Hybrid Chickadees Are Terrible at Memory Games

An evolutionary kink from crossbreeding might affect the birds' ability to survive in harsh conditions.

In the bare winter woods across North America, you can hear the clear whistles of Black-capped and Carolina Chickadees as they forage for food. The insects they normally love to eat are gone, so the birds must find seeds and stash them among the trees for later.

The Black-capped Chickadee and its southern lookalike, the Carolina Chickadee, are like squirrels in this sense: well-known for their food-caching behavior. They’ve evolved sharp brains, with some parts that grow bigger in the winter, specifically so they can remember the location of hundreds to thousands of seeds. But in the narrow strip of land from Kansas to New Jersey where the two species overlap and mate, their offspring have a weaker memory, according to a new study published in Evolution last week. In a set of experiments, only 62.5 percent of hybrid chickadees were able to solve a puzzle to uncover their food, as opposed to 95 percent of normal chickadees.

More importantly, the hybrids’ poor recall could hurt their ability to survive harsh winters. “These birds don’t migrate; they stay in their regions throughout the year, so winter survival is pretty important,” says Michael McQuillan, a biologist at Lehigh University who was the lead author of the research. “If the hybrids are less able to do this, or if they have worse memories, that could be really bad for them.”

The trend could also explain why the blended birds haven’t evolved into a distinct species over time. Black-capped and Carolina Chickadees hybridize extensively—often to the chagrin of birders, who already have a hard time telling them apart. In general, hybridization is common: It occurs in about 10 percent of animal and 25 percent of plant species, McQuillan says. Many hybrids thrive, and in rare cases like the Golden-crowned Manakin and the Galapagos “Bird Bird” finch, they can form stable new lineages.

Yet in certain cases, hybridization can rejigger genes in a detrimental way, McQuillan says. (It’s why hybrid chickadee eggs are less likely to hatch, he notes.) Still, few researchers have looked at the cognitive challenges hybrid birds face, and whether natural selection acts against them.

To test the outcomes in Black-capped and Carolina Chickadees, McQuillan and his team caught 36 birds near Bethlehem, Pennsylvania. They used genetic testing to categorize the subjects’ ancestry and placed them in an outdoor aviary with 60 built-in cubbies.

Every day, the scientists hid a live wax worm in the same cubby, and every day, individual chickadees would practice finding it (as seen in the video below). Over time, non-hybrid birds were able to search out the prey quickly. Hybrids, on the other hand, took much longer to catch on; they made mistakes and peeked into more cubbies before pinpointing the worm.

Video: Michael McQuillan

In a second brain teaser, McQuillan placed the worm in a small box covered with a transparent film. He then observed another set of chickadees as they tried to access the insect. “The hybrids were less likely to be able to solve that problem, too,” he says. If they didn’t get it within the first few minutes, they simply gave up. The non-hybrids, meanwhile, figured out how to peel the cover off with their beaks before gobbling up the reward.

So, what do these experiments say about survival in the wild? In addition to failing to find stored nuts and seeds, the hybrids might have trouble remembering dangerous places, predators, and worthy traits in mates, says Robert Curry, a biologist who studies chickadee behavior at Villanova University. To understand the full list of side effects, scientists must first identify the aberrant genes, he says. Then, they can see if there’s an impact on various lobe sizes in hybrid brains.

Curry is also curious as to whether these learning and memory problems show up in other avian species. “As climate zones shift, it throws species into contact with each other who weren’t in contact before,” he says. “And if hybridization is happening more often because of climate change, then we will see more and more examples of these limitations.” At least the squirrels will have a field day with all those forgotten stashes.