Regional spatial synchrony in density-dependent survival of a woodland passerine

Sanjana Ravindran , Alexander Eyo, Albert B Phillimore, Hannah Froy

Spatial population synchrony, the tendency of geographically separated populations to fluctuate in concert, reduces opportunities for demographic rescue and increases extinction risk, yet the demographic processes underlying it remain poorly understood. Adult survival is a key driver of population dynamics but few studies have quantified its spatial synchrony or identified the mechanisms that generate it. Spatially correlated environmental drivers such as winter weather are predicted to synchronize survival via the Moran effect, whereas density-dependent regulation is typically expected to desynchronize local dynamics. However, where population density itself varies synchronously across space, density-dependent effects on survival may instead act as a synchronizing force. Here we use a 12-year mark-recapture study of blue tits (Cyanistes caeruleus) breeding across 44 woodland sites along a 220 km latitudinal transect in Scotland to quantify synchrony in adult apparent survival and identify its drivers. Using hierarchical Cormack-Jolly-Seber models, we partitioned variance in annual apparent survival into spatial, temporal and spatiotemporal components, and quantified synchrony as the intra-class correlation (ICC) of shared temporal variance. Adult survival was highly spatially synchronous (ICC = 0.81, 95% CrI: 0.19–1.00), though the wide credible interval reflects considerable uncertainty in the precise magnitude of this synchrony. Survival was negatively associated with annual fluctuations in local breeding density, consistent with density-dependent regulation. Since local density was itself strongly spatially synchronous (ICC = 0.96), these density-dependent effects contributed to synchrony in survival. Annual fluctuations in local winter minimum temperature were also highly synchronous across sites (ICC = 0.89) and were associated with further reductions in shared temporal variance in survival, although we detected no statistically significant direct effect of temperature on survival. Our results are consistent with adult survival in this short-lived resident passerine being spatially synchronous over hundreds of kilometres, potentially driven by local weather conditions and density-dependent processes that are themselves regionally synchronous. Such synchrony in a key demographic rate heightens the collective vulnerability of spatially distinct populations to concurrent decline, underscoring the potential value of landscape-wide monitoring and management of woodland bird populations under ongoing environmental change.