Keeping cool with poop: Urohidrosis lowers leg surface temperature by up to 6ºC in breeding White storks

During the spring of 2021 (from late March to mid-June), we conducted a study on the thermoregulation and heat exchange of White storks Ciconia ciconia at a nesting colony in Extremadura, southwestern Spain (38º52’ N; 6º19’W). This colony hosted 12 active nests that were visited twice a week to obtain thermal images to study the potential role of different body parts (particularly beaks and legs) as thermal windows along a range of different environmental temperatures. We collected thermal images of each nest throughout the study period, from early incubation to the end of the breeding season. Thermal imaging sessions were performed from 1 h after sunrise until maximum ambient daily temperature was reached (which was confirmed by tracking temperature with a portable weather station; see below), measuring no more than 2 nests per day (switching the camera between focal nests during thermal imaging sessions). We employed a handheld thermal infrared camera (FLIR E95, resolution = 464 x 348 pixels, FLIR Systems, USA), equipped with a 14º lens. We automatically took a thermal image every 20 seconds and simultaneously recorded local weather data – environmental temperature, Ta (Ta_Kestrel, ºC); wind speed, (m s-1); and relative humidity (%) – every 30 s using a portable weather station (Kestrel 5400, Kestrel Instruments, USA) mounted on a vane and placed on the colony (at 1.5 m height). Although thisurohidrosis behaviour appeared to be frequent in White storks during late spring, we only could obtain thermal images of 22 urohidrosis events (four in adults and 18 in nestlings), from five different nests and seven individuals (three adults and four nestlings) during the 31th of May, and the 7th, 9th and 10th of June. A total of 173 thermal images of individuals showing urohidrosis were taken, but we discarded those images in which legs were not completely visible or those in which storks’ position could compromise accuracy of thermal measures, as incidence angle influences apparent emissivity – with increased angle resulting on declines in apparent emissivity (Playà-Montmany & Tattersall 2021). Then, we selected for the analyses those pictures in which a lateral (or almost lateral) view of the legs could be taken. Finally, 81 images from these 22 urohidrosis events met this criterion and were analyzed using the program FLIR ResearchIR (FLIR Systems, USA). For each image, we assumed an emissivity of 0.95 and set ambient temperature and relative humidity from values obtained from the weather station. Average surface temperature of the leg area covered by excreta was measured using the ‘freehand roi’ function, while average surface temperature from the adjacent region free of excreta was measured by using the ‘line roi’ function. We employed the ‘freehand roi’ function to measure urohidrosis due to the irregular shape of excreta marks along the leg. Overall, temperature measures were calculated on areas that had, on average, 20.73 ± 7.97 pixels. We calculated urohidrosis gradient as the difference between the surface temperature of the bare skin leg region and that of the region covered by excreta. When possible, we repeatedly calculated urohidrosis gradient for the same event to estimate its duration and magnitude over time.