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Raptor populations on islands are limited by resource availability and the dispersal possibilities for young birds, which are often determined by the size of the island. This leads to differences in population dynamics and viability compared to mainland populations. Human land use modifications on islands such as agriculture, forestry, excessive hunting, and urban infrastructure development may affect resource availability and increase risks to these populations, ultimately threatening their survival. Consequently, many island raptor populations have been dramatically reduced or driven to extinction and have never fully recovered. The conditions necessary for their long-term persistence remain uncertain. Gotland, a large, human-dominated island located in the Baltic Sea, is home to one of the densest populations of Golden Eagles (Aquila chrysaetos) in the world. However, the drivers of population dynamics remain unknown, and many speculations exist that require empirical testing. Approximately 86 Golden Eagle territories were identified and surveyed across Gotland, an island spanning approximately 3,200 sq.km (152 km long, 52 km wide, with an 800 km coastline). We investigated the spatial drivers of breeding dynamics in this eagle population, evaluating the effects of territorial habitat composition, overlap with White-tailed Eagles, prey density, and neighborhood effects on territorial productivity. The average productivity was 0.41 fledglings per pair, which varied annually, with approximately 72% of territories occupied and 32% being successful. Despite significant variation in habitat composition across territories, spatial differences in productivity were primarily influenced by the proportion of coniferous forest (nesting habitat), access to coastal areas (greater prey diversity), the density of the main prey species (roe deer, Capreolus capreolus), and the reproductive status of neighboring territories in a year. Several novel findings emerged: the role of roe deer as a potential prey species had been previously underappreciated, proximity to the coast was associated with increased productivity, and the variation in spatio-temporal reproductive dynamics across neighboring territories appears to influence overall population dynamics. This relationship warrants further study. We discuss the implications of these findings for the long-term conservation and persistence of this iconic island population and similar populations worldwide.
Raptor populations on islands are limited by resource availability and the dispersal possibilities for young birds, which are often determined by the size of the island. This leads to differences in population dynamics and viability compared to mainland populations. Human land use modifications on islands such as agriculture, forestry, excessive hunting, and urban infrastructure development may affect resource availability and increase risks to these populations, ultimately threatening their survival. Consequently, many island raptor populations have been dramatically reduced or driven to extinction and have never fully recovered. The conditions necessary for their long-term persistence remain uncertain. Gotland, a large, human-dominated island located in the Baltic Sea, is home to one of the densest populations of Golden Eagles (Aquila chrysaetos) in the world. However, the drivers of population dynamics remain unknown, and many speculations exist that require empirical testing. Approximately 86 Golden Eagle territories were identified and surveyed across Gotland, an island spanning approximately 3,200 sq.km (152 km long, 52 km wide, with an 800 km coastline). We investigated the spatial drivers of breeding dynamics in this eagle population, evaluating the effects of territorial habitat composition, overlap with White-tailed Eagles, prey density, and neighborhood effects on territorial productivity. The average productivity was 0.41 fledglings per pair, which varied annually, with approximately 72% of territories occupied and 32% being successful. Despite significant variation in habitat composition across territories, spatial differences in productivity were primarily influenced by the proportion of coniferous forest (nesting habitat), access to coastal areas (greater prey diversity), the density of the main prey species (roe deer, Capreolus capreolus), and the reproductive status of neighboring territories in a year. Several novel findings emerged: the role of roe deer as a potential prey species had been previously underappreciated, proximity to the coast was associated with increased productivity, and the variation in spatio-temporal reproductive dynamics across neighboring territories appears to influence overall population dynamics. This relationship warrants further study. We discuss the implications of these findings for the long-term conservation and persistence of this iconic island population and similar populations worldwide.
Insular fauna often suffers significant predation from invasive mammals, yet robust evidence of their impact from rigorous demographic analysis remains scarce. We aimed to understand the life history and population dynamics of the elusive Cape Verde petrel Pterodroma feae, an endemic seabird of the Cabo Verde archipelago, which is severely predated by cats. We analysed capture-mark-recapture data from birds mist-netted in a courtship area in 2007-2021 and monitored their nests between 2012 and 2021 on Fogo Island. Across all islands where this species breeds, we recorded cat predation rates and made additional captures of petrels in courtship areas. Demographic analysis revealed an 8.8% annual population decline of Cape Verde petrels, primarily driven by a 14.7% surplus mortality among breeders from threats in breeding areas, likely tied to the annual cat predation rate of at least 4% observed in monitored nests. Immature males exhibited lower survival rates, likely due to their increased vulnerability while seeking initial nest sites. The limited connectivity detected among and within islands, along with strong nest-site and mate fidelity rates, highlight the urgency of preventing local extinctions, which would be challenging to reverse. This study emphasizes the unsustainability of cat predation rates and the insufficiency of enhancing breeding success to counter the population decline, and it provides compelling evidence of a concerning decline of the Cape Verde petrel, primarily attributable to cat predation. Moreover, it emphasizes the significance of long-term demographic studies to understand the impact of invasive species on endangered populations.
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