Urban Ecology & Applied Conservation
A recent paper from a collaboration with the Cornell Lab of Ornithology and others reveals that since 1970, bird populations in the United States and Canada have declined by 29 percent, or almost 3 billion birds, signaling a widespread ecological crisis (Rosenberg et al. 2019). The results show tremendous losses across diverse groups of birds and habitats — from iconic songsters such as meadowlarks to long-distance migrants such as swallows and backyard birds including sparrows. This research supports the idea that birds are indicators of environmental health, signaling that natural systems across the U.S. and Canada are now being so severely impacted by human activities that they no longer support the same robust wildlife populations. The findings show that of nearly 3 billion birds lost, 90 percent belong to 12 bird families, including sparrows, warblers, finches, and swallows — common, widespread species that play influential roles in food webs and ecosystem functioning, from seed dispersal to pest control.
Barn Swallow artwork courtesy of Tim Kuhn / Audubon Photography Awards.
Urbanization during the latter half of the twentieth century has altered habitats, restructured avian communities, and influenced the range sizes and population dynamics of animal species. Neighborhood Nestwatch (NN), started in 1999 by Marra, is a citizen science program developed to study the ecology and evolution of birds along the expanding urban to rural land use gradient. NN reconnects people in such human-dominated landscapes with nature while helping scientists gain access to private property and quantify the effects of human development on wildlife – primarily birds. Scientists visit yards of participants in the Washington DC/Baltimore region during annual bird ringing visits and capture, color-ring, measure and release focal neighborhood bird species. This unique experience provides participants with captivating close up views right in their own backyards while teaching them about science and the challenges faced by birds and other wildlife in human-dominated environments. During the visit, scientists also convey basic facts about the biology and ecology of birds, provide tips for finding and monitoring nests, how to re-sight color-ringed birds, record and submit data and suggest improvements to backyards for wildlife enhancement. The citizen scientist experience continues as participants keep a watchful eye throughout the year to identify and observe “their” color-ringed birds as well as find and monitor nests. Participants access the NN website to enter their data, learn more about the ecology of birds, and communicate with Smithsonian scientists.
Through this unique scientific approach, NN has gained key insights into the survival and reproduction of birds, the impacts of non-native species and contaminants in urbanized landscapes and how data collected by citizens compares to data collected by scientists. NN has expanded all along the eastern seaboard from Springfield, MA to Gainesville, FL.
Cat photo by Mark Marek; Northern Cardinal nest.
One of the unique challenges birds face in an increasingly human-influenced world are the non-native predators, free-ranging cats. Apart from habitat loss, free-ranging cats, whether those that owners let outside or unowned stray cats, are the number one cause of bird deaths (Balogh et al. 2011, Loss et al. 2013, Loss & Marra 2017, Marra 2019). We estimate that annually between 1.3-4 billion birds are killed by free-ranging cats (Loss et al. 2013). Reducing the number of outdoor cats would greatly increase the chances that songbirds successfully survive, reproduce and help population rebound (take a look at ABC’s Cats Inside program).
Invasive plants also contribute to recent insectivorous bird declines, especially in human-dominated habitats. Nonnative plants are very common in urban areas because of horticultural preferences, but these plants do not support local biodiversity due to the fact that they do not produce insects like moth and butterfly larvae. We have been studying how nonnative plants affect the availability of insects, as well as the behavior, diet, and population growth of a common backyard bird, the Carolina chickadee (Narango et al. 2017, Narango et al. 2018).
We conduct research in the D.C. metropolitan area and have surveyed plants, insects and birds in more than 200 residential properties of participating Neighborhood Nestwatch volunteers. We use a combination of field observations (point counts, behavioral surveys, nest monitoring and bird banding), lab work (stable isotopes), and technology (radio-telemetry) to assess the role of nonnative plants on urban food webs.
Bob Reitsma showing kids the birds he is banding.
Our future research will focus on determining how different human-caused changes to the natural world combine to affect the population trajectories of birds. The scientific community has identified many of the threats birds face, from land use change and feral cats to climate change and pesticides, but our knowledge about how each of those factors affects continental bird populations in the long run is lacking. Going forward, we hope to identify the smoking guns in modern bird declines – the main threats birds are currently encountering across their ranges and life cycles.