Researchers at Tel Aviv University have found that many birds in Israel have either become smaller or longer over the past 70 years, and posit that this is an adaptation to global warming.
The decrease in body mass of some species and the increase in the body length of others boosts the ratio between surface area and volume. The researchers said these could be strategies to lose heat as the climate warms.
The study was led by Prof. Shai Meiri and Ph.D. student Shahar Dubiner of Tel Aviv University’s School of Zoology and Steinhardt Museum of Natural History. Their paper was published in the scientific journal Global Ecology and Biogeography.
Prof. Meiri explained that according to Bergmann’s rule, formulated in the 19th century, birds and animals living in cold climates tend to be bigger than similar species living in warmer places.
This is because the ratio of surface area to volume is higher in smaller animals, permitting more heat loss (an advantage in warm regions), and lower in larger bodies, to minimize heat loss (a benefit in colder climates).
Based on this rule, scientists have recently predicted that global warming will lead to a reduction in animal size, with the possible exception of birds living close to humans such as pigeons and hooded crows, which have access to food and may gain size rather than lose it (this is what has happened with many jackals and wolves that feed on things humans have discarded).
The researchers used the Steinhardt Museum’s vast bird collection to look for changes in bird morphology over the past 70 years in Israel.
They examined almost 8,000 adult specimens from 106 different species, including migratory birds that pass through Israel every year (such as the common chiffchaff, white stork, and black buzzard), resident wild birds (among them the Eurasian jay, Eurasian eagle-owl, and rock partridge), and birds that live near humans.
They built a complex statistical model to assess morphological changes in the birds’ body mass, body length and wing length.
“Our findings revealed a complicated picture,” said Dubiner.
“We identified two different types of morphological changes. Some species had become lighter — their mass had decreased while their body length remained unchanged — while others had become longer. Their body length had increased, while their mass remained unchanged.”
These changes were found in more than half of the species examined, but there was practically no overlap between the two adaptations.
“Almost none of the birds had become both lighter and longer.”
Dubiner went on: “We think that these are two different strategies for coping with the same problem, namely the rising temperatures. In both cases, the surface area to volume ratio is increased… which helps the body lose heat to its environment. The opposite, namely a decrease in this ratio, was not observed in any of the species.
“These findings were observed across the country, regardless of nutrition, and in all types of species.”
Changes in body length tended to occur more in migrants, while those in body mass were more typical of non-migratory birds, Dubiner said.
“The very fact that such changes were found in migratory birds coming from Asia, Europe, and Africa suggests that we are witnessing a global phenomenon,” he added.
“Our findings indicate that global warming causes fast and significant changes in bird morphology. But what are the implications of these changes? Should we be concerned? Is this a problem, or rather an encouraging ability to adapt to a changing environment?
“Such morphological changes over a few decades probably do not represent an evolutionary adaptation, but rather a certain phenotypic flexibility exhibited by the birds,” he said.
“We are concerned that over such a short period, there is a limit to the flexibility or evolutionary potential of these traits, and the birds might run out of effective solutions as temperatures continue to rise.”