Density‐dependent age of first reproduction as a key factor for population dynamics: stable breeding populations mask strong floater declines in a long‐lived raptor

The age at which individuals reproduce for the first time is a key demographic factor impacting population dynamics and is subject to substantial variation across animal populations. There is also widespread empirical evidence that age of first reproduction responds to changes in population density over time, especially for long‐lived birds and mammals. The density dependence in age of first reproduction has profound impacts on the size of the non‐breeding (‘floater’) part of the population. A better understanding of floater dynamics in raptor populations is urgently needed for comprehensive assessments of conservation status and management. We use the European near‐endemic red kite Milvus milvus, a long‐lived raptor of conservation concern, as an example to investigate total population dynamics with an age‐structured demographic model. Using published estimates of survival and reproduction, we model the red kite population in Germany over four decades, considering also density dependence in the age of first reproduction in different model scenarios. Based on the literature and the results of our simulations, we show that age of first reproduction for the red kite most likely responds non‐linearly to density and that this general feedback mechanism should regularly be considered in demographic simulations. Our model results have far‐reaching implications for the conservation status of the red kite, as they highlight a drastic decrease of juvenile and non‐breeding individuals in the population over time – driven both by declining vital rates and a density‐dependent shift towards a younger age of first breeding. This process is not visible when judged only by the size of the breeding population, which our model estimates to be of similar size today as in the 1980s. The total red kite population reconstructed for Germany, however, seems diminished to nearly 50% of its former size.