Since the early 1970s, the extent of urban area in the United States has more than doubled. This rapid increase in urbanization creates costs and benefits to wildlife depending on the species. Understanding the full picture of this issue requires long-term records on avian survival, not just in urban centers but also in outlying suburban and rural areas. Examining nest success within an urban-to-rural gradient over a long period of time enables a thorough look into how birds cope with urbanization.
Neighborhood Nestwatch has existed for nine years and through its network of citizen scientists has been collecting data on nest success and other variables important to bird survival. The goal of this study was to use Nestwatch data to examine nest success of five focal species across the Washington, D.C., gradient over the nine year period. In addition to discerning trends regarding bird survival, this study also statistically compared the reliability of citizen data versus data derived from scientists.
In addition to several years of nest data from both Nestwatch staff and participants, data from an artificial nest experiment using both quail and clay eggs was used as a method to characterize nest predators within the gradient. Data from 405 nests of American robins, northern mockingbirds, northern cardinals, gray catbirds and house wrens were included in this study spanning the years 2000-2008. Nests were monitored by either participants or staff. During repeated visits to nests, the number of eggs, nestlings and fledglings was recorded, as was the date of the last egg laid, the date of hatching and fledgling, and the date that the nest either fledged or failed due to abandonment, predation or other another cause. Artificial nests, small wicker baskets containing two quail eggs and one clay egg, were deployed in randomly selected Nestwatch backyards along the gradient. Nests remained for two weeks during which data were collected on disruption, destruction or removal of eggs and the origin of any markings left on clay eggs.
Overall, the results showed that predation is a central component determining nest survival and that the amount of canopy cover and impervious surface surrounding a nest influences predation risk. Somewhat surprisingly, four of the five species had higher nest success in urban areas compared to rural environments. Urban areas are sometimes referred to as “safe zones” in this respect because they contain lower numbers and diversity of predators. Artificial nest results showed that avian predators, such as American crows and blue jays, were more common than small mammals as the environment shifted from rural to urban.
More specifically, statistical tools showed that strong correlations existed when examining canopy and impervious cover at a 1,000-meter radius around a nest. This strong correlation is most likely due to the area requirements for nest predators. The house wren, which is not an open-cup nester but rather a cavity nester, had the highest probability of nest survival presumably due to the difficulty for predators to reach the nest. Finally, all data generated by citizen scientists were as reliable as that of Smithsonian scientists proving that citizen scientists provide a dependable and beneficial source of data for research and larger conservation programs.
The data on nest success suggests that urbanization may benefit some breeding birds due to reduced predation. But the researchers are quick to point out that even though urban environments may have less predation risk, they also contain unique threats to survival through factors such as exposure to contaminants, noise pollution, decreased fledgling success (all addressed in previous Nestwatch articles) and increased nest parasitism. So, once again, Nestwatch research has shown that life in an urban environment for birds is a mixed bag.
Ryder, T., Reitsma
, R., Evans, B., and P. Marra
. 2010. Quantifying avian nest survival along an urbanization gradient using citizen-and scientist-generated data. Ecological Applications,