The California drought still hasn't let up—in fact, it's projected to cost $2.2 billion according to a study from UC Davis Center for Watershed Sciences. Beyond the economic ramifications, local wildlife are struggling—both waterfowl and passerine bird populations have dropped. But even less water-dependent species have taken a hit: Raptors and owls have had a rough breeding season in southern California and many experts think that the drought played a major role.
At Audubon's Starr Ranch, there are eleven predatory bird species that normally nest on the property. "A lot of these raptors—if they lay eggs—they do it within two to three days of the same time each year," Starr Ranch manager Peter DeSimone says. But this year not only was he not seeing new signs of nesting, but several raptor species—notably the Red–tailed Hawk, which usually nests all over the property—were nowhere to be seen
He suspects that the disappearance of breeding raptors is linked to the food chain. The food supply is shrinking all along the chain and raptors are the latest victims. Most birds of prey eat a variety of small mammals and insects to sustain themselves, but the drought has put a dent in their food source. Prey populations—like mice—are shrinking because the local grasses and plants they depend on for food aren't able to grow in the hot, dry drought conditions. Put it all together and you have a recipe for starving predator birds and hungry birds leads to less breeding success—up to 90-95 percent reduction for this season according to experts.
One of the species most notably affected was the Barn Owl. Starr Ranch is well known for its Barn Owl cam project, where they monitor a nesting pair during the breeding season, but this year viewers were left without much to see. Although a pair of Barn Owls did make a nesting attempt, shortly after the first egg was laid, the male owl stopped returning to the nest and the female left shortly thereafter—neither returned.
"We don't know what caused that pair to cease its nesting attempt," says Peter Bloom, zoologist and environmental consultant. He explains that in times with limited water supply and resultant limited food sources owls and hawks are going to experience lower reproductive success, but it's unclear what happened in this situation. The female may have left because the male didn't bring enough food or she may have been affected by a threat unrelated to the drought, such as rat poison, Bloom explains. "We don't want to jump, on very little clues, to big conclusions," Bloom says.
In fact, Bloom thinks the current decline in raptor populations may have causes beyond the current drought. The dramatic drop in raptor populations isn't a statewide phenomenon—instead, it seems to be centered in southern California where there is a large urban population. "I think [the population drop] is more severe than what I would have thought a drought would produce," says Bloom. He suspects that urban threats, such as disease and rodenticides, may be playing a large role as well, but as of yet there is no scientific evidence that points to any of these as the definitive cause of breeding population decline.
To get to the bottom of this mystery in southern California, Bloom thinks a large scale observational project of raptor populations is called for. "The need for this sort of work has never been previously identified," he says. "It might be time now."