Heat tolerance for many birds under climate and land use change scenarios could be compromised in the future. Common Nighthawks (Chordeiles minor) belong to the Caprimulgiformes, a generally heat-tolerant order, but few studies have assessed heat tolerance in Caprimulgiform chicks, which might be particularly susceptible to heat stress. In the Midwestern U.S., nighthawks primarily nest on flat graveled rooftops in urban areas, as natural nesting habitats are limited. Urban rooftop-nesting nighthawks are likely exposed to higher environmental temperatures than birds nesting at more thermally buffered natural sites and evaporative cooling might be impeded by the typically high summer humidity in their Midwest breeding range. This combination of heat and humidity might negatively impact heat tolerance of nighthawk chicks. We exposed 7 to 14 day-old nighthawk chicks (n = 15) from rooftop nests to ambient temperatures up to 51° C at typical summer dew points. Chicks initiated gular flutter at a mean air temperature of 42.4 ± 3.4 (SE) °C. Evaporative water loss (EWL) rates increased significantly with increasing temperature above 44.0 ± 1.5 (SE) °C. Chicks showed little evidence of lower and upper bounds of the thermal neutral zone over the range of temperatures (30-44 °C) for which we measured oxygen consumption. Body mass loss was significantly positively correlated with temperature during heat exposure trials. Chicks tolerated ambient temperatures up to 51 °C and body temperatures up to 48 °C, which, along with the high temperatures at which gular flutter and high rates of EWL were initiated, suggest that nighthawk chicks are tolerant of high air temperatures, even with relatively high humidity. Given the high rates of mass loss and high body temperatures at hot air temperatures, chick heat tolerance mechanisms could be detrimental for rooftop-nesting nighthawks given projected increasing trends for both heat and humidity in the Midwestern U.S.

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 T_e 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 T_a within the range of T_e recorded at 28 nest sites.We used a sliding scale of increasing temperatures to determine the T_a 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 T_a by 3-5 °C every 30 min until we reached maximum target temperatures to which individual chicks were exposed; maximum target T_a ranged from 44 to 51 °C. Each chick was exposed to 2-4 target temperatures within this range; exposure durations at each target T_a 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 T_e estimates by 2 °C increments for 2016 and 2017. From the histograms, we calculated the proportion of each day within each 2 °C T_e 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 T_e bin (using the midpoint T_e for each bin) above 30 °C. We then multiplied the EWL for each 2 °C T_e bin by the proportion of the day T_e was within that 2 °C bin and summed the EWL for all 2 °C T_e bins to derive TEWL (g H₂O d^-1) for T_e above 30 °C. We multiplied the allometrically estimated EWL (g H₂O d^-1) by the proportion of the day with T_e < 30 °C to estimate daily EWL at T_e < 30 °C. We then summed the values for daily EWL at T_e > and < 30 °C to derive an estimate of daily TEWL for nighthawk chicks in the present study.