Ecogeography of group size suggests differences in drivers of sociality among cooperatively breeding fairywrens

Cooperatively breeding species exhibit a range of social behaviours associated with different costs and benefits to group living, often in association with different environmental conditions. For example, recent phylogenetic studies have collectively shown that the evolution and distribution of cooperative breeding behaviour is related to the environment. However, little is known about how environmental variation may drive differences in social systems across populations within species, and how the relationship between environmental conditions and sociality may differ across species. Here, we examine variation in social group size along a steep environmental gradient for two congeneric cooperatively breeding species of fairywrens (Maluridae) and show that they exhibit opposing ecogeographic patterns. Purple-backed fairywrens, a species in which helpers increase group productivity, have larger groups in hot, dry environments and smaller groups in cool, wet environments. By contrast, supe..., To aquire group size data, we performed two observational sampling transects along a rainfall and temperature gradient in south-eastern Australia, one over ten days in December 2018 (late breeding season) and the second over seven days in August 2019 (early breeding season). We visited local parks, conservation areas, and national parks, starting in coastal Victoria and extending north to inland New South Wales, passively observing fairywren social groups and recording their composition (group size and sex of individuals) at populations along the transect. Groups were only included in analyses (and this dataset) if the entire group was identified (though description of incomplete groups can be found on eBird via checklist identifies included in this dataset).
Climate variables were calculated from two gridded datasets available through the Australian Bureau of Meteorology. Rainfall grids have a resolution of 0.05 degrees (~5 km; Evans et al. 2020) and maximum temperature grids have a re...,