As the global climate undergoes significant, wide-ranging changes from human activities, the effect on the biosphere, or collection of ecosystems and the organisms that inhabit them, is being examined to observe ecological responses. A recent study from the University of Michigan School for Environment and Sustainability (SEAS) used an interesting data set of migratory song birds to illustrate one such set of long term changes.
‘Smashing Good Data’
The specimens used for the investigation was a from a collection of birds from 1978 to the present that died from hitting urban structures in the city of Chicago. The aerial roadkill, scooped up by the Field Museum in the windy city, provide a surprisingly complete record of several migratory song bird species and their morphological changes in the last 4 decades. Specifically, the data of interest lay with the body size and the wing length of each bird and how these size measures have changed with corresponding temperatures.
Drawers Full of Birds
The study’s lead researchers, including UM’s Benjamin Winger, Benjamin Weeks, and co-authors at the Field Museum, painstakingly measured the tarsus (lower leg / foot bone), bill length, body mass, and wing length of the more than 70,000 specimens to look for trends across the 4 decades of data.
Previous research has suggested that rapid warming periods lead to decreased body size in birds and these findings seemed to confirm this with surprising accuracy. After measuring more than 70,000 bird samples, researchers found that the trend of bird body size has decreased significantly while the length of bird wings from tip to tip has increased.
Measures in all 52 species of birds showed a decrease in body size (with 49 statistically significant decreases) while 40 of the bird species showed a statistically significant increase in wing size. Measured tarsus length decreased 2.4% while wing length increased by a mean length of 1.3%. Both physiological changes the researchers are attributing to a warming climate over the past 4 decades.
Heating Up: A Case for Efficiency
The UM researchers discuss the implications of their findings as evidence of how a warming climate affects individual species. Unlike ice core, coral reef, or tree ring samples, this is a study showing direct impact of a warming planet on animal morphology or physiology as opposed to measuring actual warming trends or atmospheric greenhouse gas concentration.
The trend towards a smaller body size of these birds suggests the relationship of warmer summer breeding grounds (around a 1 degree C increase), to a reduced body size (a small, but significant amount ~10 g in mass). Flight migration is an extremely energy intensive effort and, prevailing theory contends, having a smaller body size yields a higher surface area to volume ration for the organism allowing for greater heat exchange with the outside environment. The smaller body size correlates with increased temperatures while the larger wing size, according to the researchers, suggests an offset for the loss in body mass to account for efficient flight.
Does the impressive data showing changing body size suggest that the birds were / are actively adapting or showing ‘developmental plasticity’ as rapid adaptation to climate change? That proposed question is being tested by the researchers in concurrent studies, but an alternative idea that holds more weight may be that the warming climate is a selective pressure driving “microevolution”. Simply put, warmer temperatures may be altering bird morphology at the genetic level by selecting individuals that are smaller in mass and size. In examining this study as a marker for how animals adapt to climate change, further research will be driven by examining changes through either rapid adaptation (plasticity) or evolution (selection).
“To date, there is no direct evidence that body size decreases in birds and mammals are an adaptive response to climate change.”Teplitsky, C., & Millien, V. (2014)
Plastic or not, what is a disheartening connection is a previous large-scale study on North American song birds showing a risk of extinction from warming temperatures for up to 67% of all species. This grand species loss may indicate that if some species are indeed adapting, many, unfortunately are not.
Sources and Further Reads:
- Brian C. Weeks, David E. Willard, Marketa Zimova, Aspen A. Ellis, Max L. Witynski, Mary Hennen, Benjamin M. Winger. Shared morphological consequences of global warming in North American migratory birds. Ecology Letters, 2019; DOI: 10.1111/ele.13434
- Teplitsky, C., & Millien, V. (2014). Climate warming and Bergmann’s rule through time: is there any evidence?. Evolutionary applications, 7(1), 156–168. doi:10.1111/eva.12129
- Blois JL, Feranec RS, Hadly EA. Environmental influences on spatial and temporal patterns of body-size variation in California ground squirrels (Spermophilus beecheyi) Journal of Biogeography. 2008;35:602–613