P-68 in her den in the Simi Hills. National Park Service.

The mountain lion study on Tuesday announced the discovery of a litter of four female kittens in the Simi Hills, the small range, north of the 101 freeway, that connects the isolated Santa Monicas with the Santa Susana Mountains. The four kittens were in the den of P-62, a previously collared female who was away from the cubs when the researchers moved in on June 11 to inspect, weigh and tag them.

The new kittens — P-66, P-67, P-68 and P-69 — are the first litter to be tagged by researchers in the Simi Hills. It's the 15th batch of kittens found overall during the study, which began in 2002.

The location of the new den is on the property of Boeing's Santa Susana Field Laboratory, the former rocket engine testing facility that remains a source of environmental hazards. The National Park Service study of Santa Monica Mountains pumas tracks lions in the Simi Hills too since they are the main habitat that lions use to reach and depart from the Santa Monicas, although that often requires a perilous crossing of the 101 freeway. Advocates are pushing for funding and construction of a wildlife corridor to bridge the freeway near Agoura Hills.

P-62 herself was only first fitted with a radio collar in February, when she was captured with a 15-month-old male offspring, P-63. He has since dispersed from the mother. Those cats also were tagged on the Boeing property, the NPS says.

NPS video of entering the den. Have the sound on if you like hearing fearsome hisses from tiny mountain lion kittens:

From today's NPS announcement:

The mother is P-62, who researchers have been tracking since January. Biologists visited the den site while she was away on June 11, locating it after several previous attempts failed because radio telemetry showed that she was still at the den with her kittens. This is the first kitten den researchers have documented in the Simi Hills, between the 101 and 118 Freeways. The kittens were found on the 2,668-acre Santa Susana Field Laboratory site, with cooperation provided by the Boeing Corporation to access the closed area.

“This is the first litter we have marked at the den in the Simi Hills, which happens to be a critical habitat linkage between the Santa Monica Mountains and larger natural areas to the north,” said Jeff Sikich, biologist for Santa Monica Mountains National Recreation Area. “We are very interested to learn about how they will navigate the fragmented landscape and whether they will remain in the Simi Hills or eventually cross one or more freeways to the north or south.” The National Park Service, along with many other local partners, has been working for decades to preserve and increase connectivity for wildlife between the Santa Monica Mountains and other larger natural areas to the north. As the Simi Hills are immediately north of the 101 Freeway, any animals moving north-south into or out of the Santa Monicas must pass through this area. Except for P-62, the mother of the kittens, every mountain lion biologists have tracked in the Simi Hills (eight so far) has crossed either the 101 or 118 or both, providing valuable information about wildlife connectivity. Sikich suspected P-62 had given birth after seeing a series of localized GPS locations from her radio collar, indicating that she may be denning. Even with the GPS information, however, determining the location of the den is challenging because the mothers choose locations that are difficult to find. National Park Service biologists took tissue samples, conducted a general health check, and marked the kittens with ear tags. The blue-eyed, spotted kittens weighed between four and five pounds and were around four and half weeks of age. This is the fifteenth litter of kittens marked by National Park Service biologists at a den site. Three additional litters of kittens were discovered and marked when the kittens were already at least six months old. The National Park Service has been studying mountain lions in and around the Santa Monica Mountains since 2002 to determine how they survive in an increasingly fragmented and urbanized environment.

The park service also has observed another male, P-64, crossing under the 101 via a culvert. Again, watch where you walk when you are out in nature.