Impact of fog on California waterfowl flight activity: historical and modern insights into effects post-Clean Air Act

Since establishment of the Clean Air Act in the early 1970s, occurrence of the dense 'Tule Fog', historically prevalent throughout winter across California's Central Valley, has substantially reduced. At the same time, waterfowl body masses have generally increased. Flight is metabolically expensive, and fog visually and navigationally impairs birds in flight, likely causing them to remain aloft for longer than usual periods. If less fog results in less flight and reduced energy expenditure, then fewer winter Tule fog events could contribute to increased body masses of California waterfowl since the mid-1980's. Therefore, we aimed to assess the relationship between waterfowl flight and fog occurrence/density with historic (1991-93) and modern (2015-23) waterfowl tracking data in the Central Valley of California (CCV). Historic tracking data showed that the probability of flight increased with increasing fog density. Birds were significantly more likely to fly in fog than when there was no fog and most likely to fly in heavy fog. Modern data showed similar responses to fog with flight occurring significantly more during dawn fog events. This relationship between improved waterfowl body mass and fewer fog events may provide an opportunity to redirect scarce funding to focus on other population requirements such as improving habitats for nesting, molting and brood rearing that are currently lacking. Unanticipated benefits of the Clean Air Act should stand as a strong recommendation to maintaining this Act into the future.