Age-specificity in territory quality and spatial structure in a wild bird population

Abstract

Age influences behaviour, survival, and reproduction; hence variation in population age structure can affect population-level dynamics. The extent of spatial age structure may be important in driving spatially-variable demography, particularly when space-use is linked to reproduction, yet it is not well understood. Here, we use long-term data from a wild passerine bird population to quantify covariance between territory quality and age and assess spatial age structure. We find associations between age and aspects of territory quality, but little evidence for spatial age structure compared to territory quality and reproductive spatial structure. We also report little between-year repeatability of spatial age structure compared to structure in reproductive output. We suggest that high breeding site fidelity and frequent territory turnover by younger breeders, driven by high mortality and immigration rates, limits the association between age and territory quality and weakens overall spatial age structure. Greater spatial structure and repeatability in reproductive output compared to age suggests that in this system habitat quality may be more important in driving spatially-variable demography than age. We suggest that the framework developed here can be used in many other populations to assess the causes and consequences of spatial age structure.