Morphology and geography predict the use of heat conservation behaviours across birds

Heat conservation behaviours in birds in part involve postural adjustments to regulate the area of exposed body surfaces. However, the occurrence of these behaviours across birds, and the factors that explain their use across bird species, remains poorly known. We examined the occurrence of three distinctive bird behaviours—back rest (where the bill is tucked into the plumage), standing on one leg and sitting—across 852 bird species using a Bayesian phylogenetic comparative analytical approach, examining the phylogenetic signal in, and ecological correlates of, these behaviours. All three behaviours were far from ubiquitous across species: Only a third to a half of species in our analysis have been confirmed to use each behaviour. The behaviours all exhibited significant phylogenetic signal indicating differential usage across clades. All three behaviours were more commonly reported in bird species studied at higher latitudes. Back rest was also more often observed in species that have longer bills, relative to their body size, while standing on one leg was associated with longer relative tarsus length. Sitting behaviour was more common in heavier birds. The results suggest the use of these postures is related to environment (climate) and morphology (appendage length), and therefore most likely is associated with the need to mitigate heat loss across the exposed body surfaces. However, other potential functions of these postures, such as alleviating biomechanical load associated with large appendages and bodies, or the need to minimise energetic expenditure, are possible. A plain language summary is available for this article.