Submitted by Kwiaht
What do our Northwest bats eat? The simple answer to this question is insects, but that is about as useful as saying “animals.” There are nearly 100,000 of them live in North America.
Imagine the challenges faced by hungry bats. Island bats are good at targeting and intercepting insects on the wing, but staying aloft takes energy. Bats need to maximize the number of calories they can collect per minute of flight time. This may mean seeking a few large buttery insects such as moths, or large swarms of smaller, less calorie-rich insects such as midges and mosquitoes. Swarms of large, greasy insects such as termites are best of all, but they cannot be found most of the year. Even on a warm summer evening, flying insects are not evenly distributed. One night there is a swarm of newly-emerged mayflies over a pond, the next the action is in a wet hayfield, a newly-turned garden, or the leafy canopy above a log filled with termites.
Different species of insects hatch and mate at different times and places, so a clever bat could try to anticipate where and when to find them. This is complicated by the fact that the seasonal timing of insect swarms is influenced by weather and varies from year to year. An older bat may remember a beach where salt-water midges often swarmed in previous years, but not know when to return.
Do our bats communicate hunting knowledge to each other? Do younger, inexperienced bats follow older bats to see where they hunt? Or is it “every bat for herself?” We really do not know.
One of the challenges for island bats is adapting to the effects of climate change. Some insect species move north as the Salish Sea warms, to be replaced by insects that island bats haven’t hunted.
Like marine biology, bat research depends on technology. Humans can’t see or hear what bats are doing without electronic devices such as high-speed infrared cameras and ultrasound recorders. Software capable of unscrambling bats’ voices and identifying individual bats in a feeding frenzy over a marsh has only existed since 2009. New-generation software using cluster analysis to identify thousands of bat calls in minutes is indispensable to Kwiaht’s network of five bats “listening posts” around San Juan County. On a typical summer evening, Kwiaht’s Bat Grid will record about 2,000 bat flyovers, representing nine species of bats.
One unmistakable finding of Kwiaht’s Bat Grid is that a lake, pond or woodland clearing is visited by different bat species at different times of the year. Some bats species are more mobile than others, and only show up for a week or two at a time, while others are present nearly year-round, varying only in numbers. Large nightly variations in numbers presumably reflect weather conditions as well as insect abundance. Much nightly variation is not correlated with the weather and may reflect bats’ predictions of where feeding is best, or other factors such as the presence of owls that hunt bats.
These dense data sets still lack one important variable: the target insect species. How can researchers tell what the bats circling over a marsh one night are eating?
The conventional solution has been to set out insect traps to sample the “prey field” (what is available that could be eaten), then trap some bats and detain them until they defecate. Fecal pellets (guano) consists of finely chewed insect exoskeletons, some of which can be identified by a meticulous, sharp-eyed entomologist with a microscope. Over the past few years, however, a small number of laboratories (including Kwiaht’s Lopez lab) have begun using the residual traces of DNA in guano to identify the insects eaten, and sometimes the bat species as well.
Kwiaht researchers collect guano from beneath known bat roosts, which is much less time-consuming and significantly safer for bats than trapping. Collection sites include bat houses.
The latest development in bat research technology is a smartphone microphone and app that makes it possible to do “point and shoot” species identification of bats in flight. This makes it possible to identify bats while observing them hunting a swarm of insects, without trapping or handling any bats. It will still be necessary to trap some of the insects — for now. The next phase of bat research, still in development, involves identifying insects acoustically. Many insects also echolocate, or have distinctive flight sounds that could be recorded and decoded.
Watch for announcements next spring and summer of Kwiaht “bat viewing nights” when researchers will demonstrate the new technology for identifying bats in real time “on the fly” as they feed on swarms of insects. And ask Kwiaht about less expensive versions of this technology that can convert your smartphone into a bat detector! Batting can be as much fun as birding if you have the technology to eavesdrop on bats and identify them.