West Nile virus impacts in American crow populations are associated with human land use and climate

Abstract

West Nile virus (WNV) was first detected in the western hemisphere during the summer of 1999, reawakening US public awareness of the potential severity of vector-borne pathogens. Since its New World introduction, WNV has caused disease in human, avian, and mammalian communities across the continent. American crows (Corvus brachyrhynchos) are a highly susceptible WNV host and when modeled appropriately, changes in crow abundances can serve as a proxy for the spatio-temporal presence of WNV. We use the dramatic declines in abundance of this avian host to examine spatio-temporal heterogeneity in WNV intensity across the northeastern US, where WNV was first detected. Using data from the Breeding Bird Survey, we identify significant declines in crow abundance after WNV emergence that are associated with lower forest cover, more urban land use, and warmer winter temperatures. Importantly, we document continued declines as WNV was present in an area over consecutive years. Our findings support the urban-pathogen link that human WNV incidence studies have shown. For each 1% increase in urban land cover we expect an additional 5% decline in the log crow abundance beyond the decline attributed to WNV in undeveloped areas. We also demonstrate a significant relationship between above-average winter temperatures and WNV-related declines in crow abundance. The mechanisms behind these patterns remain uncertain and hypotheses requiring further research are suggested. In particular, a strong positive relationship between urban land cover and winter temperatures may confound mechanistic understanding, especially when a temperature-sensitive vector is involved.

Keywords: Anthropogenic; Avian; Land use; Pathogen; West Nile virus.

Fig. 1

Map of BBS sites (circles are route centers) in the eastern US that were used in this study. Large circles denote sites that were characterized as ‘urban’ (light) and ‘forest’ (black) landscapes for Fig. 3. The asterisk in the inset shows New York City, the site where WNV was first detected

Fig. 2

American crow population series [1980–2008] from the posterior predictive distribution of mean annual counts across BBS sites. Bars denote 95% credible intervals for mean across sites. Only the bold-faced years were used to estimate parameters in Table 1

Fig. 3

Proportional changes in crow abundance [(1998 count–2004 count)/1998 count] were generally negative, but declines were substantially greater in urban versus rural sites (t = 5.3, p < 0.001). Plot shows median (solid line) and 1st and 3rd quartiles, while diamonds represent mean values

Fig. 4

a Mean winter temperatures are generally warmer at sites located in lower latitudes (−1.45 ± 0.04, p < 0.001). Average winter temperature across all sites in a given year is shown by the gray line, extending from 1994 on the right to 2004. b After accounting for the broad scale variation associated with latitude, urban land cover is a significant predictor of mean winter temperature (0.27 ± 0.04, p < 0.001)