Migratory birds are species that migrate long distances on a regular basis, usually flying south in the winter to warmer climates and north in the summer to breed. Some examples of common migratory birds include songbirds like warblers and thrushes, waterfowl like ducks and geese, raptors like hawks and eagles, and shorebirds like sandpipers and plovers.
In recent decades, there has been growing concern over apparent declines in populations of many migratory bird species. However, the evidence is mixed on whether migratory birds as a whole are actually declining significantly.
Evidence of declines
There are several sources of evidence suggesting troubling declines in many migratory bird populations:
- The North American Breeding Bird Survey, which has tracked breeding bird numbers since 1966, has documented significant population declines in more than 30 common migratory bird species, including many warblers, flycatchers, and sparrows.
- Audubon’s climate report found that climatic changes could lead to declines of over 50% in the populations of over 300 bird species in North America.
- The State of North America’s Birds 2016 report found that over one-third of all North American bird species are at risk of extinction without significant conservation action.
- Long-term studies of migratory shorebirds have found substantial declines in species like the red knot and ruddy turnstone around Delaware Bay, an important migratory stopover site.
- Radar studies have detected declines in nocturnal migratory bird traffic over areas like the Midwest and Northeast U.S. since the 1990s.
There are likely multiple contributing factors behind these declines, but habitat loss and degradation, climate change, and threats during migration are considered the major drivers.
Evidence of stability or increases
On the other hand, some data sources suggest many migratory bird populations are stable or even increasing:
- The North American Breeding Bird Survey shows increasing trends for species like blue-winged warbler, osprey, and sandhill crane.
- Duck breeding populations in the U.S. remain close to historic highs according to U.S. Fish and Wildlife surveys.
- Goose populations of species like the Canada goose and snow goose have grown substantially and now number in the millions.
- The endangered whooping crane population has rebounded from just 15 birds in 1941 to over 500 today due to conservation efforts.
- Counts of raptors migrating past watch sites like Hawk Mountain Sanctuary have set new records in recent years.
These increases may be partly attributed to conservation measures such as wetland protection, restricted hunting seasons, and bans on DDT and other pesticides. Some waterfowl and goose populations have also adapted well to human landscapes and agriculture.
Challenges in tracking trends
Determining overall trends for migratory birds is complicated by several factors:
- Migratory routes and behaviors vary greatly between species, making comprehensive surveys difficult.
- Population sizes can fluctuate widely year-to-year due to factors like weather, food supply, and breeding habitat conditions.
- Long-term monitoring programs like the Breeding Bird Survey have limited geographic coverage and may miss population changes outside sampled areas.
- For many species, population trends are uncertain due to lack of monitoring data over time.
These challenges mean年子・子孫年子・子孫that constant vigilance through bird monitoring and conservation programs is needed to detect and reverse significant declines before it is too late.
Are particular groups of migratory birds more threatened?
Some recognizable patterns have emerged in terms of which types of migratory birds are faring worse than others:
- Grassland bird species have declined more steeply than any other group, with species like eastern meadowlark and bobolink showing severe population drops.
- Shorebirds that migrate incredible distances to the Arctic for breeding have also declined sharply, like red knot and semipalmated sandpiper.
- Aerial insectivores like nighthawks, swifts, and flycatchers have declined more than other feeding guilds, likely due to insect population crashes.
- Long-distance migrants that winter in the tropics tend to be faring worse than short-distance migrants, potentially due to habitat loss on migration routes and wintering grounds.
In contrast, wetland species have generally fared better in recent decades, and some generalist adaptable species like American crow and blue jay are thriving.
Are there differences between flyways?
There are some notable geographic patterns between the major north-south migratory flyways in North America:
- The Central Flyway has seen the steepest declines based on Breeding Bird Survey data, particularly in birds that breed in the northern Great Plains like Sprague’s pipit.
- Grassland species declines have been less severe in the Atlantic Flyway compared to central and western regions.
- Waterbird declines have been more pronounced in the Pacific Flyway, especially for species like long-billed curlew and marbled godwit.
- The Mississippi Flyway has seen increasing trends in many waterfowl populations, but declining trends in other species like chimney swift.
Various factors may contribute to these flyway differences, including habitat loss and alteration on breeding, wintering, and migration stopover grounds unique to each region.
Conclusions
In summary, the overall trends paint a troubling picture for many migratory bird species, but with some bright spots and success stories thanks to conservation efforts. Continued monitoring of both declining and recovering populations will be critical for guiding future management strategies. The challenges migratory birds face today – from climate change to habitat loss on multiple continents – require international cooperation on solutions. But by taking informed and timely action, there is hope we can protect these species that connect ecosystems across the hemisphere.