Continent-wide drivers of spatial synchrony in breeding demographic structure across wild great tit populations

Variation in age structure influences population dynamics, yet we have
limited understanding of the spatial scale at which its fluctuations are
synchronised between populations. Using 32 great tit populations, spanning
4○W–33○E and 35–65○N involving >130,000 birds across 67 years, we
quantify spatial synchrony in breeding demographic structure (subadult vs.
adult breeders) and its drivers. We show that larger clutch sizes, colder
winters, and larger beech crops lead to younger populations. We report
distance-dependent synchrony of demographic structure, maintained at
approximately 650km. Despite covariation with demographic structure, we do
not find evidence for environmental variables influencing the scale of
synchrony, except for beech masting. We suggest that local ecological and
density-dependent dynamics impact how environmental variation interacts
with demographic structure, influencing estimates of the environment’s
effect on synchrony. Our analyses demonstrate the operation of synchrony
in demographic structure over large scales, with implications for
age-dependent demography in populations.