Title: Genus-specific Causes and Consequences of Haemosporidian Infection Intensity in American Kestrels (Falco sparverius)

Program: Raptor Biology MS

Committee Chair: Julie Heath

Committee: Julie Heath, Stephanie Galla, Allison Simler-Williamson

Abstract: Parasites are powerful selective forces in wild populations, with consequences for host survival, condition and reproductive success. Environmental and host-associated traits can shape host-parasite dynamics, through factors that mediate parasite or vector breeding and density or host immune status. Additionally, co-occurring parasite taxa may respond differently to drivers of infection. Together, these processes can result in divergent individual and population-level infection probability, intensity, and costs of parasite burden to hosts. Avian haemosporidian parasites (Haemoproteus, Leucocytozoon, and Plasmodium) are among the most widespread and common chronic infections afflicting wild birds. Pathogenicity and effects of infection can vary across host species and parasite genera. While previous research has demonstrated fitness costs of infection at both acute and chronic levels, many studies have focused on a single genus (i.e., Plasmodium) or have relied on prevalence-based approaches, characterizing infection as present or absent, leaving the consequences of variation in infection intensity comparatively underexplored. If costs of infection scale with intensity of infection, prevalence-based approaches may underestimate the burden of infection on individual hosts, particularly for the low-level chronic infections that are common in wild adult birds. We analyzed drivers and costs of haemosporidian infection probability and intensity in the American Kestrel, a partial migrant raptor experiencing well-documented range-wide population declines, to better understand how environmental conditions, host traits, and parasite genus shape infection dynamics and their consequences for host condition and fitness.
In Chapter 1, we examined how site, season, winter temperature, sex, and migratory strategy shaped infection probability and relative infection intensity of Haemoproteus, Leucocytozoon, and Plasmodium; revealing genus-specific responses to environmental and host-associated drivers of infection. Overall parasite prevalence was high (74.3%), with nearly one-third of samples coinfected with multiple genera. Environmental conditions shaped infection patterns in genus-specific ways: Leucocytozoon and Plasmodium infection intensities were higher at a high elevation wildland site compared to a suburban-rural site, Haemoproteus infections had lower probability and relative intensity in winter compared to the breeding season, and Plasmodium infections were lower-intensity following warmer winters. Host traits also explained variation, with males generally exhibiting lower relative infection intensities compared to females and resident individuals showing lower Haemoproteus and Plasmodium infection probability compared to migrants. Finally, we detected divergent shifts in infection probability between the three surveyed genera between time periods (2009-2012 and 2023-2025).
In Chapter 2, we investigated whether genus-specific patterns in infection intensity were associated with host physiological condition and reproductive outcomes, using quantitative molecular methods to detect relationships that may have been obscured by a prevalence-based approach. Leucocytozoon infection intensity was negatively associated with relative telomere length, suggesting a cost to individual condition in adult kestrels, even at relatively low intensities. Haemoproteus infection intensity in males was negatively associated with both carotenoid concentrations and hatching success, pointing to reproductive and condition costs of a genus generally regarded as mild or non-pathogenic in raptors. There were no detectable associations between Plasmodium infection intensity and kestrel condition or reproductive fitness, despite demonstrated effects of this genus in other avian systems.
Together, these analyses demonstrate that haemosporidian infection dynamics are structured by both environmental and host-associated factors in genus-specific ways, and that even relatively low-intensity infections carry measurable costs for hosts. Detected costs of infection were also genus-specific and were associated with genera that are generally considered to be benign in adult raptors. These findings underscore the importance of multi-driver frameworks integrating host and environmental traits, as well as intensity-based, genus-resolved infection metrics, for assessing drivers and consequences of host-parasite dynamics in wild populations.