Estimating the Impact of House Sparrows on Eastern Bluebird Reproductive Success Across an Urban Gradient

Abstract

Non-indigenous species are widely reported to compromise the population growth of native species, but quantitative estimates of this are often lacking. As a result, management recommendations for non-native species are frequently rooted in subjective speculation and thus have poor predictive power. The House Sparrow (HOSP) was introduced into New York, NY from Europe in 1851 and has since expanded its range to encompass all of North America, where it usurps nests of native cavity-nesting birds, frequently destroying eggs, and killing nestlings and adults. This two-year study investigated habitat effects as well as the impact and management of HOSPs on the reproductive success in Northeast Ohio of the Eastern Bluebird (EABL), a native species whose population is managed via artificial nest box placement and maintenance. To do so, I attempted to found rural and urban EABL populations to assess whether the expected high number of discarded HOSP nests from artificial nest boxes in urban areas was sufficient to facilitate EABL nesting success. I found EABL fledgling number was two-fold higher in rural compared to urban areas, but nesting success (at least one chick fledged per nest attempt) was marginally higher in the latter. Despite reduced EABL nesting success by HOSPs, this habitat difference appears to not be primarily driven by HOSP abundance and suggests nest predation rates on EABLs may be higher in rural areas. Point count estimates of HOSP abundance were positively correlated with the number of HOSP nests I discarded from nest boxes and negatively correlated with distance from buildings offering HOSPs a stable food source. These data, including the effects of House Wrens, were integrated in a GIS-based hazard model to predict EABL nesting success across an urban-rural landscape. Key factors affecting the founding and subsequent success of EABL populations across habitats will provide managers with science-based practices to best augment populations at local and regional scales.