Migratory birds’ chromosomes reveal stress

Researchers recently discovered some differences between the DNA of migratory and resident birds that can provide insight into the stresses that migratory birds face from their journey.

As part of a study published in The Auk: Ornithological Advances, lead author Carolyn Bauer and her colleagues examined telomeres in songbirds. Telomeres are the protective cap at the ends of the chromosome that stop the cells from aging and dying. When a cell replicates its DNA, a little bit of the telomere is shortened due to oxidative stress — the increased production of free radicals such as peroxides or other chemicals. This shortening of telomeres cannot be repaired, says Bauer, a postdoctoral scholar at North Dakota State University.

Bauer, who has had a longtime interest in telomeres and stress physiology, wanted to determine the differences in telomeres of migratory and resident dark-eyed juncos (Junco hyemalis) in western Virginia. “Birds are actually pretty easy to study telomeres in,” she said, adding that you can extract the DNA right from their red blood cells, which is different than that of humans. Researchers also refer to telomeres to determine the age of an organism.

Bauer and her team went to the University of Virginia’s Mountain Lake Biological Station, a field station on a mountaintop in southwestern Virginia, during early March right before the migrant subspecies was about to fly to northern Ontario for the spring. Using mist nets and Potter traps — wire cages that the birds hop into — the team trapped the birds and retrieved blood samples.

To measure the birds’ telomeres, the researchers used a qPCR, a method of detecting specific DNA sequences in a sample, which allowed them to get comparative measures of the telomere lengths of residents and migrants. The researchers only examined one-year-old birds in order to control for age — which they determined based on plumage and the color of the birds’ eyes.

Bauer and her colleagues found that migrants had shorter telomere lengths than the residents. “That fits the hypothesis of migration,” she said. “Migrants and residents experienced the same wintering grounds. The only real difference was that one long migratory trip.” Bauer says this suggests the stress of the migratory journey could possibly cause the shortening of their telomeres or these results could suggest the migrant subspecies is not as good at repairing its DNA.

“If migrants are not as good at repairing their DNA, this may suggest that migrants place less emphasis on self-maintenance and more on reproduction and growth,” Bauer said. ”Further, while telomeres don’t have a direct effect on the birds, shorter telomeres are associated with a lifetime exposure to stress. Bauer says there have been studies that show birds with shorter telomeres have an increased chance of mortality as well.

Bauer says the next step in the research is to study the telomeres of migrant and resident junco nestlings to determine if they start at the same length. While she tried to do this in the past, getting to remote and inaccessible areas where they are born in northern Ontario is difficult. “[Studying the nestlings] is something I’d like to do,” she said. “I have to figure out how I can get up there.”

Header Image: A male slate-colored dark-eyed junco of the migrant subspecies (Junco hyemalis hyemalis) perches on a tree. ©Clay Billman