On leaden wings: Tracking heavy metal contamination in Pacific Coast raptors

As stunningly beautiful apex predators who interact with a multitude of different ecosystems, raptors are commonly used as “sentinel” taxa for anthropogenic ecosystem disturbance, as well as important cultural symbols and flagship species for global conservation. In particular, they are valuable indicators of environmental pollution due to biomagnification of toxic chemicals up their food chains. In California, while the impacts of pesticides on the diets and breeding success of the avivorous, urban tolerant peregrine falcon (Falco peregrinus) are well studied, little data exist for heavy metals. For the state’s primarily mammal-eating, less urban tolerant golden eagles (Aquila chrysaetos), a robust body of literature on mercury and lead poisoning exists in Europe, but dietary heavy metal and stable isotope concentrations are both relatively understudied in North America. In this thesis, I develop the following research topics surrounding metallic contamination in raptors: 1) A synthesis of heavy metal impacts on diurnal raptor populations across the North American Pacific Coast over the past fifty years. 2) An exploration of heavy metal pollutants in the diets of breeding raptors on the Stanford University campus, and at Stanford’s Jasper Ridge Biological Preserve 'Ootchamin 'Ooyakma (JRBP 'O'O). Specifically, I quantify concentrations of stable isotopes (carbon and nitrogen), and fifteen different heavy metals, including lead and arsenic. I use mass spectrometry to analyze dietary samples from breeding pairs of a) peregrine falcons at Stanford University’s Hoover Tower, and b) golden eagles at JRBP 'O'O. Over the past fifty years, lead and mercury have been identified at especially high levels for Pacific Coast raptors overall, but a few flagship species and regions are overrepresented in a relatively small body of literature. I argue that the prevailing view of heavy metals as a lesser threat to raptors is mainly due to a lack of standardized studies, in addition to confounding factors like the use of invasive organ sampling obfuscating toxicosis-induced mortalities in past studies. My analysis of Stanford's breeding raptors using non-invasive dietary sampling contributes to this body of knowledge. I uncovered abnormally high levels of heavy metals, equivalent to EPA standards for toxic waste, in the diets of Stanford's peregrine falcons, including contaminants like lead and cadmium. My results for Jasper Ridge’s golden eagles indicate lower metallic concentrations overall, suggesting that the peregrines' closer proximity to human infrastructure and their urban tolerant hunting behaviors may be important causes for their increased exposures. However, abnormally high levels of lead and arsenic are still present for the eagles, the latter indicating possible leaching of rodenticides into their diets. In addition, multiple heavy metals appear to follow distinct dietary pathways into each raptor, providing insights into potential prey species for future ecotoxicological research. These findings can inform conservation efforts for two critical, charismatic and threatened raptor species. The broad significance of my work is that more robust information on how raptors and their prey are responding to heavy metals can provide key insights on how we can minimize anthropogenic disturbances within our shared global ecosystem. My findings may also help guide biomonitoring efforts to pinpoint the sources and effects of metallic pollutants in the Bay Area, aligning with global efforts to promote human and planetary health.