Seasonal and between-population variation in heat tolerance and cooling efficiency in a Mediterranean songbird

Data collection

This database contains physiological data on thermoregulation in response to heat -- heat tolerance limit (HTL), body temperature (Tb), resting metabolic rate (RMR), evaporative water loss (EWL) and evaporative cooling efficiency (EHL/MHP) -- collected during winter and summer in two populations on Great tits Parus major submitted to different thermal environments (one from a montane, more thermally stable site; and the other from a lowland, warmer and more thermally heterogeneous site) in southwestern Iberia. Physiological data were collected by using open flow through respirometry (see Material and Methods for detailed protocols). 

Statistical analyses 

We evaluated seasonal and between population differences to asses the degree of phenotypical flexibility in those physiological thermoregulatory traits both above and below thermoneutrlaity. See detailed analyses below: 

We conducted all statistical analyses in R 4.1.2 (R Core Team, 2021). We used the segmented package (Muggeo, 2009) to determine inflection points in Tb, RMR, EWL, and EHL/MHP for each site and season. Then, the data were split based on inflection points for subsequent analyses below and above thermoneutrality (as in Whitfield et al., 2015). Linear and linear mixed-effects models were fitted to the data by using the lme4 package (Bates et al., 2015). We used the emmeans package (Lenth, 2022) to perform post-hoc pairwise contrasts between groups, and visually checked model assumptions in model residuals.

First, to assess seasonal and between-population variation in heat tolerance, we fitted a linear model with HTL (please see Heat Tolerance Limits sheet on dataset) as response variable and body mass, site, season, and the site×season interaction as predictors. Then, we fitted linear models to each thermoregulatory trait (namely Tb, RMR, EWL and EHL/MHP; please see Physiological Data sheet on dataset), using a single Tair stage per individual within (Tair ~ 30 ºC) and above thermoneutral zone (Tair ~ 37 ºC) of Great tits (as inflection points of all variables were below this last Tair stage), including body mass, site, season, and the site×season interaction as predictor variables.

Second, for summer measurements, we fitted linear mixed-effects models to evaluate population variation in the slopes of Tb, RMR, EWL and EHL/MHP against Tair above thermoneutrality, as we could obtain several measurements per individual above inflection points for each trait during this season. Initial models included Tair, body mass, site, and the Tair×site interaction as predictor variables, with ring as a random effect. When site emerged as a significant predictor, we additionally fitted separate population-specific models to calculate the slopes and y-intercepts of each thermoregulatory trait in response to Tair.