Which species is known to occupy and defend the largest territory?

The Gyrfalcon is renowned for occupying and defending the largest territory among the given options. This is largely attributed to its hunting and nesting habits, which require a significant expanse of land to ensure access to sufficient prey. Gyrfalcons are primarily found in the Arctic and subarctic regions, where their extensive territories can encompass vast distances in order to adapt to the challenges posed by the variable availability of food sources in such environments. Their larger size, along with their powerful flight capabilities, allows them to cover and dominate extensive areas, making them highly territorial.

In contrast, the other species listed, such as the Northern Harrier, Cooper's Hawk, and American Kestrel, tend to have smaller territories as they occupy a range of habitats that provide resources within a more localized area. Their hunting strategies and ecological needs are typically met within these smaller territories, as they target specific prey within defined locations that suit their size and hunting style.