At the end of summer each year, birds from forests across North America leave their breeding grounds and travel thousands of miles, through all kinds of weather, over mountains and oceans, to their wintering grounds in the tropics.
Since it is such a long and demanding trip, they will need to stop in unfamiliar habitats to find food and rest before continuing on their journey to a tropical land that is quite different from the temperate one they left behind. In 5 to 8 months, they will start the journey again, in reverse. It happens every year, and yet we understand so little about the journey and the birds who undertake it.
The survival of migratory birds can be limited by both time and space in many ways, so understanding the complexity of how they use their habitat is critical. This complexity is evident when considering breeding grounds and winter habitats can be thousands of miles apart and involves a variety of unknown stopover sites where birds can rest and feed during migration.
To add to the problem, only a handful of North American migratory birds have been studied closely in both their breeding and wintering grounds. Furthermore, there is almost no data on South American austral migrants who breed in South America but winter in the tropics near the equator.
Despite our overall lack of knowledge, scientists have made great strides in the past 30 years to learn more about migratory birds and the ecological conservation necessary for their survival. Prior to 1970, most research about Nearctic-Neotropical migratory birds centered on the behavior and ecology associated with their breeding season.
However, that all changed in 1977 after a Smithsonian sponsored symposium on migrant bird ecology. The symposium brought to light the fact that many migrants spend much more time in the tropics than in their breeding grounds. And in fact, these migratory birds are integral parts of the resident tropical communities, joining mixed flocks and exhibiting territorial behavior.
Suddenly, scientists were looking at migratory birds in a new way. Instead of viewing migrant birds as residents of temperate zones who travel to the tropics to avoid cold winters, researchers began to perceive them as tropical residents who migrate into temperate zones for breeding purposes. Consequently, the mass habitat destruction occurring in the tropics would have devastating effects on these birds previously thought of as residents of temperate zones.
Scores of birds that range across a large potion of North America in the summer would return en masse to a much smaller range in the tropics only to discover much of their winter habitat had disappeared.
Since the right kind of habitat is critical for birds to survive and reproduce, it makes sense that habitat management is an important focus of bird conservation. The quality of resources and the protection they provide is important to understand when trying to discern birds' migration timetables and flight paths. But understanding what makes a high quality habitat for each species of bird is not an easy task.
Not only do conservationists need to consider patterns on a landscape level—such as the types and abundance of plants available for nesting and cover—they must also consider broader, large-scale geographic factors such as the size of the habitat required and the overall distribution of the birds and their predators and parasites. For most species, we have no idea what size of habitat is required to maintain a successful population.
Scientists gather population statistics, such as how many birds hatch in an area compared to the number of birds that die in that same region. But once the birds fledge, tracking the juvenile population is an entirely different problem. Studies have shown many birds move to a different habitat after fledging. What happens to them when they leave? Where do they go? How far do they travel? And it's not just the fledglings who complicate the task of associating population success with a habitat.
Some birds use a different habitat for breeding than for nesting. And other birds, particularly in western North America, migrate to a different habitat in Mexico to molt after nesting, and they may even produce a second clutch in the molting habitat. The sheer geographic scale of migrating birds' is daunting and poorly understood at this point.
Logically, conservation efforts should focus on the high quality habitats, but how do we identify which habitats are "high quality?" One way is through intensive studies that measure survival, abundance, and health of birds in a habitat over a long period of time. But once again, the "migratory" aspect of these birds comes into play. Birds like the chestnut-sided warbler (Dendroica pennsylvanica) move around in the winter, rather than settling in one place. The rose-breasted grosbeak (Pheucticus ludovicianus) and the yellow-rumped warbler (Dendroica coronata) might faithfully visit a particular winter site for several years, and then suddenly not appear at all.
Pictured from left to right: chestnut-sided warbler, rose-breasted grosbeak, yellow-rumped warbler.
Naturally, scientists want to know where they have gone and why. But with bird species like this, it is difficult to track individuals by banding, so alternatives must be found. Radio transmitters and geolocators can shed light on migratory bird movements and stopover times, but they are too heavy for most birds to carry. In a few cases, scientists have begun using stable isotopes to map links between breeding and wintering habitats. But again, these kinds of studies need to have significant depth and be conducted over a long period of time.
An ongoing study in Puerto Rico by Faaborg, Arendt, and Dugger has revealed disturbing trends in migrating birds to that area, particularly warblers. Researchers have been capturing birds with nets since the 1970s, and in the past eight years have noted a severe decline in their bird counts. Migratory black-and-white warbler (Mniotilta varia) and ovenbird (Seiurus aurocapilla) counts have dropped to less than 20% of their original populations on the island.
Pictured from left to right: black-and-white warbler, ovenbird.
There are many theories as to why this is happening. It could be due to increased rainfall in area. However, resident populations of birds in the same area are also on the decline, which indicates overall conditions in the forest are worsening. West Nile Virus could also provide an explanation for some of the decline, but warblers typically have not been impacted severely by the disease.
Another theory is that a warming global climate has moved the winter habitat range for the birds closer to their breeding range, so they are no longer traveling to Puerto Rico. These are all good theories, but the point is that without a long-term study conducted in the wintering habitat of these birds, we just don't know why their populations are declining in Puerto Rico.
We do know that human activities at the local level are affecting all migratory birds. Population declines have been linked to urban development, logging, and clearing land for agriculture. Additionally, cell phone towers, wind farms, and buildings are taking a toll. The move towards carbon-neutral energy, while beneficial in many ways, is in some cases negatively impacting birds. There is definitely a trade-off.
For example, wind farms are a good alternate energy source, but they need to be located away from flight paths of migratory birds. Ethanol encourages farming and keeps energy production within our country, but the conversion of forests and meadows to agricultural land destroys a variety of habitats used by birds throughout the year.
On a broader scale, large changes to the ecosystem such as global warming and acid rain are at work. One of the strongest indicators that climate change is affecting birds is the recent shifts in the migration and breeding schedules of many bird species. Studies have shown global climate change has advanced the migration schedules of birds in both North America and Europe. This is important to note because the advanced timetable forces birds to choose between migrating early and staying to reproduce.
So what can we do about this lack of knowledge? First and foremost, we need long-term, replicated studies of migratory bird species during all phases of their annual life cycle to collect data about their habitat needs and migration patterns. Additionally, long term population trends need to be analyzed in conjunction with ecological data. But these are not easy tasks.
As stated before, migratory birds divide their time between multiple locations: a breeding site and a winter site thousands of kilometers apart, and a multitude of stopover sites when moving between the two. That's a great deal of land to cover in any kind of study.
One recommendation is for the U.S. Fish and Wildlife Service and the Canadian Wildlife Service to jointly fund an intense, geographically-broad assessment of winter migrant distribution. This study would include habitat quality evaluation and stable isotope studies. The effort would allow researchers to get a good idea of the distribution of birds, identify optimal habitats, and draw much needed connections between breeding and wintering grounds.
Additionally, there is a need for experimental studies that allow conservationists to shape and manipulate habitats. These kinds of studies will help us improve and cultivate high quality habitats in the future.
If these studies are necessary and provide information critical to conservation efforts, and there are researchers willing to perform the work, why don't they exist? In a word, money. Management studies like these require repetition and aren't considered cutting-edge science. This severely hurts a project's chances of obtaining federal funding. For baseline studies of migratory birds and their habitats to succeed, priorities for federal funding need to be changed or new avenues of funding must be found.
Joint ventures may be the answer. Joint ventures are regional partnerships of public and private sector organizations working together to conserve bird populations and their essential habitats. These efforts combine national and international initiatives and conservation efforts with local land use concerns. Local, regional, and federal agencies are stakeholders, as well as private organizations, corporations, and tribes.
Science-based conservation planning and evaluation are at the heart of these ventures, and they address the needs of all priority bird species in a region. Joint ventures have already produced a number of spatial models to evaluate current and predicted bird distribution. They are also developing population targets that can be used to evaluate species viability considering a variety of factors that affect birds throughout their migration cycle.
There is a definite shift from focusing on simply doing more—more protection, more management, more restoration—to a quantified approach—how much more? And where? Agencies such as the U.S. Geological Survey and the U.S. Fish and Wildlife Service are implementing goals based on bird population targets, like the ones developed by joint ventures, instead of acreage targets for conservation priorities.
The solution for many local-level problems is good habitat management of the breeding, wintering, and migration stopover sites. Preserving the natural habitats that exist today will be critical for birds as we try to develop or reclaim suitable habitat for bird species going forward. Modeling possible changes in habitat across time and space can help us anticipate bird distribution in the future, and predictions about climate change and the consequent changes in vegetation can be combined with analysis of how birds will respond to these changes. This will help us determine which birds are tolerant to change and which ones will need the most help.
Global changes provide a far more complex challenge. If global climate changes are responsible for regional population declines, restoring habitat isn't going to help. On a global scale, major changes in human behavior are required. But this isn't going to happen without better data. While there is convincing evidence that human activities influencing climate change are affecting the ecosystem and species across the globe, more information is needed to perform better analyses on the specifics of the causes and effects.
Since birds aren't concerned about country borders, international cooperation is essential. Conservation strategies must include measures to protect all habitats used during the entire annual cycle of migratory birds, regardless of country, state, or province. Since we lack so much baseline information, the development of management guidelines with the goal to save all species in an area is needed. For example, setting aside large parks that support tropical residents will also provide sanctuary for migrants. Shade coffee plantations can also be managed to serve multiple bird populations.
Although we have learned a great deal about the ecology of migrant land birds in the last few years, more conservation guidelines are needed. And conservation efforts should focus on those birds that need our help the most. Determining how and when a migratory bird species population is in trouble is difficult, but necessary for conservations trying to make decisions about habitat conservation. Where does a particular bird spend the winter? What habitat does it need to not only survive, but also reproduce?
More reliable knowledge is needed on what is limiting populations, but we can't wait for that knowledge. Conservationists need to make day-to-day decisions today using the knowledge we do have. But that doesn't mean we shouldn't be pushing forward.
We need to develop ways to monitor populations and identify declining species early, so we have time to conduct research to figure out why populations are declining and implement changes, if possible. Protecting populations, providing safe migration paths, and ensuring plenty of quality stopover habitats are preserved are all goals we can achieve if we start today.
This article summarizes the information in this scientific paper:
Conserving migratory land birds in the New World: Do we know enough? 2010. John Faaborg, Richard T. Holmes, Angela D. Anders, Keith L. Bildstein, Katie M. Dugger, Sidney A. Gauthreaux Jr., Patricia Heglund, Keith A. Hobson, Alex E. Jahn, Douglas H. Johnson, Steven C. Latta, Douglas J. Levey, Peter P. Marra, Christopher L. Merkord, Erica Nol, Stephen I. Rothstein, Thomas W. Sherry, T. Scott Sillett, Frank R. Thompson III, and Nils Warnock. Ecological Applications 20:2, 398-418.
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