Migratory_Birds

Global warming is certainly one of the hottest issues in today’s world, and it is a phenomenon that shows no signs of cooling down. According to NASA’s Goddard Institute for Space Studies, average temperatures around the world have climbed 1.4 degrees Fahrenheit (0.8 degree Celsius) since 1880, with most of this increase only occurring in recent decades due to excessive carbon pollution [1]. Our planet’s climate has been continuously changing throughout its long history, so what’s the big deal about a number this small? Well, just a few of its impacts are rising sea levels, more rapid spread of infectious diseases, and less food and water all over the world. For compelling consequences that may feel closer to home, just look to the birds flying above or singing in the nearest tree. Birds have long been used as indicators of environmental disruption because they are easily visible and audible to human observers, have predictable patterns of behavior, and serve as a popular hobby amongst birdwatchers. In fact, much of the data used by conservation biologists to study the long-term health and status of bird populations around the world has been generated not necessarily by experts, but by amateur bird enthusiasts with a good set of binoculars. The current prognosis for birds: bad, and quickly getting worse.

Why Migrate?

Bird migration allows many avian species to take advantage of seasonal variations in natural resources. By inhabiting different areas during different times of the year, bird populations are able to optimize both food availability and nesting locations. The longer days of northern summers, for example, provide extended time for breeding birds to feed their young and also help some types of birds produce more offspring at one time. As cooler weather and longer days arrive in the fall, many birds return south to warmer regions where food supply fluctuates very little from season to season [2].

Figure 1: American redstarts are one of the many bird species that is threatened by climate change. Some of the redstart’s favorite foods are caterpillars and magnolia seeds, but they themselves can be tasty snacks for predators such as foxes and flying squirrels! (Smithsonian Science)

Approximately 1,800 of the world’s 10,000 bird species are long-distance migrants, and in North America alone, about 350 of its 650 bird species head to the tropics for colder months [3,4]. Just how far south a bird migrates depends on the type, sex, and age of the bird. Nonetheless, whether we consider a warbler, hummingbird, swift, or shorebird, all of these migratory masters know how and when is best to fly. Most of the timing and control of migration is programmed at birth [5]. The rest is acquired over time. Birds learn to recognize various landmarks and form mental maps with the help of Earth’s magnetic field. Their annual treks can also be susceptible to environmental cues, and are made progressively more difficult as the environment itself now rapidly changes.

Hotter, Harder Migrations

Although bird species all over the globe are suffering from the effects of climate change, long-distance migrants are particularly vulnerable. Unlike birds that stay in the same place year-round, migratory species rely heavily upon separate breeding, wintering, and stopover sites. Changes to any one of these habitats can put them at risk, pushing them out of sync with crucial elements of their natural surroundings. For example, many birds are unable to advance their arrival date at breeding ground destinations to coincide with the prime time for finding insects on which they feed. Such a mismatch caused the decline of pied flycatcher populations in the Netherlands by up to 90 percent [6]. Pied flycatchers are nesting more quickly after their spring arrival in Europe, but still cannot time nestlings’ food demands with peak insect supplies, which now occur even earlier due to global warming.

Other species are facing similar problems due to earlier arrival dates. A 2012 study from the UNC College of Arts and Sciences analyzed when 18 different species of birds arrived at various points along their migration journeys. On average, each species reached various stopping points 0.8 days earlier per degree Celsius of temperature rise. Some birds began their journeys as much as three to six days earlier per degree of increased temperature [7].

Habitat loss is another grueling challenge facing long-distance migrants. Escalating temperatures in Arctic regions, for instance, could decrease tundra regions by 70 percent and severely impact species that breed there [8]. The critically endangered Siberian crane is perhaps one of the clearest cases of climate disruption. This wetland species breeds in the vanishing tundra of Russia and Siberia and spends the winter along China’s Yangtze River, where precipitation has decreased due to warmer temperatures. It is therefore no surprise that the Siberian crane population has plummeted to only 3,000 individuals worldwide. Loss of habitat similarly predisposes the golden bowerbird to extinction. An occupant of the cooler mountain areas of Australia’s humid tropics, the habitat of the golden bowerbird is predicted to shrink by 97.5 percent with a future warming of 3 degrees Celsius and a 10 percent decline in rainfall [8].

Figure 2: North American songbirds face many dangers each year during southern migration due to landscape change, from global warming to pesticide pollution, manmade structural barriers, and habitat loss.

How Birds Help Humans

The ability to fly (or swim in the case of penguins) means that migratory birds are able to relocate to different regions [9], yet that same mobility can leave them particularly vulnerable to climate change. Missed connections between peak breeding and peak food supply as well as habitat loss, due to warmer temperatures, pose a threat to bird migration. Add these risks to the perils that birds inherently face by traveling thousands of miles, including physical exertion as well as predation, and many long-distance migrants will face extinction. Scientists have already reported severe decreases and/or total reproductive failure in many bird populations.

Why should we care? First, the ecological services that birds provide to humanity have incredible economic value. Birds pollinate plants, disperse seeds, and control populations of pests and parasites. Our avian friends also provide people all over the world with eggs and meat for sustenance, and feathers for comfort and warmth. Second, birds provide early warning signs of health hazards in the environment. Twentieth century coal miners, for example, brought canaries into coal mines to forewarn the workers of toxic gases such as methane and carbon monoxide. Birds also alerted people to the dangers of DDT-based agricultural chemicals in the 1960s, when predatory species like the bald eagle and peregrine falcons began dying at an alarming rate [3]. Third, greater species diversity on our planet ensures more natural sustainability for all life forms. The list goes on and on.

We regularly hear that future changes to our climate will lead to drought, melting glaciers, and more frequent heat waves. Less obvious, however, is that global warming will shift patterns of bird migration and destroy many habitats in which birds breed and thrive, which in turn will reduce the economic and environmental benefits described above. To protect our health and the prosperity of our own habitats, it is critical to consider the significant consequences of climate change from all perspectives: human, avian, or otherwise.

Laura Smith is a PhD student in the Biological and Biomedical Sciences at Harvard Medical School.

References

1. National Geographic: “Global Warming Fast Facts” http://news.nationalgeographic.com/news/2004/12/1206_041206_global_warming.html

2. “The Cornell Lab of Ornithology: All About Birds” http://www.birds.cornell.edu/AllAboutBirds/studying/migration/

3. National Wildlife Federation Report (2013): “Shifting Skies: Migratory Birds in a Warming World”

4. Sekercioglu CH. (2007). Conservation ecology: area trumps mobility in fragment bird extinctions. Current Biology 17 (8): 283–286.

5. Ogonowski MS, Conway CJ. (2009) Migratory decisions in birds: extent of genetic versus environmental control. Oecologia, 161: 199-207.

6. World Wildlife Federation Report (2006): “Bird Species and Climate Change”

7. Hurlbert AH, Liang Z. (2012) Spatiotemporal variation in avian migration phenology: citizen science reveals effects of climate change. PLoS ONE, 7(2): e31662.

8. Carrillo-Rubio L. “Flying Is No Escape: Migratory Birds and Climate Change” http://www.climate.org/topics/climate-change/migratory-birds-climate-change.html

9. Audobon: “Birds & Climate FAQs” http://birdsandclimate.audubon.org/FAQs.html

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