Bird Population Studies

We investigate the historical factors and current conditions influencing variation in the survival, reproduction, and behavior of birds. We analyze long-term data from focal studies and citizen-science data to understand changes in bird populations in response to factors such as weather, predation, competition, and disease, as well as human activities such as pollution and urbanization. To do this, we explore ways to push the boundaries of public participation in ornithology to gather data across wide geographic scales; we lead multidisciplinary, collaborative studies; and we develop new techniques to analyze millions of data records on birds and the environment.

Project Highlights

Bird Distribution and Abundance

By analyzing long-term data from citizen-science projects and other sources, we identify how environmental factors influence birds’ habitat preferences, species ranges, and abundance.

Birds and Climate Change

Climate has an enormous influence on where birds survive and reproduce. In the short term, weather can influence the timing of migration, territory establishment, breeding, and egg laying. Over the long term, species have adapted to seasonal weather trends. As global climate patterns change, many harbingers of spring are occurring earlier each year. We combine data from citizen-science projects with long-term data on weather to examine climate's role in the changes we are seeing in the ranges of some bird species, as well as the timing and outcomes of breeding.

Data Analysis Toolkits and the Avian Knowledge Network

Using new techniques to analyze millions of data records, we study how bird distribution and abundance change through time and geographical space. We start by identifying large-scale patterns. This seemingly basic task is challenging because bird observations are unevenly distributed across the continent and are influenced by observers’ ability to detect birds. In collaboration with statistical and computer scientists, our researchers are developing novel methods to analyze data collected by both ornithologists and citizen-science participants around the world. The Cornell Lab is leading an initiative to digitally archive tens of millions of data records through the Avian Knowledge Network and to make these data more widely available. This will enable scientists to create a clearer picture of where bird species live and how their distributions change.

Species Coexistence

Can closely related species coexist? According to a longstanding debate in evolutionary ecology, closely related species will compete until one excludes the other. But there's also the possibility that closely related species could overlap more than would be expected by chance because their evolutionary history constrains them to live in similar environments. Irby Lovette and Wesley Hochachka examined breeding distributions of North American wood-warblers along with data from a wood-warbler phylogeny (family tree). These data revealed that, in general, the degree of relatedness between two species had little effect on whether they coexisted at a site. Only very distantly related species pairs showed a greater likelihood of co-occurring than expected.


We investigate the factors that drive the spread and impact of disease, using citizen-science data to track continentwide changes in bird populations, combined with studies of behavior, genetics, and avian immune responses.

House Finch Eye Disease

In the winter of 1993–1994, people in the Washington, D.C., area began seeing House Finches at their bird feeders with a strange new disease. The area around the finches’ eyes was red and swollen, and in some cases the birds had become blind. The cause of the disease was identified as a common bacterial pathogen of domestic poultry. The bacteria had unexpectedly mutated and jumped to House Finches. Within three years, roughly 60% of House Finches in eastern North America were dead. The disease has persisted since then, and House Finch numbers have yet to recover completely. Bird Population Studies researchers developed a citizen-science monitoring program called the House Finch Disease Survey to document the spread of the disease and used additional citizen-science data to describe its impacts. Further work investigated why the pathogen has been so successful and the disease so persistent. The goal is to gain a better understanding of the ecology of other diseases in other organisms, including humans. This work involves close collaboration with researchers at five universities.

West Nile Virus

When West Nile virus arrived in North America in 1999, scientists began to document the sharp declines it caused in American Crow populations. Deaths of smaller, less common birds have been more difficult to detect; details of which species are affected, and how severely, are still not well known. We are analyzing data from citizen-science databases, including the Christmas Bird Count, North American Breeding Bird Survey, and Project FeederWatch. Our goals include understanding which bird species have been affected by West Nile virus, whether ecological factors such as bird species diversity have altered the virus's effects across the continent, and whether populations of affected birds are recovering.

Human Impacts

Through intensive focal studies and analysis of citizen-science data, we investigate how natural and human-induced environmental variation, such as habitat availability, toxins, and disease, affect the abundance of birds.

Birds and Habitat Fragmentation

Fragmented landscapes, in which natural habitat exists as isolated patches, are a common side-effect of human activities. We study the effects of fragmentation with the help of citizen-science participants. Our Birds in Forested Landscapes project involved volunteer birders at 3,800 sites across North America. The project yielded valuable insights about habitat fragmentation and factors such as occupancy by birds and local extinction rates. Results were published in scientific journals including the Journal of Animal Ecology, the Proceedings of the National Academy of Sciences, and Ecology and Society. The results have also been compiled into guidelines for land managers interested in conservation of tanagers, forest thrushes, and Golden-winged Warblers.

Effects of PCBs on Bird Song

Polychlorinated biphenyls (PCBs) are worldwide chemical pollutants to which bird populations may be especially sensitive. Even at low concentrations, PCBs cause neurological damage and mimic estrogen activity, thereby altering sexual development and reproduction. Hormonal balance plays a key role in bird song, so it might be possible to monitor PCB exposure by studying songbirds. To test this hypothesis, researcher Sara DeLeon recorded songs and collected blood and feather samples from Black-capped Chickadees and Song Sparrows along a gradient of PCB contamination along the Hudson River in New York. Her preliminary results suggest that nonlethal levels of PCBs in the environment do affect bird song, a finding with implications for wild birds in polluted landscapes.

Effects of Acid Rain and Mercury on Birds

Led by scientist Stefan Hames, we are studying the biochemical pathways by which acid rain and mercury deposition reduce the availability of calcium, an important nutrient for nesting birds and their eggs. Data collected by participants in the Birds in Forested Landscapes citizen-science project indicated that Wood Thrushes are less likely to breed at sites heavily impacted by acid rain; the absence of high-calcium prey may be the cause. Further pollution research focuses on mercury contamination in birds, a condition that is surprisingly pervasive in upland habitats, particularly in combination with acid rain. With support from the Leon Levy Foundation, our researchers are developing a model of mercury contamination in New York forests and identifying regions and birds at high risk, including Wood Thrushes and Red-eyed Vireos.

Birds and Climate Change

Climate has an enormous influence on where birds survive and reproduce. In the short term, weather can influence the timing of migration, territory establishment, breeding, and egg laying. Over the long term, species have adapted to seasonal weather trends. As global climate patterns change, many harbingers of spring are occurring earlier each year. We combine data from citizen-science projects with long-term data on weather to examine climate's role in the changes we are seeing in the ranges of some bird species, as well as the timing and outcomes of breeding.

Variation in the Lives of Birds

We investigate the dramatic variation in life-history strategies enabling birds to survive and reproduce from place to place and from year to year. Using long-term data from multiple locations, we seek to better understand the factors that generate these and other geographic patterns.

Evolution of Variation in Life Histories of Birds

We investigate reasons why the pace of life among birds varies so consistently with latitude. These patterns include slower metabolism and embryonic development, smaller clutches, and longer incubation periods in tropical birds than in their cousins at high latitudes. Most researchers have examined biotic explanations, such as differing food availability or predation rates. We focus on non-biological factors like differences in temperature and day length between the tropics and the temperate zone. Poultry scientists have pioneered understanding how temperature and photoperiod affect the physiology and development of birds. Following their lead, we take a tiered approach to investigate how incubation in wild birds is influenced by temperature and photoperiod. In turn, we study how local incubation patterns affect embryo development. We analyze data from citizen-science programs, such as NestWatch, and the NSF-funded Golondrinas de las Americas network, consisting of study sites focused on Tachycineta swallows across the Western Hemisphere.

Cooperative Breeding: Acorn Woodpeckers

The remarkable Acorn Woodpecker of western North America lives in family groups of up to 15 individuals of both sexes and all ages. These permanent groups defend a territory together, store food together, and cooperate to raise young. For more than 30 years, Walter Koenig has studied individually marked Acorn Woodpeckers in central coastal California. Life in these groups is complicated, but it leads to an interplay of cooperation and competition that makes Acorn Woodpeckers unique in the avian world. Several related males compete to mate with several breeding females, all of whom lay their eggs in a single nest cavity. Offspring from these joint nests help raise the group's young for up to several years. Acorn Woodpeckers are also highly dependent on acorns, which they store, often by the thousands, in storage trees or granaries. This dependence provides much of the motivation for our Population Synchrony: Acorn Production by California Oaks study.

Population Synchrony: Acorn Production by California Oaks

Oaks are well known “masting” species—acorn production varies greatly from year to year, but is highly synchronized among trees. As a result, in good acorn years there is a bounty of acorns over a wide area, while in a poor year few if any are produced. Acorns are a critical food for many kinds of wildlife, including Acorn Woodpeckers. Walter Koenig leads a team in surveying acorn production across California and conducts a detailed study of oaks in central coastal California. His aim is to understand variability in acorn production, including why productivity differs, how far synchrony in acorn production extends, and what effects the variability has for California’s wildlife.

Human Dimensions

To help engage larger, more diverse audiences in bird-related recreation and citizen-science efforts, we study human demographics, motivations, and behaviors that can inform our decisions about how to develop and communicate new opportunities.

Environmental Recreation and Public Participation in Science

Nature recreation benefits people's physical and mental health and connects people with the environment. Millions of Americans are interested in wild birds: they compile lists, travel long distances to see particular species, watch birds at feeders, monitor nests, and photograph birds. We use questionnaires to examine the relationships between demographic patterns and individual motivations, preferences, and skills in these hobbies. This information will help the Cornell Lab better serve the community of bird enthusiasts and help involve under-represented groups. For example, we are investigating gender patterns related to a range of competitive and non-competitive bird hobbies in the United States and United Kingdom.