July Nature Almanac: AcroBats Over Boulder
By Ruth Carol Cushman and Stephen R. Jones
July 2021
On a July day in 2017 wildlife rehabilitator Donna Nespoli got an urgent call. A bat with two babies had fallen from a tree that had been cut down. Donna took in the family and thought they looked different. Sure enough, they were tricolored bats (Perimyotis subflavus), a mostly eastern US species that had never been documented as breeding in Boulder County. Since then, more evidence of breeding has been found, and tricolored bats have been confirmed as Boulder’s tenth summer resident bat species.
Resting tricolored bat (Perimyotis subflavus). Photo by Philip Jordan, USFS.
Last summer we were with Rick Adams, President of the Colorado Bat Society and author of Bats of the Rocky Mountain West and Into the Night, when he set up a bat detector at the edge of a shrinking pond in Burke Park on Ponca Drive in Boulder. These small flying mammals make high frequency calls that bounce off surrounding objects including their food -- tiny insects. The bats use the echoes to determine the size, range, position, speed, and direction of their target’s flight. The calls also help them avoid colliding with other bats or obstacles in their way. The bat detector then translates these echolocation squeaks which are inaudible to human ears into clicks we can hear and sonograms that Rick can use to determine which species and how many are in the air.
Rick says big brown bats (Eptesicus fuscus) are often the first to arrive because they feed on diurnal beetles which emerge before many of the flying insects do. To our surprise and joy, that night he also recorded the tricolored bat, as well as the more common silver-haired bat (Lasionycteris noctivagans), hoary bat (Lasiurus cinereus), western small-footed myotis (Myotis ciliolabrum), and little brown bat (Myotis lucifugus). The four other species of bat breed in Boulder County that we did not detect that night are long-eared myotis (Myotis evotis), fringed myotis (Myotis thysanodes), long-legged myotis (Myotis volans), and Townsend’s big-eared bat (Corynorhinus townsendii).
Western small-footed myotis (Myotis ciliolabrum). Photo by Rick Adams.
A single little brown bat can catch 600 mosquitoes and other insects an hour. A battalion of 150 big brown bats can protect local farmers from up to thirty-three million corn rootworms each summer. In addition to insect control, bats serve as pollinators and are especially important to desert and tropical ecosystems where they pollinate and fertilize such goodies as mangoes, bananas, and agaves.
The first thing these beneficial creatures need to do after awakening each night is to get a drink to replenish water lost during their daytime sleep. Water is especially vital to lactating females. Without sufficient water, both mom and pup may die. Because of global warming, many water sources are drying up. Rick fears this could lead to a catastrophic reduction in bat numbers and extinction of some species.
Starting at dusk, frenzied flitting above local ponds continues until about an hour after dark. Then the bats alternate resting and eating until just before dawn. They spend the day roosting in such places as tree crevices, on the underside of leaves, under rocks, and, occasionally, in buildings. To combat possible intruders, Rick recommends sealing small cracks where a bat could squeeze in.
The acrobatic show will continue until late September or early October after which migration and mating occur. By November, Boulder’s bats will either have migrated south where they stay active year-round or have moved into underground caverns or mines to hibernate. Researchers have found that in some species males wake from hibernation during winter to copulate with a still hibernating female. Sperm is stored in the female’s uterus until spring when a single pup is born to each impregnated female. Rick has weighed bats just before hibernation and says one big brown bat went from 12 grams to 28 grams in a short period of time. By April, when they emerge, they will have lost that extra weight and be ravenous for food and water.
For now, bats are faring fairly well in the west as we have so far escaped the devastating white-nose syndrome that has decimated eastern populations. To help monitor for white-nose syndrome in Colorado, Colorado Parks and Wildlife ask that anyone report winter sightings of active or dead bats (learn more from CPW). In fact, some eastern species, such as the tricolored bat, are spreading west, and we hope none will bring the syndrome with them. Adventurers and cavers can also help prevent the spread of the disease by not using any gear that has been used in eastern caves on explorations in Colorado and throughout the west.
Although many people fear bats, they are not dangerous and do not attack humans. Some do carry rabies, so never touch a sick bat. If you are bitten, get medical attention as rabies, left untreated, is fatal.
If you missed Rick’s May 2021 presentation “Carpe Noctem,” you can watch it on Boulder County Audubon’s YouTube channel.
Other July Events
Fireflies glitter and glow early in July in grassy wetlands near Sawhill Ponds and in a few other muggy marshes as mosquitoes whine and dine on firefly watchers.
Butterfly populations peak, and annual counts are held around July 4.
Above treeline, pikas start making hay piles on the tundra as yellow-bellied marmots sunbathe on mountain boulders. White-tailed Ptarmigan chicks hunt for insects, and American Pipits, White-crowned Sparrows, and Horned Larks hatch.
Wildflowers carpet the tundra and subalpine meadows. Stars of the show include rosy paintbrush, chiming bells, and Parry’s primrose.
Broad-tailed Hummingbirds start to return to lower elevations, and fierce battles ensue at feeders, especially when pugnacious Rufous Hummingbirds arrive from the northwest.
Ruth Carol Cushman and Stephen Jones are authors of Wild Boulder County and The North American Prairie.