Dr. Ian Newton, an international expert of bird ecology and former president of the British Ecological Society, spoke about the human impact on bird migratory patterns at Dartmouth on Wednesday, November 4th.
Image of a King Eiders bird
Newton began his talk by describing the reasons that made the bird problem interesting: migrant bird population sizes are influenced by conditions in more than one area, and migrant birds face mortality risks associated with travel. Many migrant populations are declining and more so in some areas than in others, and migrants are more difficult to study than resident populations since they are on the move and no one is sure where they spend their winters.
Newton detailed two types of problems that may affect bird populations. First, the problem may lie in the winter traveling months when high mortality occurs. Alternatively, the problem may be that the migratory bird population far exceeds the numbers that the breeding habitat can hold, also known as its carrying capacity. In this case, the birds may either move elsewhere or accumulate locally as non-breeding circles where only a portion can breed; the latter scenario is more common.
Newton spoke about the effects of stopover events on populations, including predation and disturbance effects as well as “catastrophic mortality associated with migration” including in-flight storms and blizzards.
Stopover locations are places where migrant birds stop on their migration journey, and research has shown that stopover locations have a major effect on bird populations. For instance, the red knot bird species stop over at Delaware Bay and feed up primarily on the eggs of horseshoe crabs before going to Antarctica’s tundra to breed. The population of horseshoe crabs there began declining due to use by humans as fish bait. Since then, the survival rate of the red knot has decreased by half, and continues to decline today. “This shows the need to conserve critical areas,” said Newton.
Moreover, stopover locations may contain bird predators such as sharp-chin hawks, which can exact a heavy toll on migrant birds. Newton pointed out that there is not only a direct effect of predation (i.e. physical consumption of migrant birds) but also an indirect effect which may be even greater. The prevalence of predators forces migrant birds to be extra vigilant and aware of their surroundings, which greatly reduces their feeding time used to build up fuel for the rest of the journey. Forty percent of a migrant bird’s time is spent looking around for predators rather than looking for food. The resulting reduction in fat reserves reduces birds’ body condition and subsequent survival and breeding success.
Another issue is catastrophic mortality associated with migration. In-flight mortality associated with suddenly encountering a rain storm or sand storm can kill hundreds of thousands of birds. In 1904, more than 750 million Lapland Lonspurs were killed by a snowstorm. Around the same time, sudden refreezing of open water in Canada killed 100,000 King Eiders just as they began singing their songs to celebrate the end of winter.
Newton calls for the environmental preservation of stopover locations. In addition, because extreme weather is likely to increase with global climate change, humans must take responsibility now and begin to look for ways to preserve migrant bird populations before it is too late.
To listen to a podcast recording of this lecture, visit Newton on Bird Migratory Patterns.