Using specialized infrared cameras and ultrasonic microphones to record bats flying through river corridors in northern California, the researchers discovered that bats use short, extremely quiet “micro-calls.” At times, no echolocation calls at all were detected, even from bats flying within 10 feet of the highly sensitive audio recorders.

https://news.wfu.edu/2018/05/02/bats-go-uiet-during-fall-mating-season/

The bats have returned! The California Department of Fish and Wildlife’s Yolo Bypass Wildlife Area is welcoming back the largest colony of Mexican free-tailed bats in California. The bats return each summer to give birth to their young and soar over the floodplain in a nightly bug-eating bonanza.

https://cdfgnews.wordpress.com/2018/05/18/reservations-now-available-for-2018-bat-night-tours/