| Summary: | Population numbers result from the combined effect of exogenous (weather, food, predation, anthropogenic impact) and endogenous (behaviour, density) factors on the population, which often act simultaneously resulting in spatial variation in population distribution and density. The effects of these factors can mask each other and operate across different spatiotemporal scales making the study of population regulation challenging. Studies on variation in demography and habitat quality across gradients in breeding density can give important insights into population regulation. The largest lowland basin in Iceland has a unique gradient of wader breeding density, an ideal setting for population regulation studies. Using the variation in Whimbrel, Numenius phaeopus, abundance recorded during a long-term monitoring programme and a three-year study in South Iceland, I aimed to better understand the possible mechanisms regulating this population. To do so, I investigated population trends and the relationships between demographic processes, habitat features, and behaviour of this species. Adult abundance decreased from 2012-2021 possibly due to habitat degradation. Brood abundance did not show a trend (2013-2021) but showed high environmental variability. Adults used a variety of semi-natural habitats, and their annual abundance was not directly explained by the process of ecological succession, potential prey abundance, or potential predator pressure. Adult numbers influenced the frequency of the aerial display behaviour with Whimbrels displaying more often at low densities during pre-incubation, probably for mate attraction. Displaying at all densities occurred during the rest of the breeding season and may serve other functions such as resource defence. Broods avoided areas of anthropogenic origin, and some semi-natural habitats were more important than others during chick-rearing and migration preparation. Food availability is likely a driver of variation in habitat quality and more broods were produced at a higher ...
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