Both Osprey parents expend considerable effort protecting the nest from intruders, including other Ospreys and potential predators.
Ospreys choose structures that can support a bulky nest, and that are safe from ground-based predators. Nest sites can be safe from predators either by being difficult for a predator to climb (e.g. on a cliff) or by being over water or on a small island.
Canon 50D, EF70-300mm f/4-5.6 IS USM