James W. Cornett, Special to The Desert Sun9:57 p.m. PDT May 12, 2015
They were dead.
Not long ago, I received a visit from a tree trimmer. He informed me that he had been removing dead fronds from a palm in a parking lot. The palm had not been trimmed for several years. He and his workers had removed the entire "skirt" as it is sometimes called. Huge chunks of the skirt had been cut and pulled from the trunk and allowed to fall more than a dozen feet to the ground.
As he and his workers began collecting the dead fronds to throw in his truck, he found two bats. Both appeared dead as they had presumably been crushed beneath the mass of fallen fronds. As he had apparently never removed a full desert fan palm skirt before, the presence of the bats surprised him. He was also disturbed that his activities had resulted in their death. That was why he came to my office. He hoped that they might be simply injured or temporarily dazed.
I went out to the pile of dead palm fronds. The trimmers had put the two bats on a frond. They were in fact dead, not just dazed or injured. I was surprised because they were western yellow bats, known to scientists as Lasiurus xanthinus. This is a relatively rare species of bat and designated by the State of California as a Species of Special Concern.
Most of what we know about the western yellow bat comes from a recent study completed by scientists Danielle Ortiz and Cameron Barrows, published in the peer-reviewed journal Southwestern Naturalist. In their article, Ortiz and Barrows related how western yellow bats are often, though not always, associated with our native desert fan palm oases. To be precise, their study focused on 41 palm oases most of which are located right here in the Coachella Valley.
Ortiz and Barrows discovered that not just any palm oasis was acceptable to western yellow bats, the bats spend daylight hours roosting in palm skirts but the skirts and oases seem to need certain characteristics. In general, Ortiz and Barrows were more able to confirm roosting western yellow bats in higher elevation palm oases possibly because such locations were in less arid surroundings and had cooler temperatures. Such variables may be important in an environment that is getting increasingly dryer and warmer.
The bats also seemed to prefer palm oases where there was greater variability in the length of the dead frond skirts. Ortiz and Barrows speculated that an assortment of skirt lengths no doubt provides a greater opportunity for a bat to find the perfect place to spend the day. And, of course, a recently burned palm oasis has no skirts whatsoever. Ortiz and Barrows found no species of bat preferred an oasis that had recently caught fire even if only a portion of the oasis had burned.
Finally, western yellow bats were most likely to be detected in palm oases that had new growth, that is, oases with lots of young palms growing on the oasis floor. It seems having a vibrant younger generation around is a harbinger of good things to come and, somehow, the bats respond positively to this.
Cornett is a desert ecologist and author of Desert Palm Oasis: A Comprehensive Guide.