On the island of Kauai, chickens have not just crossed the road.
They are also crowing in parking lots, hanging out at beaches and flocking in forests.
“They’re absolutely everywhere,” said , an evolutionary biologist at Michigan State University who has been studying these truly free-range birds. “They seem to be living a whole diversity of lifestyles, from eating garbage and cat food to being fed by tourists at the beach to foraging on native arthropods.”
In , Dr. Gering and his colleagues tried to untangle the genetic history of the Kauai feral chickens, which turn out to be not only a curiosity for tourists, but also a window into how humans domesticated wild animals. The reservoir of genetic traits could also prove useful for breeders.
Modern breeds of chickens are, by and large, bigger versions of the red junglefowl, a Southeast Asian cousin of pheasants that was domesticated more than 7,000 years ago. (There also appear to be some genes mixed in from the related gray junglefowl.)
American scientists published in 2004 . And in 2010, an international team coordinated by Leif Andersson, a professor of medical biochemistry and microbiology at Uppsala University in Sweden, . The comparison of chicken DNA, Dr. Andersson said, sought to answer, “Which are the most striking changes that have taken place during chicken domestication?”
One discovery was that all of the domesticated chicken breeds have the same mutated form of a gene that is the blueprint for a hormone receptor in the thyroid gland. The scientists are still investigating how the mutation changed the bird, although Dr. Andersson said it might play a part in chickens’ ability to lay eggs continuously instead of just in certain seasons.
Later research, dating back 2,000 years recovered from European archaeological digs, found that the mutation was not always ubiquitous. That disproved the notion that the mutation was crucial to domestication.
Chicken researchers say the uncertainties arise in part because of limited knowledge of the wild red junglefowl — the 2004 genome, for example, came from a single female. Also, the wild birds today may be different from those that were originally domesticated. Chickens and red junglefowl readily interbreed, and the wild birds may have become more chickenlike over time.
“There is still surprisingly a large amount that we don’t know about the domestication process that made this bird so valuable to us,” Dr. Gering said.
The Kauai chickens provide a new perspective — domestication in reverse.
Local lore is that many of the Kauai chickens are descendants of birds that escaped when Hurricane Iwa in 1982 and then Hurricane Iniki in 1992 blew open coops. (Feral chickens are found on other Hawaiian islands, but not in overwhelming numbers. Some speculate that Kauai is overrun because mongooses, which like to eat eggs, were never released there. Dr. Gering said another reason could be that the two hurricanes only sideswiped the other islands.)
Crowing of a Red Junglefowl
The shorter, “throttled” crowing of the ancestor to farm chickens.
Annual bird counts organized by the Audubon Society support that notion: The numbers of chickens on Kauai jumped a few years after each hurricane.
But the genealogy of chickens on Hawaii stretches back much farther. The first people there were Polynesians, who embarked on long ocean voyages to specks of islands in the expanse of the Pacific. They arrived at Hawaii at least 800 years ago.
Wherever the Polynesians went, they took their chickens, which were much like the ancestral red junglefowl.
Fossils of chickens dating to the Polynesian era, long before Capt. James Cook, the British explorer, arrived in Hawaii in 1778, have been dug up in a cave on Kauai. From the fossils, scientists led by Alan Cooper, director of the at the University of Adelaide, have extracted mitochondrial chicken DNA, part of the data they used to . (Mitochondria, the energy factories in the cell, possess their own genetic code, separate from the main strands of DNA in the cell nucleus.)
The same Australian researchers concluded Polynesians never made it all the way to South America, because South American chickens do not have the Polynesian genetic signposts.
In the new research, the scientists sequenced DNA from 23 chickens from eight regions of Kauai. As the influx of farm chickens encountered the older Polynesian red junglefowl population, Dr. Gering wondered, “How is that population evolving?”
The mitochondrial DNA of a few of the chickens matched that of the Polynesian chicken bones from the Kauai cave; more had the DNA of recent European breeds. Not all of the feral chickens had the mutated thyroid hormone receptor of modern domesticated chickens. The birds’ appearances also indicate that ancient traits persist. Some look as if they have just walked off the farm, but many others, with burnt orange and black plumage for the males, look like red junglefowl from the forests of India or Vietnam.
Crowing of a Farm Rooster
Domestication led to the familiar morning “cock-a-doodle-doo.”
Dr. Gering’s collaborator Dominic Wright, a biology professor at Linköping University, noted that many of the roosters sounded more like red junglefowl.
Both types of birds crow, but the last syllable of the junglefowl’s crow is shorter. “ right at the end,” Dr. Wright said.
Other crowings fell in between in duration. “It’s so diverse,” he said. “It certainly seems to be a lot of the original wild reservoir.”
Dr. Wright is also interested in the floppy red combs on the chickens’ heads. Domestic hens have been bred to be larger and lay eggs prolifically — traits that would seem to be attractive to roosters hoping to spread their genes. The prolific hens advertise their ability with a large comb. And yet, the feral hens on Kauai have quickly reverted to a more ancestral body shape — smaller in size and smaller combs.
“That implies there are other trade-offs that are advantageous,” Dr. Wright said. “It must have been really strong selection.”
In follow-up research, the scientists would like to observe more of the characteristics of the feral chickens — How many eggs do they lay? How often? Do they grow quickly like the farm breeds? — and then try to connect the genes responsible for the evolution of the hybrids. Dr. Wright is mating chickens and red junglefowl to precisely study how traits and behaviors are passed on.
Dr. Gering speculated that until recent decades, the Kauai chickens were largely like the ones that the Polynesians brought long ago, living in small parts of the island and modest in number. Then they began mating with the escaped farm chickens or their descendants, with greater fecundity and a wider range of habitats.
“We think that’s why we’re seeing them now at Walmart and all over the place,” Dr. Gering said.