Even though it was, in most ways, identical to the present planet, the Earth still looked very different at the bottom of the last ice age 20,000 years ago. The globe was around 4°C cooler on average, and ice sheets covered large portions of the Northern Hemisphere, including Canada and Scandinavia. One thing you might wonder, given how much of the planet was barely habitable, is what migratory species did.
Given the loss of all that habitat to mile-thick glacial ice and a reduced winter-summer contrast courtesy of Earth’s orbital cycles, some researchers have hypothesized that bird migration wasn’t much of a thing then. Is it possible that bird species turned this behavior on and off through the ice ages?
A team led by Yale’s Marius Somveille tested this idea with a model of the factors controlling migratory behavior—and it predicts patterns surprisingly similar to the present day.
Migration on ice
Migration is essentially an annual investment, a bet that the incredible exertion of the journey will pay off with superior food resources and habitat. It’s what’s often called an “energetic optimization.” The model used in this study simulates everything as an energy cost-benefit calculation, taking into account the number of bird species competing in an area and the amount of photosynthetic growth forming the base of the food web there. It simulates the current distribution of species pretty well, capturing all the major patterns we observe. And there’s good reason to think birds can adapt their migratory behavior in response to change, given that we’ve already seen some of that occurring as the world warms.
To wind back the clock, the researchers utilized results from a climate model simulation of the last 50,000 years, which was also fed into a model of global vegetation. Against this backdrop of climate and food availability, the model could simulate how bird migration would be expected to shift over time.
The results can be broken down into two stories: one for the Americas and one for the rest of the world. The Northern Hemisphere sees the largest change in ice cover, and that’s where most of the action is, but North America hosted a larger ice sheet than Europe or Asia. Because of that, simulated bird migrations shift more in the Americas.
In the Americas, about 20 percent fewer bird species migrate in the simulated coldest ice age climate. They instead keep the same range year-round. And, with a large ice sheet occupying much of the northern region, those that do migrate travel about 40 percent shorter distances on average.
Little change
Elsewhere, though, the differences are actually pretty minor. Migration to the glaciated portion of Northern Europe is also curtailed, but the number of species migrating isn’t any lower. And with alternative destinations available, there’s also no real change in average distance traveled.
Adding more complexity to the model could modify things a bit, of course, or at least highlight additional patterns within the big picture. But the researchers say it shows that migration would still be advantageous for most bird species in an ice-age world.
They also note that this means there could be interesting behavioral differences to be found between migratory bird species on different continents. “For example,” they write, “the need for communication calls during migratory flights might be higher in the New World to compensate for the fact that species’ migratory behaviors have been particularly variable over time.”
As for the problem of modern global warming, this sort of flexibility could be beneficial for the migrating species that have it. And a model like this could provide a prediction of how the migrations of those species will change in the future.
Nature Communications, 2020. DOI: 10.1038/s41467-020-14589-2 (About DOIs).
