Irrational fears threaten bats and trees

The Secret Lives of Bats is appropriately named because our knowledge of bats is limited by the fact that they are active at night while we sleep and their activities are shrouded by darkness.  The author of this engaging little book, Merlin Tuttle, devoted his life, from the time he was a teenager, to the study and conservation of bats.  Although he learned a great deal about bats in the 60 years he has studied them, it is his accomplishments in bat conservation that are most inspiring and impressive.

A few bat facts

"Chiroptera" from Ernst Haeckel's Kunstformen der Natur, 1904
“Chiroptera” from Ernst Haeckel’s Kunstformen der Natur, 1904

There are nearly 1,300 species of bats and they are distributed all over the world, with the exception of Polar Regions.  We can’t describe the entire range of variation in the characteristics of such a diverse order (Chiroptera), so we will describe them only in general terms:

  • Bats are the only flying mammal. Their pups are born live and are generally fed breast milk by their mothers.
  • Many bat species live in caves and migrate from cold caves where they hibernate in winter to warmer caves where their pups are born.
  • Seventy percent of bat species eat insects. Bats in the US are insectivores.  Most remaining bat species eat fruit and live in tropical regions.  There are three bat species, called vampire bats that eat exclusively blood.   There are also a few species of carnivorous bats such as those that eat fish or frogs.
  • Many bat species live 20 to 30 years.
  • Most bat species use echolocation to find their prey in the darkness. See a video of a bat finding a moth, using echolocation HERE.

Local bat facts

There are 17 bat species in coastal California.  A study of foraging bats in 22 of San Francisco’s municipal parks found 4 species of insectivore bats.  The amount of forest edge and distance to water were the factors best explaining species richness and foraging activity.  The study found no correlation between bat foraging and the percent of native of plants, implying that there is no correlation between insect populations and native plants.

Why are bats important?

Insect-eating bats reduce insect populations, which reduces agricultural pests and disease-carrying mosquito populations.  A study in Arizona and New Mexico found that crop pests made up to two-thirds of free-tailed bats’ diets.  Another study found free-tailed bats feasting on migrating moths in Texas thousands of feet aboveground.  Tuttle estimates that “a single bat easily can consume more than 20 moths in a night, each carrying 500 to 1,000 eggs that otherwise would be laid on crops.  A density of 5,000 to 10,000 caterpillars per acre of cotton exceeds the threshold for pesticides.  Yet 20 moths can lay from 10,000 to 20,000 eggs.  If even half hatched to become caterpillars, they still could force a farmer to spray an acre of crop.”  (1)  Reduced populations of crop pests means fewer pesticides, which reduces farmers’ costs and toxicity exposure to consumers.

Common fruit bat. This photo makes it clear that the wings of a bat are also its hands. Creative Commons
Common fruit bat. This photo makes it clear that the wings of a bat are also its hands. Creative Commons

Fruit-eating bats are important pollinators and dispersers of seeds.  There are some species of plants that can only be pollinated by bats because of the shape of the flower and the fact that it blooms only at night.

Although birds are also dispersers of seeds, the germination success of the seeds they disperse is probably less than those dispersed by bats because most bird species poop while perched, unlike bats that usually poop while flying.  Seeds deposited on open ground are more likely to germinate than seeds deposited in the shade of tree canopy.  Therefore, bats probably play a vital role in the reforestation of fallow agricultural land.

Bad raps about bats

So, as useful as bats are, why are their colonies often threatened with destruction?

Bats departing from Congress Ave Bridge, Austin Texac
Bats departing from Congress Ave Bridge, Austin Texas

In the past, ignorance of the valuable functions performed by bats was the usual reason why their colonies are destroyed. The fact that the lives of bats are largely unseen contributes to that ignorance.  The colony of Brazilian free-tailed bats in Austin, Texas is a case in point.  In 1984, 1.5 million Brazilian free-tailed bats took up residence under a bridge in Austin, Texas.  “Newspaper headlines screamed, ‘Bat colonies sink teeth into city.’  They claimed that hundreds of thousands of rabid bats were invading and attacking the citizens of Austin.”  (1)

Tuttle was the curator of mammals at the Milwaukee Public Museum in Wisconsin at the time.  He had recently founded Bat Conservation International.  He quit his job and moved his fledging enterprise to Austin, where he was not warmly received.  But Tuttle is an engaging fellow and his knowledge of and fondness for bats is contagious.  Tuttle is equally modest, so he gives the bats full credit for convincing the public that bats need not be feared.  Within two months of his arrival, he turned the media coverage around.

The colony of Brazilian free-tailed bats in Austin is now internationally famous and a major tourist attraction.  Every evening at dusk, crowds form to witness the departure of the bats to forage for the evening.  My family has witnessed this moving event.  It is, indeed, a spectacle, on par with watching and listening to the raucous honking of huge flocks of Aleutian geese departing at dawn from their roosts for agricultural fields in Humboldt County.  Nature makes the best performance art.

Our federal Centers for Disease Control and Prevention informs us that we have little to fear from rabid bats:  “Most bats don’t have rabies. For example, even among bats submitted for rabies testing because they could be captured, were obviously weak or sick, or had been captured by a cat, only about 6% had rabies.”

Anthropogenic problems and bad solutions

There are only three species of vampire bats in tropical regions, but only one is considered a problem because it has a preference for mammals.  The population of this species of bat has grown because of the introduction of large herds of domesticated cattle.  The bats deplete the blood of the cattle and can spread diseases.  Reluctantly, Tuttle admits that these bat populations must be controlled.

So, his mission where vampire bats are causing problems for ranchers was to educate the ranchers about how to identify and find the bats.  There are other species of bats living in these areas that are performing valuable functions and unfortunately they roost in big colonies that are easy for the ranchers to find.  The ranchers were dynamiting or destroying these colonies because they were unable to distinguish them from the bats that were causing their problems.  Unfortunately, the vampire bats roost independently, hiding in trees.  So, they are more difficult for the ranchers to find.  With Tuttle’s help, tactics were devised to find the individually roosting vampire bats in order to reduce their populations.

New challenges for bats

Tuttle and his compatriots have accomplished a great deal in the past 60 years to increase our knowledge of bats and the important roles they play in nature.  He has convinced many people that it is not in their interests to destroy bat colonies on their properties.  However, he closes his book with two new challenges to bats:

  • Wind turbines are killing as many bats as they are birds. We must carefully study the design and placement of wind turbines to reduce this threat to our winged neighbors.
  • White-nosed syndrome is a fungus that has killed tens of thousands of bats in caves, particularly in the North East of the US. The fungus seems to live in a fairly narrow temperature range, so we are hopeful that it will not continue to spread rampantly.

The danger of misinformation

Millions of bats needlessly lost their lives because people were afraid of them.  Much progress has been made to inform the public of the value of bats.  Is this starting to sound familiar?  It should.  Millions of eucalyptus trees have been destroyed because people were needlessly afraid of them.  Please help us spread the word about the value of our trees.

(1) Merlin Tuttle, The Secret Lives of Bats, Houghton Mifflin Harcourt, 2015.  A favorable review of this book is also available in the New Yorker.

Bats in Glen Canyon Park are being evicted NOW!

We are reprinting with permission an article about bats in San Francisco’s parks from the San Francisco Forest Alliance.  The article reports observations of bats as well as a study of bat populations in San Francisco’s parks.  The study found that both the number of bats as well as the number of species of bats was related to the amount of forest edge in each park and the availability of water.  The study reports that the forest edge contains more insects which are the primary food of the bats. 

We are posting this story today because hundreds of trees in Glen Canyon Park in which bats and many other creatures live are being destroyed as we speak.  Here is a video of the destruction which started yesterday and continues today, along with a narrative of how and why this needless destruction is happening.  Please watch this moving video to understand why we are so committed to opposing the pointless destruction of our trees by extremist agendas that are damaging the environment and harming the animals that live in our open spaces.   (Edited to Add:  And here is a video showing the second day of the demolition project.)

The study did not find any relationship between the number of bats nor the number of species of bats with the percent of native plant coverage.  We speculate—although the study does not—that the absence of correlation between bat populations and native plant populations suggests that there are not more insects in native plants than at the forest edge.  Although native plant advocates claim that more insects are found in native plants, this is yet more evidence that this claim is not true. 


Bat in Glen Canyon Park Bats are insect-eating machines. According to the USGS, “Bats normally eat about half their weight in flying insects each night.” So even for those who don’t find these night-flying mammals charming, it’s good to know there are bats among us.

San Francisco has at least four species of bats, all of which eat insects. According to research by Jennifer Krauel, which involved recording bat sounds to determine which species they were, Mexican Free-tailed bats are the most common. Parks with water – like Glen Canyon – also have Yuma Myotis bats. The other two species she found (more rarely) were Western Red Bats, and Little Brown Bats, and she found them in just a couple of places.

2012-04-07 at 19-41-44 batHer research indicated that “amount of forest edge and distance to water were the factors best explaining species richness and foraging activity.” It also showed that bats in San Francisco remain active through the winter and don’t hibernate or move elsewhere.

If you’re interested in reading her paper, it’s here as a PDF: Jennifer Krauel thesis on bats in SF


Glen Canyon’s bats are often visible at dusk. They’re most evident in the Fall, though they’ve been seen at other times of the year. (The pictures above are from February and April, those below from October.)

Here’s a note on bat-viewing from one visitor to Glen Canyon.

“It was late in the afternoon, and late in October. We were standing around the entrance to the park on Alms Rd. As dusk fell, bats emerged from the tall eucalyptus trees. Quite suddenly they were in the air right above us. I pulled out my camera, which is not really good in poor light but I tried to take some pictures anyway. Here’s one:

bats 1

“They’re difficult to spot in the picture, but all those black smudges are bats that were moving too fast for my pocket-camera. Here’s the same picture, cropped, with the bats circled in yellow:

bats 1a

“They dispersed over the canyon. Here’s another picture from a few minutes later (and the one below it shows where the bats are).

bats 2

bats 2a

“It was fantastic. I haven’t seen this many bats anywhere in San Francisco.”


We did a little research, and found a Stanford report that emphasized the importance of large trees to a particular species of bats, Yuma Myotis… bats that Krauel’s research had actually found in Glen Canyon Park.

“Yuma bats that forage in the preserve travel several miles to roost in large trees in Portola Valley and Woodside, suburban communities on the San Francisco Peninsula. The average diameter of the bats’ chosen trees is about a yard across — more than three times wider than the average tree in those areas.”

(The link to the abstract of the actual Stanford research paper is HERE.)

That’s the size of the big eucalyptus trees in Glen Canyon Park – including those that SFRPD wants to chop down.


Bats are an important part of an eco-system, and fill a role few other creatures do: They hunt night-flying insects like mosquitoes that birds don’t catch because they’re sleeping. This is especially important now as West Nile virus, a mosquito-borne disease, has been spreading.

Having bats in a landscape contributes to its bio-diversity. All species of bats are protected in California.

(Some people are concerned that bats carry rabies – and it’s true no one should handle bats, especially grounded bats that may be sick, with their bare hands. But according to the California Department of Fish and Wildlife, less than 1% of bats are infected. [Click HERE to see their note.] The risk of getting rabies from a bat is less than the risk of being struck by lightning.)


We’re concerned about the impact of the planned tree removals on Glen Canyon’s bats.

  • All species of bats are protected, and removing the trees will impact their habitat by reducing the number of safe roosting spots, especially for Yuma Myotis bats that need both large trees and nearness to water.
  • The contractor will be chopping down the trees in the daytime. Bats roosting there are likely to be killed – if not in the process of the tree-felling, by being forced to fly blinded and confused in the daytime and fall prey to hawks, crows and ravens.

How is SF RPD going to ensure the protection of these bats?

And in what ways will felling large trees near the stream alter the ecology of the canyon?