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

Storks (Ciconiidae) are renowned for defecating on their legs when exposed to high temperatures, a phenomenon known as ‘urohidrosis’. Previous work suggested that this behaviour can cool down the blood supply to the legs and thus prevent hyperthermia in captive storks when overheated. However, no study has quantified the magnitude or duration of its evaporative cooling effect in free-ranging birds. Here, we combine urohidrosis data with thermal imaging and microclimate data to investigate the thermoregulatory significance of urohidrosis in White storks Ciconia ciconia during the breeding season. We show that urohidrosis can reduce leg surface temperature by up to 6.7 ºC (4.40 ± 1.04 ºC). Yet its cooling effect was of short duration (lasting no more than 2.5 min) and decreased with time since defecation. Thus, for urohidrosis to significantly contribute to heat dissipation, storks must perform it repeatedly when overheated. Indeed, individuals can perform up to 11 urohidrosis events per ..., 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...,