What critical limiting factor, addressed by human intervention, dramatically aided the recovery of *Aix sponsa* populations?

The installation of artificial nest boxes for secure nesting real estate.

The population decline of *Aix sponsa* in the late 19th and early 20th centuries was driven by two major factors: intense hunting due to their brilliant plumage and the widespread destruction of mature nesting trees via logging. The understanding that their most specific limiting factor was the availability of secure nesting real estate—natural tree cavities often located over water—was crucial for successful conservation management. Therefore, a primary intervention involved targeted human assistance in the form of installing artificial nest boxes. The availability of these structures within a reasonable distance of water became the single greatest factor influencing local population success, allowing the species to rebound significantly once their specialized habitat needs were met, even as their visual appeal remained a point of interest.