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

Citation

Newberry, Gretchen (2022), Urban rooftop-nesting Common Nighthawk chicks tolerate high temperatures by hyperthermia with relatively low rates of evaporative water loss, Dryad, Dataset, https://doi.org/10.5061/dryad.5mkkwh75c

Abstract

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.

Methods

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 (RMRmL 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.