What factor complicates the prevention of diseases with wild birds as reservoirs?

The prevention of diseases with wild birds as reservoirs is complicated primarily because these birds migrate freely without restrictions. This migratory behavior allows them to travel great distances and spread pathogens across vast regions, often without environmental barriers that might otherwise limit the spread of disease. This mobility makes it challenging to track and manage outbreaks, as these birds can be carriers of diseases without showing symptoms, thus posing a continuous risk to other wildlife, livestock, and even human populations.

In contrast, the other options do not accurately capture the complexity involved in disease prevention relating to wild birds. For instance, attempting to control wild bird populations in urban areas presents significant difficulties due to their adaptability and legal protections, while the notion that wild birds have no known diseases is incorrect, as many are known carriers of various pathogens. Additionally, the monitoring of wild birds can be quite complex due to their widespread distribution and the dynamic environments they inhabit, making it hard to gather consistent and comprehensive data. Therefore, the unrestricted migration of wild birds is the predominant factor complicating disease prevention efforts.