Title: Live and Let Fly: Assessing Threats to California Condors

Program: Biology MS

Committee Chair: Todd Katzner

Committee: Todd Katzner, Julie Heath, Trevor Caughlin

Abstract: California condors (Gymnogyps californianus) are a critically endangered species that face threats, both anthropogenic and natural, that slow recovery of their population. The leading causes of death of condors are lead toxicosis, predation, trauma, fire, and collision and electrocution at power lines. As a long-lived and slow-maturing species, each death is consequential and understanding when and where threats occur can greatly benefit effective management and mitigation. Although the focus of extensive mitigation efforts, lead toxicosis remains the leading cause of death for condors. There has been less effort dedicated to addressing the second-leading anthropogenic threat, interactions with power lines. Such efforts may yield substantial benefits if strategically targeted to the areas of greatest risk.
We used GPS data from 140 condors in southern California between 2013 and 2023 to identify where on the landscape condors most frequently interact with power lines and we evaluated landscape features at these locations to identify correlates of locations where birds are most at risk of death from electrocution or collision. We identified 4.27 km2 (0.02% of condor range and 0.18% of power lines in condor range) of “danger zones” where risk was highest and that could be prime candidates for effective mitigation. Condor perching near power lines was positively associated with terrain ruggedness, while low-altitude flight near power lines was negatively associated with that ruggedness. Both these risky behaviors were most commonly associated with forested areas.
We evaluated temporal trends for all of the leading causes of death of condors to identify annual and seasonal patterns that could inform risk management and mitigation for this high priority species. Across the entire population, deaths from lead toxicosis continues to increase in frequency, likely because as the population grows, older birds are relying less on clean proffered food and instead foraging on carrion they find themselves. Death from trauma temporarily increased around 2012; the reason for this is not clear. Finally, death from fire was most likely to occur in the summer, the start of the fire season. Comparisons between subpopulation suggested that the steepest increase in lead toxicosis was in the central California subpopulation. The southern California subpopulation showed peaks in deaths related to trauma in 2012 and predation in 2010.
Together, these findings highlight that threats to condors vary spatially and temporally, and they suggest avenues for risk mitigation. Identifying power lines in “danger zones” lets managers know where to target application of mitigation such as power line retrofits and line markers. Similarly identifying temporal trends overall, and differences in subpopulations that occupy different areas supports development of adaptive mitigation strategies to target threat-specific action when and where it can have the greatest impact.