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Birds usually practise ‘unihemispheric’ sleep, meaning that they rest one half of their brain at a time, keeping them semi-aware. But they’re still at high risk of predation. Strategies to minimise the chances of being eaten include resting on water or on predator-free islands, roosting in dense shrubs or high up in trees, or hiding in cavities such as chimneys. Birds will often opt for safety in numbers, grouping together in hundreds or even thousands.