Ecoimmunology and Demography of Recruitment in Swainson’s Hawks

Abstract

Swainson’s Hawks are a long-lived migratory species that breed throughout North America. Swainson’s Hawks frequently forage and nest in modified-agriculture landscapes, which often support large prey abundances. Breeding surveys across the state of California estimated a 91% decline in the number of breeding pairs in 1979, prompting a listing-status of “threatened” in 1983. A long-term monitoring project for the species was developed in Butte Valley, California in association with this state-wide survey and has run continuously from 1979 to the present. Examination of this long-term breeding and reproductive data shows a continuum of territory occupancy and productivity values. This heterogeneity becomes especially apparent when examining differences between individual territories. I explored the differences between territories and its role on natal conditions using immune system development as a framework. The hypothesis being that nestlings benefitting from favorable conditions are able to invest in stronger immune systems. Mixed effects analyses indicated a significant link between extended territory occupancy history, an index of habitat quality, and stronger immune systems in nestlings in 2013 but not in 2015. I also examined long-term trends in population size across the last three decades. I used multi-state models to test hypotheses involving trends in recruitment rates and to estimate first-year survival. Models that accounted for the permanent ban of “monocrotophos” pesticide use across the wintering range received the most support. Indicating that the disuse of this pesticide in 2000, was particularly important for this species. This result emphasizes the necessity and value of conservation work.