Camera trap data of small mammals at experimental dishes

We conducted cafeteria experiments in 2012/13, 2018/19, 2019/20 and 2020/21. In early October, we presented 18 dishes with 10 individually-marked seeds of Fagus sylvatica, two of which were fitted with a VHF-transmitter and eight with a 25-cm wire tag with a colorful flag. To confirm the species identity of visiting rodents, we installed camera traps at each experimental dish in autumn 2019 and 2020. Rodent species composition recorded at the experimental dishes varied between 2019 and 2020, but taxonomic groups other than small mammals were extremely rare in both years. In 2019, camera traps recorded 723 independent detections of small mammals (including 524 encounters of Apodemus spp. and 160 of Myodes glareolus) and in 2020, 79 small mammal detections (including 33 encounters of Glis glis and 13 of Myodes glareolus) during 15 and 23 days of operation, respectively. Encounter rates on camera traps were in line with species composition of life trapping and highlighted the enormous variation of visitation rates at experimental dishes between subsequent years. The only camera trap observation of a larger mammal was triggered by Meles meles in 2020. In addition to the frequent encounters of small mammals at the experimental dishes, we also detected two birds in 2019 and another one in 2020.

Methods
We installed 18 camera traps (Victure HC600) 60 cm above-ground facing the experimental dishes of our cafeteria experiment and used motion-activated photographs for the identification of mammals and birds in October 2019 and 2020. We manipulated the lenses to ensure sharp images at close distance by turning each lens about 45° counterclockwise. Our custom-built camera rack consists of 20x20 cm plexiglas for the attachment of the camera, screwed on a 1 m threaded rod (10 mm diameter) stuck into the ground (about 30 cm deep). We stick the rod through drilled holes at the center of two aluminum rulers that form a cross on top of the forest floor to enhance stability and reduce vibrations caused by wind. A threaded nut ensures the camera lens to be installed in the preferred distance to the ground (60 cm in our case).

The resulting photographs depict a small segment of the forest floor (about 35*50 cm) with animals being photographed at short distances, to enhance the identification of small mammals and birds. We chose the maximum sensitivity of the PIR-Sensor to maximize detection probability and minimum illumination of the LED-flash to avoid overexposed photographs. Cameras were set to take a series of three images and a 5-seconds video when triggered. We did not use a sleep-function to minimize the risk of missed animal encounters. 

Camera traps were left for 15 days of operation in 2019 and 23 days in 2020 and made 15,593 and 13,605 images, respectively. Images were manually checked and labeled via the software digiKam (Version 5; https://www.digikam.org). We did not label the avi-files, but reviewed more than 9,000 video sequences to aid species identification. We summarized detections of the same taxon within 5 minutes for each location to define independent trigger events or encounters by avoiding repetitive detection of the same individual lingering in the focal view of the devices. We could not determine one encounter in 2019 and another 15 encounters of small mammals in 2020, due to blurred or bad quality images. All other animal encounters could be identified at the genus- or species-level.

我们于2012/13、2018/19、2019/20及2020/21四个年度开展了自助餐式取食实验（cafeteria experiment）。每年10月初，我们在18个实验投放点各放置10粒带有独立标记的欧洲山毛榉（Fagus sylvatica）种子：其中2粒搭载甚高频发射器（VHF-transmitter），剩余8粒则佩戴带有彩色旗标的25厘米金属线标记。为确认到访啮齿类的物种种类，我们于2019年和2020年秋季在每个实验投放点布设了红外相机陷阱（camera traps）。2019年与2020年实验投放点记录的啮齿类物种组成存在差异，但两年间除小型哺乳类外的其他分类类群均极为罕见。2019年，红外相机在15天的有效布设期内记录到723次小型哺乳类独立触发事件，其中包括524次姬鼠属（Apodemus spp.）活动记录与160次欧洲红背䶄（Myodes glareolus）活动记录；2020年的23天有效布设期内则记录到79次小型哺乳类触发事件，其中33次为睡鼠（Glis glis）活动记录、13次为欧洲红背䶄活动记录。红外相机记录的到访率与活体诱捕的物种组成结果一致，同时也凸显了相邻年度间实验投放点的动物到访率存在巨大差异。2020年的红外相机仅记录到1次大型哺乳类活动，触发者为狗獾（Meles meles）。除实验投放点频繁出现的小型哺乳类外，我们还在2019年记录到2次鸟类活动，2020年又记录到1次。

Methods
我们于2019年和2020年10月布设了18台Victure HC600型号红外相机，将其安装在距离地面60厘米的高度，朝向实验投放点，通过触发式拍摄照片以识别哺乳类与鸟类。我们将每台相机的镜头逆时针旋转约45°，以确保近距离拍摄时图像清晰。我们自制的相机支架采用20×20厘米的有机玻璃（plexiglas）作为相机安装底座，将其固定在一根长1米、直径10毫米的螺纹杆上；该螺纹杆被插入地下约30厘米深处。我们将螺纹杆穿过两根铝制直尺（aluminum rulers）中心的钻孔，使两根直尺在林地表层形成十字结构，以此提升支架稳定性并降低风力引发的振动。通过螺纹杆上的螺母可将相机镜头固定在预设高度（本实验中为60厘米）。

拍摄得到的照片仅覆盖林地表层的一小块区域（约35×50厘米），且动物均在近距离内被拍摄，以此提升小型哺乳类与鸟类的识别精度。我们将被动红外传感器（PIR-Sensor）调至最高灵敏度以最大化检测概率，同时将LED闪光灯（LED-flash）的亮度调至最低以避免照片过曝。相机被设置为触发后连拍3张照片并录制一段5秒的视频；我们未启用休眠模式，以降低漏拍动物活动的风险。

2019年红外相机的有效布设时长为15天，共拍摄15593张照片；2020年为23天，共拍摄13605张照片。所有照片均通过digiKam软件（版本5；https://www.digikam.org）进行人工核查与标注。我们未对AVI格式的视频文件进行标注，但核查了超过9000段视频序列以辅助物种识别。我们将同一地点、5分钟内记录到的同一分类单元（taxon）的活动归为一次独立触发事件或到访记录，以避免将同一动物在视野内停留时的重复检测算作多次记录。由于图像模糊或画质不佳，2019年有1次到访记录无法确定物种，2020年则有15次小型哺乳类到访记录无法确定物种。其余所有动物到访记录均可鉴定至属或物种水平。