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WSB: Age and sex affect survival of translocated grouse
Two decades of Gunnison sage-grouse translocation have revealed that when it comes to survival, you can’t teach an old grouse new tricks.
Tony Apa has been involved with boosting Gunnison sage-grouse populations before they were ever listed on the Endangered Species Act.
His first major task after being hired by the Colorado Parks and Wildlife involved translocating birds from the main population in Gunnison Basin in central Colorado, to augment a smaller population in Poncha Pass just to its east in 2000 and again in 2002.
Gunnison sage-grouse (Centrocercus minimus) live in seven populations in Colorado and one in Utah, though they were historically found in New Mexico and Arizona as well. The U.S. Fish and Wildlife Service listed the ground-nesting birds as federally threatened in 2014 due to habitat loss, fragmentation and degradation. The Gunnison Basin holds the largest remaining population, while the others are considered satellite populations, so-called because they are smaller than the main one and not connected through habitat.
A number of other translocations have occurred after Apa’s initial involvement in translocating sage-grouse two decades ago. These movements, using a number of different strategies, often occur on an emergency basis when there are declines in satellite populations. In a study published in the Wildlife Society Bulletin, Apa and his colleagues looked for trends in the various movements to inform best practices in future translocations.
In general, they found translocated grouse do best when wildlife managers move them quickly. That may involve trapping them in the evening, for example, and releasing them into the new environment the next morning.
After examining the data they collected, they also determined that translocating young females in the fall ensured the highest survival rate. “Adult males have the lowest survival,” he said.
The researchers aren’t completely sure why, but they believe young grouse survive better because they are more adaptable to new surroundings. Younger grouse may not have become set in their ways, while older grouse show stronger fidelity to seasonal habitats. Even young resident birds had higher survival rates than translocated adults, Apa said.
In addition, females typically survive better than males, since males are more at risk of predation due to their more colorful plumage. Females are also more cryptic, whereas males spend time out in the open displaying their feathers in the spring for nearly two months in an effort to attract mates.
The season sage-grouse were released also impacted survival. Fall releases had the best survival by a slight edge for all grouse, for example, possibly because the birds had time to get used to different climate conditions in their new homes. “They come into the spring ready to breed,” Apa said. Releases in the fall may also help survival because even the young birds haven’t yet developed any fidelity to their home habitats before translocation.
But season didn’t seem to matter for yearling females—those moved in the spring had nearly as high survival as those moved in the fall.
Translocation success also varied based on where the birds were moved. The Crawford population, which was geographically closest to Gunnison Basin—the source population for all translocations—had the highest survival.
Birds moved to Glade Park/Piñon Mesa had the lowest survival rates. This surprised the researchers, as the area is typically thought of as having some of the best sagebrush habitat. Apa speculated that it may have to do with the fact that the area is relatively high elevation compared to Gunnison Basin. Birds translocated from the latter ecosystem may have already chosen nest sites there. Then, they are moved to a new area that often still has snow on the ground in the spring, confusing their seasonal sense of timing.
Apa said that this finding reinforces the idea that birds moved to Glade Park/Piñon Mesa might survive better if they’re translocated in the fall, as the extra time may give them a change to acclimatize before the spring breeding season.
The researchers also found that the main source population of grouse at Gunnison Basin is shrinking, creating complications for wildlife managers looking to translocate the birds. If this trend continues, Apa said that it’s probably worth discussing the use of a captive breeding program to augment the various populations rather than moving them from Gunnison Basin directly to the satellite populations.
“When we don’t have enough birds in the source population, we can’t do translocations,” he said.
This article features research that was published in a TWS peer-reviewed journal. Individual online access to all TWS journal articles is a benefit of membership. Join TWS now to read the latest in wildlife research.