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Seasonally breeding animals often exhibit different social structures during non-breeding and breeding periods that coincide with seasonal environmental variation and resource abundance. However, we know little about the environmental factors associated with when seasonal shifts in social structure occur. This lack of knowledge contrasts with our well-defined knowledge of the environmental cues that trigger a shift to breeding physiology in seasonally breeding species. Here, we identified some of the main environmental factors associated with seasonal shifts in social structure and initiation of breeding in the red-backed fairywren (Malurus melanocephalus), an Australian songbird. Social network analyses revealed that social groups, which are highly territorial during the breeding season, interact in social “communities” on larger home ranges during the non-breeding season. Encounter rates among non-breeding groups were related to photoperiod and rainfall, with shifting photoperiod and increased rainfall associated with a shift toward territorial breeding social structure characterized by reductions in home range size and fewer encounters among non-breeding social groups. Similarly, onset of breeding was highly seasonal and was also associated with non-breeding season rainfall, with greater rainfall leading to earlier breeding. These findings reveal that for some species, the environmental factors associated with the timing of shifts in social structure across seasonal boundaries can be similar to those that determine timing of breeding. This study increases our understanding of the environmental factors associated with seasonal variation in social structure and how the timing of these shifts may respond to changing climates.

Fire disturbance in tropical savannas is integral to maintaining habitat heterogeneity and biodiversity, but its impact on avian species is highly variable. Savannas in northern Australia have recently been invaded by gamba grass (Andropogon gayanus), a perennial tussock grass that fuels late season fires at eight times the intensity of native vegetation. As gamba grass rapidly outcompetes native species and promotes more frequent and intense fires, it greatly decreases landscape heterogeneity and alters the effect of fire in tropical savannas. To investigate how a small passerine, the red‐backed fairywren (Malurus melanocephalus), responds to fire disturbance and gamba grass cover, we studied their fine‐scale habitat use throughout the dry season before and after a high intensity fire. We used two spatially distinct approaches, radio‐telemetry and a transect‐based population census, to quantify fairywren habitat use at the group and population level, respectively. Radio‐telemetry and transect surveys revealed no direct mortality associated with the severe bushfire during the middle of the study season, suggesting fairywrens are resilient in the short‐term to fire disturbance. Our results indicate that fairywrens are largely flexible in their habitat use – instead of relocating after fire, they re‐centre their home range around the most photosynthetically productive habitats, dominated by saplings. While we found substantial variation in habitat use among social groups, red‐backed fairywren groups generally avoided dense habitat areas dominated by mature gamba grass. We conclude that red‐backed fairywrens are resilient to fire and flexible in their habitat use in the short‐term; however, in the long‐term, gamba grass may pose a threat to population viability. The importance of flexible behavioural strategies in tropical passerines will increase as fire regimes are exacerbated by invasive species and climate change.