Sandhill cranes are freshwater birds. These omnivorous birds eat insects, aquatic plants, cultivated grains, and, at times, berries, and tubers. Most populations are migratory and winter in southern United States and northern Mexico. During the spring, they migrate north to their summer breeding grounds in much of western Canada and Alaska. Migration often takes places in large flocks of thousands of birds.
The cranes prefer to migrate during the daytime. As the rays of sunlight warm up the ground, the air warms and rises upwards generating thermal uplift which allow the cranes to gain altitude of thousands of feet. The cranes then glide for several kilometers, slowly losing their altitude, before encountering another thermal uplift to regain altitude and repeat the whole process again and again. This process helps the cranes conserve energy, which makes it possible for some birds to travel as far as Siberia for breeding. This remarkable feat is made possible, in-part, by their anatomy, i.e., great wingspan of up to 6 feet. These evolutionary adaptations which allow birds like Sandhill Cranes to migrate long distances are changing in response to climate change.
Bergmann’s rule states that among warm blooded animals, such as birds and mammals, species found in higher latitudes tend to be larger while species occupying lower latitudes tend to be smaller. Bigger body size in higher latitudes, i.e., cold places, allows for better heat retention. This blog will explore the impacts of global warming on colour polymorphism in birds and bird anatomy as explained by Bergmann’s rule.
The global average temperature has increased by 0.8° Celsius since 1880. Warming temperature has a widespread impact on the North American birds. In one large-scale project, scientists studied the effects of global warming on 52 species of migratory bird in North America.
They inspected over 70,000 museum specimens and found that all species experienced a decline in body size over the last four decades. Simultaneously, nearly all species in the study grew longer wings to compensate for increased energy requirements caused by shrinking body size.
Birds in other parts of the world are also experiencing a decline in body size and mass. The Zebra Finch is native to Australia and it occupies most of the continent. Scientists in Australia examined the effects on temperature on early development of Zebra Finch chicks. As predicted by the Bergmann’s rule, Zebra Finch chicks that hatched during warmer periods had lower body mass compared to chicks hatched during cooler periods.
The anatomical changes caused by the warming temperature have severe consequences for some species. Take European population of Red Knot for example. This medium-sized shore bird has experienced an unparalleled temperature increase due to global warming in its Arctic breeding grounds. As a consequence, the body sizes of newly hatched chicks have decreased over last three decades. Short billed Red Knots, caused by shrinking body size, are unable to dig most nutritious prey thereby decreasing their survival in their wintering grounds in Mauritania. More research is needed to understand the long-term implications of shrinking bird body size.
Some 3.5% of all the bird species in the world show color polymorphism, i.e., two or more distinct color patterns within a species. Warming temperature and climate change, however, is favoring a particular morph in some polymorphic species. Take the Tawny Owls for example. This European species has two common morphologies with a brown and a grey variant. Scientists observed that as winters became milder over the last few decades, the abundance of brown morph increased. It has been hypothesized that selection for the dark morph under changing climate may be driven by the protection that it provides against UV-radiation and warming temperature.
Some bird species are experiencing an even more dramatic change. The Collard Flycatcher is a European songbird. Male Collard Flycatchers have a big white ornamental patch on their forehead. The patch signals to the rivals that you are fearsome rival and a strong companion for a female. Climate change is turning this evolutionary tool on its head. Swedish scientists noticed that, historically, male Collard Flycatchers with larger forehead patches were at an advantage for mating. But, the warming temperature over the decades shrank the forehead patch size and males with bigger forehead patch lost the mating advantage to males with a smaller patch. At the same time, large-patched males became less likely to survive warming winters.
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Jayanth Sharma, accessed on June 11, 2020, retrieved from https://upload.wikimedia.org/wikipedia/commons/2/2f/Tawny_Fish_Owl.jpg
Frank Vassen, accessed on June 11, 2020, retrieved from https://commons.wikimedia.org/wiki/File:Ficedula_albicollis,_Bia%C5%82owie%C5%BCa_1.jpg
Dennis Jarvis, accessed on June 11, 2020, retrieved from https://commons.wikimedia.org/wiki/File:DSC09803_-_Zebra_Finch_(36408993773).jpg
Chuck Homler, accessed on June 11, 2020, retrieved from https://commons.wikimedia.org/wiki/File:Red-Knot-for-Wiki.jpg
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