Cosmic-Ray Propagation. Ii. Diffusion in the Interplanetary Magnetic Field

Abstract

Cowbirds are avian brood parasites that demonstrate resistance to West Nile Virus (WNV). However, because cowbirds depend on surrogate bird species to rear their young that are susceptible to the disease, WNV may indirectly reduce cowbird population size. Cowbirds are generalist brood parasites allowing them potential to switch to more abundant host species, provided WNV does not affect all hosts equally. We tested for a reduction in the effective population size of cowbirds in an area near Chicago, Illinois, where WNV caused high bird mortality during 2002 and 2003. Using genetic and host nest census data collected before and after the spread of WNV into the area, we assessed cowbird genetic diversity and host species abundance to understand how this host brood generalist parasite responded to WNV, a disease that causes high mortality in many bird species. Our genetic analysis suggests that the cowbird population size remained stable throughout the spread of WNV into the area. Host abundance analysis also suggests that certain host species disappeared from the breeding area after the introduction of WNV but others increased in abundance, providing evidence for the potential of host switching. Understanding how cowbirds responded to WNV host infections will improve our ability to control declining bird populations in the U.S. where habitats are shrinking and host extinctions are more likely.