Is a new chickadee on the horizon?

Class is in session for a group of chickadees at their temporary quarters inside an aviary in Lehigh’s department of biological sciences.

Today, the students are beginning a 10-day lesson on spatial memory. A wooden board hanging on one of the aviary’s walls contains 60 small holes, each covered with a ball of cotton. One of those cotton balls conceals a worm. One at a time, each bird must find the hole hiding the worm.

Over the next nine days, the test will be repeated daily, with a new worm hidden each day inside the same hole.

The experiment, says Michael McQuillan, is designed to measure each bird’s ability to remember where it has stored food—a trait vital for chickadees and other animals that scatter and hoard their food to survive in the wild.

It is also designed to shed light on speciation, the process by which new species arise over time.

McQuillan, a Ph.D. candidate in biological sciences, is particularly interested in hybridization, which occurs when parents from two different species mate. Their offspring, called a hybrid, may or may not become a new species.

McQuillan’s chickadees include the black-capped chickadee, whose range covers Alaska, Canada and the northern United States, and the Carolina chickadee, which lives in the southeastern U.S. The two species of songbirds do not migrate and are similar, but distinguishable, in appearance. The black-capped variety is darker and larger, its tail is longer, and its black bib has sharper edges.

The ranges of the two purebred species overlap in the Lehigh Valley. The intermingling has led to interbreeding and produced hybrid chickadees, which McQuillan is also testing.

Hybridization occurs in about 10 percent of animals and 25 percent of plants, says McQuillan. Some hybrids thrive, but most do not. The mule, a cross between a horse and donkey, is valued for its strength and endurance but is unable to reproduce and thus to speciate. And roughly 60,000 years ago, says McQuillan, scientists believe human beings interbred with Neanderthal men.

“Hybrids are not always inferior to the parent species,” says McQuillan. “Sometimes they are superior, and this can lead to outcomes that affect evolutionary trajectories. If we knew why hybrids do or don’t have the ability to reproduce, we could figure out how speciation occurs.”

McQuillan has proposed a new hypothesis to explain why hybrids fail to thrive and why natural selection, one of the basic tools of evolution, might act against hybrids. Hybrids, he says, might be cognitively deficient, that is, slower to learn than their parents.

“The genes in parent species have been evolving for a long time and work really well,” he says. “But hybridization mixes genes up in a new way that hasn’t yet been tested by evolution. Because of this, the genes can interact in a way that causes fitness disabilities and genetic incompatibilities.”

These incompatibilities can be manifested as sterility and lack of viability. McQuillan plans to conduct further experiments in hopes of determining whether they also cause cognitive deficiencies.

For his research proposal, McQuillan received the George W. Barlow Award last fall from the Animal Behavior Society. The award is one of four named awards given by the society to graduate students each year, and it is the only named award given for research outside the field of conservation.

Testing cognitive deficiencies

The ability to achieve spatial memory is heritable, says McQuillan, and it is critical for bird species that survive harsh winters by returning to the caches where, in warmer times, they have stored nuts, seeds and other food. The black-capped and Carolina chickadees both have impressive spatial memories, he says, but the record so far is held by the Clark’s nutcracker, which is known to cache as many as 33,000 seeds a year and to remember most of their hiding places.

So far, the experiments with the cotton balls seem to bear out McQuillan’s hypothesis about the cognitive deficiencies of hybrid birds relative to their parents. Preliminary results show that the hybrid birds are slower to learn the spatial memory tasks. By the fourth, fifth and sixth day, the two purebred species have improved the speed at which they find the lucky cotton ball, while the hybrid’s pace hovers near the average random speed.

McQuillan captures his test birds in the wild with a thin, 6-foot-high, nearly transparent mist net and releases them back into their environment two to three weeks later after finishing the spatial memory tests. He says the hybrid chickadees seem to have a low hatching success rate, with only one of nine birds hatching in some egg clutches. He wants to conduct more tests to determine if the hybrids’ relatively deficient memory skills represent a determining factor in their ability to thrive.

The next experiment will compare how fast each of the three chickadee types learns new skills. This time, the birds will perch alongside small blocks extending out from the wall of the aviary. Atop each block, a washer will cover a hole where a worm has been placed. The washer will be transparent, allowing the bird to see the worm. The bird’s instinct is to peck at the washer, but it must learn that only by sliding the washer from one side to the other will it be able to get to the worm.

“This is a novel problem for the birds,” says McQuillan. “They have never encountered an apparatus like this before in their environment. But if the environment changes, the bird will have to learn quickly to adjust. The ability to learn cognitively is vital for survival and fitness.”

To help determine whether chickadees acquire cognitive skills genetically or in response to their environment, McQuillan plans a follow-up test with chickadees raised in the lab.

“We’ll give them the same tests we give to the other birds,” he says. “Every behavior has a genetic and an environmental component. By doing tests in a lab, we eliminate the latter and control for the former.”

McQuillan’s faculty adviser is Amber M. Rice, an evolutionary biologist in the department of biological sciences.

McQuillan finds biology fascinating, he says, because he has always been interested in evolution.

“Evolution helps me understand how the world works, how life works, how all the animal and plant species came to be. It’s really rewarding to be able to develop questions about the natural world, to design experiments to answer those questions, and to be creative at the same time.”
 

Story by Kurt Pfitzer

Photos by Christa Neu