Avian Thermoregulation Adaptation Strategies

Climatic selection and constraint on the evolution of the desired phenotype may imply several scenarios, depending on which periods are most critical for thermoregulatory performance⁶² . Some scenarios assume that phenotype is shaped by extreme temperature events (e.g. either the warmest or the coldest days or months), which cause severe mortality of organisms that can easily overheat or overcool during these critical timeframes^{16 (link),18 ,45 (link),62} . Alternative scenarios assume that the phenotype is selected by the average temperature across year, as animals spend less time on cooling or heating, and thereby performs better in foraging or reproduction. We therefore retrieved both average, upper and lower monthly temperatures measured within species ranges to test our hypotheses under these alternative scenarios.
We obtained temperature data for each species from spatial analyses within the ‘sf’ (version 1.0-8)⁶³ and ‘raster’ (version 3.5-15)⁶⁴ R packages, using global raster layers of temperatures available in World Clim database (version 2.1)³⁴ . These rasters (Tmin, Tavg and Tmax; see below) had a resolution of 30” and were consist of monthly averages from a period of 58 years (1960-2018). The temperature metrics have been calculated within polygons of species ranges available in form of multi-polygon vector layers extracted from the BirdLife International database (version 2020.1)³⁵ .
We first excluded polygons identified as uncertain species presence, uncertain season of presence, non-native presence or species extinct in a region, leaving us with 9962 species (out of 9,993 species) with complete geographic data. Second, having polygons with only a certain, native and extant species presence we grouped them by the species (according to the phylogenetic taxonomy^{32 (link)}) and the season of presence (either breeding season, winter or year-round presence) and then we aggregated them to obtain single polygons specific to species and season (Supplementary Fig. 19). Third, using breeding and year-round species ranges, where species live at hotter period of the year; we calculated their zonal means of monthly temperature maximums (Tmax) and took the largest monthly value for each species (maximum temperature of all months). We also calculated their zonal means of monthly temperature averages (Tavg) and took the largest monthly value for each species (average temperature of hottest month). Fourth, we analogously used winter and year-round species ranges, where species live at colder period of the year. Then, we calculated their zonal means of monthly temperature minimum (Tmin) and took the lowest monthly value for each species (minimum temperature of all months). We also calculated their zonal means of monthly temperature averages (Tavg) and took the lowest monthly value for each species (average temperature of coldest month). Fifth, we took all (breeding, winter and year-round) species ranges and we calculated their zonal means of monthly temperature averages (Tavg) and averaged all monthly values to obtain average temperature of all months for each species. We also retrieved absolute latitude from the centroids of the above species ranges (breeding, winter and year-round, summarized to a single polygon per species), which described a simple geographic variation across species. Sixth, the obtained temperature measures (minimum temperature of all months, average temperature of coldest month, average temperature of all months, average temperature of hottest month and maximum temperature of all months) were used in models predicting the phenotype. Where we used these measures as response variables when predicting the temperature within species range (i.e. the environmental temperature to which the species is adapted), we transformed variables with two different formulas to normalize left-shewed distribution (Supplementary Fig. 20).
The temperature measures reflected the full range of global thermal environments occupied by birds. For example, the maximum temperature of all months ranged from −3.8 °C (in the emperor penguin Aptenodytes forsteri), through 29.9 °C (median, in the Minas gerais tyrannulet Phylloscartes roquettei) to 43.8 °C (in the Basra reed-warbler Acrocephalus griseldis). In contrast, the minimum temperature of all months ranged from −35.3 °C (in black-billed capercaillie Tetrao urogalloides), through 14.1 °C (median, e.g. in Yellow-breasted apalis Apalis flavida) to 24.8 °C (in the Seychelles warbler Acrocephalus sechellensis). Notably, our multiple measures of temperature indicated distinct aspects of seasonality in thermal conditions that may require different phenotypic adaptations across avian lineages.