From the brink of extinction to regulation: northern Europe's white‐tailed eagles now face density dependence and climate constraints after rapid population growth

Population growth reflects the combined influence of regulation and density‐independent factors operating through demographic processes. Under exceptional circumstances (e.g. populations recovering from near‐extinction), growth may initially be weakly regulated but typically slows as negative density dependence (NDD) sets in. Despite some early‐20th‐century protective measures, the white‐tailed eagle Haliaeetus albicilla ( WTE) was on the brink of extinction across Europe (including several local extinctions) due to multiple stressors, including human persecution and pollution. Following protection, populations began recovering in the 1950s–1960s and continued to grow for 60–70 years, with accelerated growth from the 1970s to around 2000. In northern Norway, the species remained relatively numerous. Yet in our study area (Steigen, Norway), the number of breeding pairs increased from < 10 (1997) to 46 (2023), peaking at 119 (2003). This recovery under protection and initially low population density provides a natural experiment to examine population dynamics in a long‐lived species. During this period, climatic conditions changed, including an increase in winter sea‐surface temperature (SSTw). Population growth, the proportion of successful nests, and chicks nest −1 showed evidence of NDD, and SSTw (one‐year lag) had adverse effects on reproduction. Most relationships were nonlinear, with the lowest values occurring at high temperatures and population sizes. Nonetheless, NDD was the only significant effect on growth. The probability of unsuccessful breeding and singletons increased with population size and SSTw (two‐year lag). Still, the climate effect was opposite and more pronounced for singletons. NDD primarily determined the probability of producing ≥ 2 chicks. Despite recovery, several past stressors persist, with NDD now playing a pivotal role that may limit future growth and complicate predictions of population dynamics. Improved understanding of regulatory mechanisms, climate‐driven changes in prey availability, and region‐specific drivers is essential for elucidating WTE population trajectories from local to regional scales.