associated with reduced fledging success and increased cost of thermoregulation (Dawson et al. 2005; Ardia 2013; Sudyka et al. 2022). The insulation properties of nest cavities may differ across cavity types, for example between nest boxes and tree or rock cavities (Maziarz et al. 2017). Therefore, the type of nest cavity can have fitness-consequences through their different cavity microclimate (Ardia 2013; Mueller et al. 2019).

As natural cavities might be limited, nest boxes are supplemented frequently as alternative to natural nest cavities. Yet, the extent of protection from the environment and the insulation properties can vary between different types of nest cavities and with their orientation. Tree cavities in forests, for instance, provide better insulation than nest boxes and consequently experience more constant internal temperatures (Maziarz et al. 2017) and east- and south-facing nest boxes are warmer in the morning compared to north- and west-facing nest boxes (Ardia et al. 2006; Sudyka et al. 2023). While several studies found microclimatic differences between cavity types in managed habitats, little is known about cavity microclimate in high alpine environments. Yet, the consequences of poor insulation may be particularly strong in these habitats, as conditions are generally harsh, and ambient temperature fluctuates strongly. Therefore, understanding the microclimate in nest cavities is the first step in evaluating the suitability of artificial nest cavities and can provide insight into potential improvements in the design of nest boxes to mimic natural conditions.

We measured temperatures in vacant nest cavities of White-winged snowfinches Montifrigilla nivalis an alpine cavity breeder. Snowfinches breed in deep rock crevices in cliffs but also use human-made structures such as nest boxes and metal tubes of ski lift pylons. The aim of the study was to describe the thermal properties of these nest cavities and compare them between cavity types and orientations. Therefore, we measured the daily temperature amplitude, i.e., the difference between minimum and maximum temperature in a 24h time window, and the difference between inside and outside temperature. We expected that (1) natural nest cavities provide more constant temperatures resulting in a lower temperature amplitude compared to human-made cavities because rocks have higher thermal inertia compared to wood or metal. Furthermore, we expect (2) that temperature profiles from east- and south facing cavities differ from west- and north facing cavities as they differ in the amount of solar radiation received.

Material and methods

Data collection

To assess the thermal conditions of nest cavities, we measured inside and outside temperatures in nest cavities using miniature temperature loggers (iButton DS1922L Thermochron). Cavities had previously been used as nest cavities by snowfinches but were vacant during the time of measurement which took place during the breeding season (May to August) in 2021. We assume that the vacant cavities are suitable nest cavities at the time of our measurements because these cavities were used as nest sites either earlier in the same year or in previous years. Especially in the case of ski lift pylons and nest boxes, there are usually many cavities available close to each other, meaning that a vacant cavity might simply not be used because there are other cavities available close by. Temperature loggers in rock crevices (n = 4) and pylons (n = 6) were placed approximately 20 cm deep in the cavities or attached to the back wall inside the nest boxes (n = 9). Outside temperatures were measured in a shaded area at similar elevation within a maximum distance of 500m from the cavities. Temperature measured in the shade should reflect ambient temperature and avoid extreme temperature being recorded due to sun incidence on the logger. Outside temperature loggers were attached on the bottom of the box or on the bottom of the metal tube for ski lift pylons, beyond the reach of the sun. For rock crevices, we measured outside temperature either attaching the logger on a north-facing wall of a nearby building or provided shade with a stone pile. We attached a 3mm fleece pad at the bottom of the logger to avoid direct heat transfer from the surface. Temperature was measured every 10 min. for 4–21 consecutive days (average: 14.5 consecutive days). We recorded the cardinal direction to which the cavity entrance was facing and divided them in two categories: east- and south-facing (NE, E, SE and S, n = 9) and west- and north-facing cavities (SW, W, NW and N, n = 10).

Statistical analysis

We used the software R (R Core Team 2020) for the data analysis. All temperature measurements were aggregated to hourly means. To quantify the temperature variability of cavities, we calculated the difference between minimal and maximal temperature per day, i.e., the temperature amplitude from midnight to midnight, for inside and outside temperature. In addition, we divided the data into daytime (8:00 a.m. until 5:59 p.m.) and night-time (6:00 p.m. until 7:59 a.m.) and calculated the difference between inside and outside temperature during day and night.

We fitted a linear mixed effect model to the temperature difference measurements with cavity type, orientation, and time (day vs night) including all two-way and the three-way interaction as predictors. The model included the outside temperature as a fixed effect because temperature amplitude or differences may change with ambient temperature and we included a unique nest identity as random factor to account for repeated measures of the same cavity. The linear model was fitted using the brm function from the brms package (Bürkner 2018). We used four chains and 2′000 iterations with a burn-in of 1′000 iterations and an average step-size of 0.12 for fitting the models. Rhat values were used to assess convergence of the Markov chains and standard diagnostic residual plots were used to check model assumptions (i.e., normal distribution of residuals).

Results

Temperature amplitude and insulation

The temperature amplitude was highest in ski lift pylons (mean ± sd: 10.9 ± 4.90 °C) and nest boxes (mean ± sd: 8.7 ± 3.40 °C). In rock crevices, temperature remained more constant (mean ± sd: 1.0 ± 0.86 °C). Inside temperature amplitude was lower than outside temperature amplitude in rock crevices (mean amplitude inside: 1.0 ± 0.86 °C, mean amplitude outside: 6.1 ± 3.40 °C), indicating that these cavities buffer ambient temperature fluctuations and provide a constant thermal environment (Fig. 1A/D, Table 1). This results in higher inside temperature relative to ambient temperature during night but usually lower inside temperature during daytime compared to outside temperature. In ski lift pylons, inside temperature exceeded outside temperature amplitude (Fig. 1D). On sunny days, temperature in ski lift pylons exceeded ambient temperature by more than 10 °C and reached up to 32 °C (Table 1). At night, mean inside temperatures did not differ from mean ambient temperatures in ski lift pylons (Fig. 1C/F). In nest boxes, inside and outside temperature were similar (mean difference: 0.20 ± 1.7 °C), indicating little insulation but in contrast to ski lift pylons, inside temperature did not exceed outside temperature during sunny days.

Fig. 1
figure 1

Hourly inside and outside temperature during 5 consecutive days for rock crevices (A), nest boxes (B) and ski lift pylons (C). Daily temperature amplitudes of inside temperature and outside temperature for each cavity type (D). The horizontal bars represent the median, boxes span 50% of the measurements and whiskers include all measurements within 1.5 times the interquartile range of the boxes. Difference between inside and outside temperature in east-and south-facing cavities and north-and west-facing cavities during day (E) and night (F). Positive values indicate that temperature was higher inside than outside the cavity. Segments depict the 95% credible interval based on the linear mixed effect model for average day and night temperature. nrock crevice = 101 days in 4 crevices, nnest box = 87 days in 9 nest boxes, nski lift pylon = 90 days in 6 pylons

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Table 1 Summary of temperature characteristics per cavity type. nrock crevice = 4, nnest box = 9, nski lift pylon = 6. In brackets is the outside temperature measured at the moment of the reported minimum / maximum inside temperature
Full size table

Cavity orientation

For all nest types, east- and south-facing cavities were generally warmer during daytime than north- and west-facing cavities (Fig. 1E). In nest boxes and ski lift pylons, inside and outside temperature were similar at night, irrespective of their orientation (Fig. 1F). In contrast, the orientation effect persisted in natural cavities with positive differences between inside and outside temperature in east-and south-facing cavities at night (Fig. 1F).

Discussion

In line with our expectation, our results suggest that natural nest cavities provide more constant inside temperatures compared to nest boxes and ski lift pylons. Despite a small sample size, our results clearly indicate that the temperature profiles of the three cavity types are distinctive. Similar results have been reported from forests and orchards where natural nest sites in tree cavities were compared with nest-boxes and wood stacks (Grüebler et al. 2014; Maziarz et al. 2017).

Strong temperature fluctuations can have negative fitness consequences for cavity nesting songbirds (Pérez et al. 2008; Ardia 2013). On one hand, low cavity temperatures might increase the cost of incubation and thermoregulation of adults and nestlings (Mueller et al. 2019), thereby reducing nestling growth and fledgling success (Dawson et al. 2005; Pérez et al. 2008; Ardia 2013). In cold environments, warmer temperatures might therefore lead to a physiological relief resulting in reduced cost of thermoregulation. On the other hand, high cavity temperatures can potentially have negative consequences on breeding success through hyperthermia or increased water loss (Cunningham et al. 2013), particularly in cold adapted species with limited heat tolerance (O'Connor et al. 2021). Therefore, further research about the fitness consequences of temperature variation during the nestling phase are crucial to better design artificial nest which provide suitable thermal conditions and to assess potential consequences of climate warming through nest microclimate.

We expect that the temperature amplitudes the birds experience might be slightly less extreme than what we measured, as the nest might provide some additional insulation. Nevertheless, the thermal environment for nestlings seems to differ both in absolute temperature and in daily pattern between rock crevices and human-made cavities.

Different nest cavity types with different microclimates may provide snowfinches with diverse thermal conditions to select from to get suitable nest sites for reproduction if they are able to assess the thermal conditions of cavities. However, as temperatures extremes are becoming increasingly common (Gobiet et al. 2014), it may get more difficult for birds to predict the suitability of a cavity based on early season conditions. Extreme temperature fluctuations, as observed in ski lift pylons might exceed the thermal tolerance and thus reduce reproductive success through lethal or sublethal effects of an unsuitable thermal environment (Cunningham et al. 2013; O'Connor et al. 2021). Successful breeding attempts of snowfinches in nest boxes and ski lift pylons (personal observation) suggest that human-made cavities generally provide suitable conditions. However, the number of fledglings from ski-lift pylon broods and their survival has not been quantified yet. Previous research suggests, that snowfinches prefer east- and south-facing nest cavities early during the breeding season (Niffenegger et al. 2023). Here, we show that these cavities are warmer compared to north-and west-facing ones. Therefore, snowfinches might reduce thermoregulatory cost using these nest sites when ambient temperature is still low.

In conclusion, we show that cavity type and orientation influence temperature profiles in a high elevation habitat and suggest that insulation properties should be taken into consideration when nest boxes are provided.