Productivity drives the dynamics of a red kite source population that depends on immigration

Abstract

Local population dynamics are driven by local processes such as temporal variation of productivity, survival, emigration and population stage structure, and by processes originating from outside the local population, such as immigration. Populations may operate as sources that contribute more individuals than have died or as sinks that depend on neighbouring populations. Knowing demographic processes driving the dynamics of a local population and the significance of a local population in a system of multiple populations is crucial for understanding population dynamics and requires detailed demographic analyses. We studied demographic drivers in a red kite Milvus milvus population located in Germany that was monitored for 34 years using integrated population modelling. We specified the model in such a way that the numbers of experienced breeders, local recruits, locally born non-breeders and immigrants are estimated explicitly, applied a retrospective perturbation analysis to identify the demographic drivers and assessed the source-sink status of the population. The study population increased on average by 1% per year. The number of breeders was about double than the number of locally born non-breeders, and the number of experienced breeders exceeded the number of local recruits and immigrants by a factor of six to nine. The retrospective analysis identified productivity, i.e. the number of fledglings per breeding pair, as the main demographic driver, followed by adult survival and immigration. As other studies show close links between food supply and productivity, it is likely that food supply plays a critical role in red kite population dynamics. The study population contributed more individuals than it lost through mortality, but due to emigration of locally born individuals it was not self-sustainable and depended on immigration. This quantifies the population as a dependent source and shows that red kite populations are linked across large spatial scales.