As Calving Season Gets into Full Swing, So Does the Food Source for Predators
Tim Mowry / Fairbanks Daily News-Miner / May 23, 2010
FAIRBANKS — Alaska’s moose population will more than double in the next three weeks, but it won’t take long for bears and wolves to take a bite out of that boom.
The monthlong moose calving season in Alaska began almost two weeks ago, and state wildlife biologists say approximately 120,000 moose calves will be born by the first week of June.
Of that number, though, only about 30,000 will survive a year, said Fairbanks moose research biologist Rod Boertje, who has been studying moose in Alaska for nearly 30 years. Most of the calves that perish will be killed by black bears, grizzly bears or wolves soon after they are born.
“Over 40,000 will be killed in the first six weeks,” Boertje said.
According to Fish and Game studies, the average mortality rate for moose calves one year old or less in the Interior is 65 to 70 percent.
“Out of those 120,000 calves, about 80,000 are killed by predators and another 9,000 die from other causes,” Boertje said.
Of the nonpredatory causes of death, drowning is the most prevalent, Boertje said. Calves also die from sickness, starvation, abandonment and genetic defects.
With a 50 percent calf survival rate, Game Management Unit 20A in the Tanana Flats south of Fairbanks has one of the highest moose calf survival rates in the state.
“We’re real lucky in 20A. We only lose half,” Boertje said. “If you only lose half (of the calf population) you’re doing really good. There are a lot of areas where we lose over 80 percent.”
Biologists estimate Alaska’s pre-calving moose population at approximately 200,000, and about 60 percent of those moose are adult cows three years old or older, Boertje said. On average, about 80 to 85 percent of adult cow moose (i.e. three years or older) in Alaska get pregnant each year.
“It’s pretty rare for a cow not to give birth,” Boertje said.
Because many cows have twins, the number of calves born exceeds the number of cows giving birth. Based on those numbers, biologists estimate the average annual moose calf crop in Alaska at around 120,000 calves.
Cow moose give birth over about a four-week period from mid-May to early June, though a few moose give birth later because they are bred later. The latest birth documented by biologists was early July, Boertje said.
The peak of the calving season in the Interior is May 21-23, and this year’s calf crop is on time. The first of the 48 radio-collared, pregnant cow moose Boertje is tracking as part of a reproductive study on the Tanana Flats in Game Management Unit 20A gave birth on May 11.
“That’s right on schedule for the first one,” Boertje said. “Fifty percent are born before the peak, and 50 percent are born after the peak.”
As of Friday, 22 of the 48 collared cows birthed their calves, which means the peak of the calving season should be today, Boertje said.
An average newborn moose calf weighs about 40 pounds when it is born, Boertje said. Twins usually weigh 5 to 8 pounds less than single calves.
Calves can stand on wobbly legs a few hours after they’re born, but they are mostly dependent on their mothers for protection.
“They’re not running around like caribou are in the first few hours, but they can stand,” he said. “We do catch them at two weeks of age just by running them down. You can’t do that in three days with a caribou calf. Caribou can turn into a whitetail doe and leap over you at a week’s age.”
Moose calves don’t have to be as mobile as caribou because they have a large mother to protect them, and they don’t have to keep up with a moving herd, Boertje said.
“They prefer to lie down and have mom protect them, and the mom prefers they do that, too,” Boertje said of moose calves.
Calves begin nursing almost immediately after birth.
“I have seen them stand within minutes,” said biologist Vic Van Ballenberghe, who has studied moose in Denali National Park and Preserve for 30 years and is the author of “In Company of Moose,” a book about the moose he has studied in the park. “They’re wobbly, but they try to stand up as soon as they can, and they know where to go to nurse. There’s no hesitancy. They’re programmed to know where to go.”
Newborn calves begin eating vegetation within a few days, though the mother’s colostrum is the calf’s major source of nutrition for the first month.
“Literally at two, three or four days old, they’re trying to eat a few leaves,” Van Ballenberghe said. “By the time they’re two weeks old, they’re eating quite a few leaves.”
A cow’s milk production peaks about four weeks after giving birth and tapers off “pretty rapidly,” Van Ballenberghe said, adding that calves will continue nursing until late September.
When Van Ballenberghe studied moose in Denali Park in the 1980s and ’90s, the moose calf survival rate was only 10 to 20 percent. Grizzly bears were the main predator of newborn moose calves, killing approximately 51 percent of calves that were killed during the first two months compared to only 6 percent killed by wolves.
“There was a period when grizzly bear predation was really, really heavy,” Van Ballenberghe said. “During the 20-plus years we studied moose in Denali, it was really apparent what was going on.”
Calf survival is “quite a bit higher” in the park now, a fact that Van Ballenberghe attributes to a dramatic decrease in the moose population in the 1990s as a result of heavy bear predation on moose calves.
“I think moose got scarce enough that it wasn’t worth the effort to bears to spend all that time searching for cows to find calves,” he said.
Bears kill more moose calves than wolves do in most areas, biologists have determined through studies. That’s no surprise, said biologist Mark Keech, who studies moose in the McGrath area, where black bears are the predominant predator of moose calves.
“It’s an opportunity, and bears are opportunists,” he said. Bears, especially black bears, far outnumber wolves in most areas of the state, he said.
The moose calf survival rate near McGrath jumped 15 percent in the five years after the Department of Fish and Game relocated more than 100 bears — mostly black — in 2004-05 as part of a black bear control program.
The moose calf survival rate before black bears were removed was about 30 percent, Keech said. In the five years following the bear removal, the survival rate increased to about 45 percent.
“That’s essentially what bears kill, calves up to two months of age,” Keech said.
It’s no mystery why bears and wolves kill moose calves.
“They kill them to eat them,” Van Ballenberghe said.
Biologists study calf kill sites and remains to determine what killed them. Scat, tracks, hair and wounding patterns are used to determine if it was a bear or wolf that killed the calf. Bears often scrape up dirt or tundra and bury the calf’s radio collar, while wolves often chew the collar and carry it away from the kill site.
“When you come back to a calf that’s been killed by a bear, it’s 90 percent consumed,” Keech said. “The only thing left is a little bit of hide, the hoof tips, the teeth and bits of skull.
“Generally that’s true with wolves, too,” he said.
Judging from a preliminary estimate this spring, it appears that black bears have rebounded and are just as plentiful now in the McGrath area as they were before the relocation program, Keech said. He is putting radio collars on 50 newborn moose calves this spring to do another mortality study to see if bears are killing as many moose calves as they were.
Of the 19 newborn moose calves Keech has collared this spring as part of a mortality study, two had died as of Friday. Both of those appeared to be killed by wolves.
Following their noses
On two occasions, Boertje has watched grizzly bears sniff out newborn moose calves from more than a mile away, “like a bloodhound.”
The first was in 1984. “I did a grizzly bear study and followed these two mating grizzly bears for 42 days in the spring,” Boertje said. “When they were mating, they were next to each other for 10 straight days in a very close area, less than an acre because the boar basically keeps the sow in a corral, so for a week or so they were not getting food.
“When they parted in late June it was a month after (moose) calving and it was a low density moose area so there were very few calves in the area — most of them were dead,” he said. “When they parted, one went north and one west. They both killed moose calves. They basically put their nose to the ground and ran until they found a moose calf.”
Another time while doing moose calving surveys, Boertje flew over a valley and spotted a grizzly bear almost two miles away from a cow and a newborn calf.
“The bear had its nose to the ground and was sniffing, and I wondered what was going to happen, if that bear would find that calf,” Boertje said. “I flew over the next day and that bear was on that calf.
“I think they can distinguish calf odor from a lone adult,” he said.
No safe haven
Van Ballenberghe did a study in Denali to determine what cow moose look for in a calving site. He has documented cow moose giving birth everywhere from river bottoms to above treeline.
Cow moose “have a really good sense when they’re going into labor,” he said. “Sometimes they travel a long distance to find a spot that they think is safe.”
Two factors appeared to determine where moose gave birth.
“Our hypotheses was they were selecting sites for predator avoidance or for forage,” he said. “The kind of sites they were selecting were more apt to be for predator avoidance.”
That typically meant spots with a field of view and some cover. One cow he followed — the most productive he’s ever seen — had twins for nine consecutive years in the same spot.
“She would return every year to this one limited area to give birth within a quarter mile of where she had in the previous year,” Van Ballenberghe said. “She was very successful in rearing those calves. She’d typically lose one and raise one to a yearling.”
Finding a safe place to raise a moose calf isn’t easy in Alaska, Boertje said.
“They’ve got to find a place where grizzly bears don’t go, and they go everywhere at this time of year,” he said.