Current nest box designs may not be optimal for the larger forest dormice: Pre‐hibernation increase in body mass might lead to sampling bias in ecological data

Abstract Biologists commonly use nest boxes to study small arboreal mammals, including the forest dormouse (Dryomys nitedula). Hibernating dormouse species often experience pronounced seasonal variations in body mass, which might lead to sampling biases if it is not taken into account when designing nest boxes. In my study of the forest dormouse, I noticed that the entrance hole of nest boxes had been gnawed. I hypothesized that this behavior was exhibited by the individual dormice of higher body mass, who were unable to pass through the entrance holes. To test my hypothesis, I categorized the individual dormice present inside nest boxes based on their body mass and then compared the seasonal body mass dynamics with the timing of the gnawing behavior. I also compared nest box occupancy by the forest dormouse before and after the gnawing behavior. Interestingly, I found that the gnawing behavior was displayed exclusively when part of the dormouse population increased considerably in body mass, which supports my hypothesis. Additionally, nest box occupancy decreased significantly from 20% before to 4.6% after the gnawing behavior. I suggest that researchers include nest boxes with entrance holes larger than 4 cm in future studies of the forest dormouse to prevent the possible exclusion of the conspecifics that have higher body mass before hibernation. This type of sampling bias might also concern studies of other species, such as the fat dormouse, that similarly show pronounced seasonal variations in body mass. I recommend that biologists consider the seasonal body mass dynamics of the target species when designing nest boxes to minimize bias in ecological data and improve management actions.


| INTRODUC TI ON
Biologists have long used artificial nest boxes as a convenient tool for obtaining occupancy, abundance, and reproductive data for various taxa, including birds and small mammals (e.g., Goldingay, Quin, et al., 2020;Menkhorst, 1984;Monti et al., 2019;Williams et al., 2013). Obviously, nest boxes should be specifically designed to suit the goals of an intended study. It is important to ensure that nest box design is tailored to species-specific requirements (Zingg et al., 2010; see also Goldingay, Rohweder, et al., 2020). For example, a nest box designed for a small sugar glider would not be efficient for studying a much larger bobuck (Menkhorst, 1984).
While the average characteristics, such as size, of a species are presumably the first factors involved in determining the optimal nest box dimensions, there can be variations within a species, which can further complicate the case. More specifically, individuals of a species might seasonally change in body mass because of changes in physiological demands and food intake. Although this type of seasonal variation is minor in many species, some show a highly dynamic body mass. In such cases, if the dimensions of a nest box, particularly the entrance hole diameter, are designed based on the average body mass of a species at its minimum, it may not be suitable for when body mass is at a maximum. In the worst scenario, this may lead to sampling biases in ecological data due to the uselessness of nest boxes in certain periods, for certain individuals of the population, or both.
No previous studies have investigated the possible biases in ecological data that result from nest boxes designed improperly for a species with high seasonal variations in body mass. However, researchers have recently described some of the ways in which nest box design can considerably affect the ecological processes and phenomena under study. Clark et al. (2020) observed that the American kestrels experience a significantly higher prey delivery failure rate at the boxes with small holes compared to boxes with large holes. Møller et al. (2014) reported that there was a significant, positive relationship between clutch size and the base area of the nest box. Moreover, Saunders et al. (2020) found Carnaby's black cockatoos (Calyptorhynchus latirostris) produced chicks of smaller mass in artificial hollows of smaller volume. Nevertheless, for example, a considerable proportion of ecological studies on birds of prey using nest boxes in the Northern Hemisphere before 2012 have not provided any details on nest box design such as size, shape, and material (Lambrechts et al., 2012), which makes it impossible to compare such studies and draw conclusions on the effects of nest box design on ecological studies.
Since the forest dormouse (Dryomys nitedula), an arboreal rodent, has a highly seasonally variable body mass, nest boxes used to study this species must be designed with more caution. Most of the debates thus far over the potential effects of nest box design on the outcomes of biological studies have focused on birds, which often show relatively small temporal variation in body mass of adults (e.g., Haftorn, 1989;Lehikoinen, 1987;Wu et al., 2014). By contrast, some species of Gliridae show a quite pronounced change in body mass during activity season. For example, both the forest dormouse and the fat dormouse (Glis glis) can nearly double in body mass from the beginning of activity season to just before hibernation (Fietz et al., 2005;Juškaitis, 2015). This highlights the importance of designing nest boxes that are suitable for the target species throughout the year.
In my study of the forest dormouse using nest boxes, I noticed that the entrance holes of some of the deployed nest boxes had been gnawed. This behavior has been reported for some mammals (Goldingay et al., 2015;Le Roux et al., 2016), but not for the forest dormouse. I hypothesized that this behavior was exhibited by the individual dormice of higher body mass, who were unable to pass through the entrance holes. In other words, I argue that nest boxes used in ecological studies of the forest dormouse must have a larger entrance hole to prevent the possible exclusion of conspecifics based on their body mass, thus yielding less biased data. If the hypothesis is correct, the frequency of the gnawing behavior should be greatest in the season when dormice have higher body mass.
Ultimately, this increase in weight might also lead to a reduction in nest box occupancy by the forest dormouse in the same period. To test my hypothesis, I compared the seasonal body mass dynamics of the forest dormouse with the timing of the gnawing behavior. I also compared nest box occupancy by the forest dormouse before and after the gnawing behavior.

| MATERIAL S AND ME THODS
This field study was conducted in three plots within a study area located in the east of Zanjan province, Iran, and to the south of the Alborz Mountain Range (elevation 1,600 m). The study area consisted of private lands with agricultural land use. Other native species than the forest dormouse that could theoretically occupy nest boxes include the Caucasian squirrel (Sciurus anomalus), the beech marten (Martes foina), songbirds, the great spotted woodpecker (Dendrocopos major), and other bird species, although the dimensions of my boxes seem to be too small for the Caucasian squirrel.
Squirrels and woodpeckers might in particular gnaw on the box entrances. The forest dormouse (Figure 1a) is the only dormouse species inhabiting the study area.
In mid-April 2019, I deployed 65 nest boxes across the study area, of which 42 were in plot one, 11 in plot two, and 12 in plot three. The approximate areas of plots 1, 2, and 3 were 60,000, 8,000, and 10,000 m 2 , respectively. The plots were ca. 500-1,100 m apart. I wired the nest boxes to trees >20 m apart, at a height of 1.5-3.5 m. The internal dimensions of nest boxes were 25 * 11 * 11 cm, the same as suggested by Juškaitis (2015). However, the entrance hole was oval, with 38 and 40 mm diameters ( Uncategorized individuals were those who escaped before I could capture them or confidently assign them a weight category. I used F I G U R E 1 (a) A forest dormouse (Dryomys nitedula) in Zanjan province, Iran. (b) The nest of a forest dormouse between the branches of a tree in my study area

| RE SULTS
The

| DISCUSS ION
In general, the present study confirms the hypothesis that the forest dormice tend to gnaw on the entrance holes of nest boxes when they have a higher body mass before hibernation and thus face difficulty passing through the entrance holes. It is a common finding that animals will gnaw on nest box entrances in an attempt to gain access (see Goldingay et al., 2007). Juškaitis' study (2015), in which body mass is measured using scales and is maximized before hibernation season.
Considering the local fauna, it is quite evident that the gnawing behavior was displayed by the forest dormouse, not other species.
In the first place, the shape and the size of upper and lower incisors deduced from the chew marks best matched that of the forest dor- The only logical explanation for this behavior seems to be that dormice gnawed on the entrance holes of nest boxes to make them larger, and thus more suitable, while they were experiencing an increase in body mass before hibernation. I am quite certain that this behavior is not a sign of wood consumption as a food source.
Firstly, previous diet analyses of the forest dormouse did not report wood as a food item for the species (Juškaitis & Baltrūnaitė, 2013;Nowakowski & Godlewska, 2006). Dormice (Gliridae) lack a caecum and are less adapted to digest cellulose using enteric symbionts than other small mammals (Vorontsov, 1967, as  Another point that should be considered about this study is that there is notable geographic variation in body mass of the forest dormouse (see Juškaitis, 2015;Stubbe et al., 2012). Dormice from northern habitats tend to be of lower body mass, probably due to shorter activity season, and as a result, they might not need larger entrance holes.
In conclusion, unsuitable nest box design may, in some cases, be a hidden source of bias, yet it is overlooked in many studies. One

ACK N OWLED G M ENTS
Thanks to private landowners for allowing us to conduct fieldwork on their lands and to volunteers for assisting us in field procedures. I would also like to thank Rimvydas Juškaitis for his advice in the initial stages of the project.

CO N FLI C T O F I NTE R E S T
None.

O PE N R E S E A RCH BA D G E S
This article has earned an Open Data, for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/dryad.sj3tx 9664 and https://datad ryad.org/stash/ share/ _adguv 3-V6ndz f5YWf FqSez -00OTA wIWHp XtYUe -54k.