DOI: 10.5061/dryad.tht76hfbd
title: A heat-sensitive songbird’s risk of lethal hyperthermia increases with humidity

description: Increasingly frequent and intense extreme heat events are making
 heat-related avian mass mortality events more common, but the role of
 elevated humidity as a contributing factor remains unclear. Here, we
 quantified the effect of humidity on risks of lethal hyperthermia for Blue
 Waxbills (Uraeginthus angolensis), the species most common among the
 victims of South Africa’s first documented heat-related mortality event
 involving wild birds. We quantified body temperature (Tb), metabolic heat
 production, and evaporative heat loss at air temperatures (Tair)
 approaching and exceeding normothermic Tb in dry (1.1 ± 0.9 g m⁻³) and
 humid (21.3 ± 0.4 g m⁻³) air. The humid treatment was associated with
 significant reductions in evaporative cooling capacity and overall heat
 tolerance, with maximum tolerable Tair ~2 °C lower (45.7 °C) compared to
 the dry treatment (47.9 °C). A model of end-of-century exposure for the
 waxbills reveals that elevated humidity will increase the risks of lethal
 hyperthermia 3- to 7-fold in some parts of southern Africa.
description: # A heat-sensitive songbird’s risk of lethal hyperthermia increases with
 humidity Dataset DOI: [10.5061/dryad.tht76hfbd](10.5061/dryad.tht76hfbd)
 ## Description of the data and file structure Files are all associated
 with Blue Waxbill (*Uraeginthus angolensis*) physiological response
 variables to two flow-through respirometry experiments. Experiment or
 protocol 1 involved waxbills (n = 10) being subjected to dry air (~0 g
 m⁻³) and a stepped air temperature profile starting at 28°C and
 incrementally increasing until birds reached their thermal endpoints.
 Experiment or protocol 2 involved waxbills (n = 10) being subjected to
 humid air (21 g m⁻³), and a stepped air temperature profile starting at
 34°C and incrementally increasing until birds reached their thermal
 endpoints. The objective was to determine how birds physiologically
 respond to high temperatures under dry and humid conditions, to
 extrapolate whether humidity increases lethal hyperthermia risks during
 heatwaves in humid environments. ### Files and variables #### File:
 NLiddle_Biology_Letters_Data_and_Script_GDrive_submission.zip
 **Description:**  1 R script and 11 .csv files associated with the study
 titled, “A heat-sensitive songbird’s risk of lethal hyperthermia increases
 with humidity”. The data files are all derived from the original file,
 titled “BWall.csv”, and are provided for easy running of the script. All
 variables are the same as those in BW_all. The script is divided into two
 sections: 1) Linear mixed effects models and 2) t-tests on maximum
 response variables across the two protocols, as well as plot code. #####
 **File: BW_all.csv** **Description:** All measured physiological responses
 with increasing air temperature across the dry and humid air respirometry
 protocols. There are no blank values in the provided files. **Variables:**
 \[1] "Date" "protocol" "Bird"
 "Sex" "Mass" "Tcab" \[7] "Ta"
 "Tb" "VCO2" "VH2O" "waEWL"
 "msVCO2" \[13] "msH2O" "waRMR"
 "msRMR" "EHL" "EHL.MHP"
 "Treatment" Date: yyyy/mm/dd format. Protocol: describes whether
 subjects belonged to the dry or humid air respirometry treatments. bird:
 unique ID associated with each bird. IDs are all prefixed with “BW”, then
 “D” or “H” for dry or humid protocol, respectively, followed by an integer
 denoting the number of birds sampled. Sex: the biological sex of the bird.
 Possible values - “male”, “female”, “unknown”, “unknown (M?)” Mass: mass
 of the bird in grams (g) Tcab: calibrated air temperature of the metabolic
 chamber in each stepped air temperature profile in °C. This is the
 temperature manually input into the system during experiments by the
 observer. Ta: actual measured air temperature (°C) inside the enclosed
 metabolic chamber housing the bird. Tb: bird body temperature (°C) as
 measured by a PIT tag and downloaded from a receiver VCO2: measured carbon
 dioxide (CO~2~) production (ml/min) from each bird as measured from
 excurrent air flows from the metabolic chambers. VH2O: measured water
 (H~2~O) production (mg/min) from each bird as measured within the
 excurrent air flows from the metabolic chambers. waEWL: calculated
 evaporative water loss (g/hr) produced from each bird, as calculated using
 VH~2~O. msVCO2: calculated mass specific arbon dioxide (CO~2~) production
 (ml/g/min) from each bird per Tcab msH2O: calculated mass specific ater
 (H~2~O) production (mg/g/min) from each bird per Tcab waRMR: calculated
 resting metabolic rate (W) from each bird for each bird per Tcab msRMR:
 calculated mass-specific resting metabolic rate (W) from each bird per
 Tcab EHL: calculated Evaporative Heat Loss from each bird per Tcab
 EHL.MHP: calculated Evaporative Heat Loss/Metabolic Heat Production from
 each bird per Tcab Treatment: similar to “Protocol” by denoting the
 approximate absolute humidity was per protocol. Possible values- “0 g.m⁻³”
 and “19 g.m⁻³” ## Code/software ##### File: Comparing dry and humidity.r
 Script is R-compatible, and can be run in RStudio Version 10.0.26100.4061.
 All input files are comma-delimited files (.csv), which can be opened and
 inspected in Microsoft Excel or other spreadsheet-compatible programs and
 software. Required packages: library(lmerTest) library(MuMIn)
 library(lme4) library(car) library(sjPlot) library(sjmisc)
 library(ggplot2) library(jtools) library(tidyverse) library(ggpubr)
 library(rstatix) library(dplyr)