When it comes to American barn owls, forget spring cleaning. 

The best time of year to clean out nest boxes to ready them for breeding pairs is the fall months of September through November, according to research out of the University of California, Davis, that analyzed nearly a century of banding and other records. 

In a paper published in the Journal of Wildlife Management, researchers found that the median egg laying date for barn owls (Tyto furcata) in California is Feb. 20, so cleaning nest boxes in the fall is recommended.

“The risk is you could disrupt a nest that has already started,” said lead author Ryan Bourbour, a Ph.D. candidate in ecology in the Department of Animal Science at the time of the study. “We want to reduce disturbances to nesting pairs even in the weeks prior to egg laying.”

American barn owls offer a natural way for land managers and agricultural operations to control pests, because the raptors eat mice, gophers and rats. One breeding pair can consume up to 2,000 rodents annually, according to the UC Davis School of Veterinary Medicine.

But natural tree cavities and old barn structures are not as plentiful as they once were, so installing nest boxes can attract the barn owls. And land managers who spend money installing these “nest box networks” have long asked about the best time of year to do maintenance and cleaning. Planning cleaning and maintenance well before the start of the breeding season is “part of making that nest box worthwhile,” Bourbour said.  

Pellets add up 

After eggs hatch, nestlings spend their early days eating and living in the nest boxes until they are able to leave the nest, said Breanna Martinico, another paper author and an animal science Ph.D. candidate in ecology. 

The nestling period of roughly 65 days is considered long. 

“That’s two months where owls are living and growing exclusively in that nest box,” said Martinico, who is also a UC Cooperative Extension human-wildlife interactions advisor in Napa, Lake and Solano counties. 

As many as five or six nestlings are typically in a box, and they eat up to four or five rodents each day. Nestlings swallow their prey whole, and what isn’t digested — fur and bones — is coughed up in round or oval-like pellets.

“They’re in there for seven to eight weeks just regurgitating these pellets,” Bourbour said. “A lot of pellets pile up over the course of a breeding season and a lot of these boxes need to be cleaned out.” 

From anecdotal to data

Discussions about how early the breeding season starts have mostly been anecdotal. To get a better picture, the paper’s authors analyzed 96 years of banding records from the United States Geological Survey Bird Banding Lab and 39 years of intake records from California Raptor Center at UC Davis. Both databases helped them estimate the typical egg laying time of year in California, Martinico said.

Having this information can help land managers ensure maintenance is done and nest boxes are safe for the next breeding pair, helping barn owl populations while also benefiting agricultural operations. 

“We can give them tools to manage barn owl nest box networks effectively and maximize pest control through owls,” Martinico said. 

Additional co-authors include Emily Phillips, Jessica Schlarbaum and Joshua Hull in the Department of Animal Science, Michelle Hawkins in the Department of Medicine and Epidemiology at the UC Davis School of Veterinary Medicine and Sara Kross from Columbia University. 

Funding came from the National Institute of Food and Agriculture’s Western Sustainable Agriculture Research and Education program.

The journal article is entitled Banding records of nestling barn owls reveal optimal timing for nest box maintenance in California.