The summer sun rises early on the east side of the Cascade Crest. A half-dozen biologists rise with it to prepare for the long day ahead. Their task is a daunting one, for they hope to find a few very small needles in a very large haystack. Their objective is to locate bats roosting in a few trees somewhere in a great expanse of conifer forests and high prairie in southern Oregon.
The biologists are exploring the roosting habits of little-studied long-legged myotis (Myotis volans), especially those of pregnant females and nursing mothers. Unlike most bats, and for reasons that are far from clear, long-legged myotis females often appear to give birth alone in hidden, solitary roosts.
A few such roosts must be in the Winema National Forest and surrounding private timberlands. Finding them would seem to demand a great deal of luck. But the team has a plan. The night before, the biologists stayed up late to capture several bats at a watering hole. They glued tiny radio transmitters to the backsides of three long-legged myotis and released them. Today, they must detect the signals from the bat-borne transmitters, then hike across rugged terrain until they discover the roosts.
The biologists are from the University of Kentucky. Michael Baker, a postdoctoral scholar in the university’s Department of Forestry, is leading the tracking effort. The research is supported by Bat Conservation International and the Northwest Bat Cooperative, a novel organization of federal and state agencies, private industries and conservation organizations dedicated to the long-term conservation of bats in forested lands of the Pacific Northwest.
Forests throughout the Pacific Northwest have undergone substantial changes in their composition after 150 years of logging and fire suppression. Concern about the largely unknown impacts of these changes on wildlife – particularly bats – is what brought members of the cooperative together. We hope to learn enough about how bats use managed forests to make bat conservation a standard part of woodlands management.
Splitting into several groups, the team sets off, driving toward patches of high ground, such as Horsefly Mountain. “High points,” as they are referred to in the field, provide a good starting spot for tracking bats across mountain slopes and valleys. Here the signals from radiotagged bats are most likely to be detected without interference from such obstructions as steep ravines and rocky bluffs.
The biologists move quickly into a forested landscape dominated by tall, thick trees with yellowish to orange bark. These are ponderosa pines, the main source of timber east of the Cascades – and the primary trees used by maternity colonies of the long-legged myotis.
“Long-legged” is actually a bit of a misnomer, since the species is more easily identified as a mid-sized, brown bat with a noticeable amount of hair on the underside of its wing, adjacent to the chest and abdomen, and a protruding cartilage (or keel) along the outer edge of the tail membrane.
These researchers have been tracking adult females of the species to roosts for three summers, two of them in the ponderosa pine forests in and around the Wenatchee National Forest of Washington state. Adult females are of special interest because pregnant and nursing bats have extremely high energy demands that require reliably abundant food supplies, and they have extremely narrow requirements for acceptable roosts. Understanding these critical habitat needs for pregnant and nursing mothers is one of the most important steps in conserving a species.
The team found that long-legged myotis rarely use live trees, roosting most often by day in dead trees, or snags. Counts of bats leaving snags at twilight suggest that nursing females gather in colonies ranging from a few individuals to more than 400 to raise their young under large plates of exfoliating bark high up on ponderosa snags.
Before giving birth, however, pregnant females behave much differently. They spend far less time in the colonies and often roost alone beneath the bark of dead grand or white fir trees. Why such different behavior before and after giving birth? What advantage could pregnant females possibly gain from roosting alone, unlike most other bats? We do not yet have answers, but the questions are both intriguing and important.
The bark of grand and white firs is much thinner than that of ponderosa pine. Presumably, all else being equal, roosts in trees with thinner bark should be less well insulated than those with thick bark, resulting in wider temperature fluctuations inside the roosts. The biologists are exploring this notion by monitoring the environment inside the roosts, via temperature dataloggers placed beneath the bark of pine and fir trees.
The situation is complicated, however, because fir trees typically grow at lower, and probably cooler, locations than ponderosa pines. And fir trees, because of their association with stream bottoms, account for much of the roosting habitat available to bats in the stream corridors (known as “riparian management zones”). The importance of these zones in sustaining water quality and wildlife habitat in forested regions of the Northwest has long been debated. There can be no doubt about the importance of this and similar studies of wildlife that live along the mountain streams of this region.
Meanwhile, after hours of chasing transmitter signals and hiking through the summer dust and heat, the biologists finally locate all three radiotagged bats.
One of the three – a nursing female – chose to day-roost in a ponderosa pine snag that had not been used by any other tagged bats earlier in the summer. The other two, both pregnant females, roosted separately in the cracks of small rocks in a boulder field. The use of rock roosts by pregnant females was somewhat unexpected and suggests that these pregnant bats are much more flexible in their roosting behavior than previously thought.
Tonight, someone will remain at the newly discovered tree roost to count the bats at dusk and confirm whether the nursing mother has led the researchers to yet another maternity site. Our knowledge about these bats comes slowly and with considerable difficulty, but by working together, we are making steady progress.