Urban rooftop-nesting Common Nighthawk chicks tolerate high temperatures by hyperthermia with relatively low rates of evaporative water loss

To find chicks, we surveyed Google Earth for gravel rooftops near point count sites where nighthawks were present (Newberry and Swanson 2018a) and then searched identified rooftops for nesting birds and nest sites. Systematic searches of rooftops for nests involved laying out a grid network with 1 m x 1 m squares on graveled areas of the rooftop and walking all gridlines until adult birds flushed. When adults flushed, we carefully searched the immediate vicinity for eggs or chicks. We removed 1-2 chicks (n = 15) from each nest (n = 10) at 7-14 days post-hatching. We estimated Te at rooftop nest sites using a modification of the technique described by Walsberg and Weathers (1986). We designed operative temperature thermometers from 10 cm x 12 cm ovoids (copper toilet floats) with the outside surface painted flat gray. We measured rates of evaporative water loss (EWL; g H2O hr-1) and oxygen consumption (RMR, mL O2 min-1) during heat exposure experiments using open-circuit respirometry. We exposed chicks to Ta within the range of Te recorded at 28 nest sites.We used a sliding scale of increasing temperatures to determine the Ta at which panting or gular flutter began. We began measurements with a 30 min exposure to 30 °C (within the thermoneutral zone for nighthawks, Lasiewski and Dawson 1964) and increased Ta by 3-5 °C every 30 min until we reached maximum target temperatures to which individual chicks were exposed; maximum target Ta ranged from 44 to 51 °C. Each chick was exposed to 2-4 target temperatures within this range; exposure durations at each target Ta ranged from 15-30 min. The total duration of heat exposure trials was ≤ 3 h for all chicks. To calculate total evaporative water loss (TEWL, g H2O d-1) under current and future temperature scenarios, we first plotted histograms of Te estimates by 2 °C increments for 2016 and 2017. From the histograms, we calculated the proportion of each day within each 2 °C Te bin for all nest sites in 2016 and 2017. We used EWL equations (Fig. 2) for chicks from this study to calculate EWL (g H2O hr-1) for each 2 °C Te bin (using the midpoint Te for each bin) above 30 °C. We then multiplied the EWL for each 2 °C Te bin by the proportion of the day Te was within that 2 °C bin and summed the EWL for all 2 °C Te bins to derive TEWL (g H2O d-1) for Te above 30 °C. We multiplied the allometrically estimated EWL (g H2O d-1) by the proportion of the day with Te < 30 °C to estimate daily EWL at Te < 30 °C. We then summed the values for daily EWL at Te > and < 30 °C to derive an estimate of daily TEWL for nighthawk chicks in the present study.