Last weekend I checked in on the burrowing owls at Cesar Chavez Park. Their winter habitat at the park’s northeastern corner has been surrounded by orange temporary fencing—a good idea, given the volume of foot, bicycle, and dog traffic. At least one owl was visible, standing quietly among the scurrying California ground squirrels, swiveling its head back and forth. (Park visitors can’t resist feeding the squirrels; the owl area is littered with peanut shells.)
My understanding is that these birds show up only in the winter. Their local breeding range is east of the hills. In both summer and winter quarters, ground squirrels provide housing for the owls, which are not so much burrowing as borrowing. But other burrowing owl populations have been known to dig their own.
Whether they excavate or expropriate, burrowing owls have long been known to scatter bits of manure—cow, horse, bison, whatever’s handy—round the entrance of their homes. This was believed to mask the smell of fledgling owls from potential predators like coyotes, badgers, and skunks. A plausible story, but not until recently empirically tested.
A little over four years ago, Douglas Levey at the University of Florida advanced a competing hypothesis in the journal Nature. Levey’s article was entitled: “Use of dung as a tool by burrowing owls.”
“Bait” might have been more appropriate.
Levey and his colleagues did their fieldwork in central Florida, home to a disjunct population of burrowing owls. They proposed that instead of using olfactory camouflage, the birds might be collecting manure to lure the dung beetles that make up 65 percent of their diet. The biologists tracked the fates of 50 ersatz nest burrows, stocked with quail eggs and enhanced with cow dung or not, and counted beetle parts in the owls’ pellets at active nests with and without manure. Their findings: predators cleaned out almost all the dummy nests, control and experimental alike, while the owls consumed ten times more dung beetles at nests with manure than without.
Beetle-baiting is not as far-fetched an idea as it might seem. Herons have been observed using feathers or bits of bread to attract fish within striking range. Why couldn’t owls do something similar with cowpats?
There’s always another hypothesis, though. It had been noticed that it was always the male owl who collected the stuff and spread it around. Could this be a form of display—a way of alerting passing females to the superior fitness of the territory—holder? Then again, the manure could simply be the male owl’s way of announcing to rivals that his burrow was occupied.
So Matthew Smith and Courtney Conway of the University of Arizona did their own field studies in southeastern Washington, in ranchland near Kennewick (where those controversial Native American remains turned up), Richland, and Pasco. Their methods included monitoring own territories beginning in February, before males and females paired off; manuring vacant burrows and vice versa; creating experimental nests with chicken eggs; and using pitfall traps to measure insect biomass near treated and untreated burrows.
Smith and Conway reported in Animal Behaviour that the male owls didn’t begin collecting and scattering manure until after the females had joined them. That seemed to rule out the display hypothesis. Although nest depredation rates were lower than in the Florida study, nests with and without dung had the same probability of being raided-a second strike against concealment by smell.
The other two hypotheses fared better. The owls were more likely to move into burrows when manure was removed than when it was added. And burrows with dung scored higher in insect biomass-mostly beetles, but with a high proportion of grasshoppers and crickets. It’s not clear how well dung beetles were represented.
As rare as they are, I doubt that burrowing owls are interfering with the vital work of the dung beetles. The ecology of manure is a more complex field than you might think. California lost at least three dung beetle species (whose exoskeletons have been identified in the La Brea tar pits) along with the mammoths, ground sloths, and the rest of the Pleistocene megafauna. The second-tier survivors like pronghorn and tule elk were not productive enough to keep the beetles going. So the remnant native dung beetle fauna was poorly equipped to process the leavings of all those cattle and sheep.
Australia, where beetles that had evolved on a diet of kangaroo pellets found themselves similarly overwhelmed by European hoofstock, responded by introducing heavy-duty dung beetles from Asia and Africa. Somewhat later, California followed suit. A California Dung Beetle Project was active at UC Davis for some years, releasing 680,000 captive-bred beetles before funding ran out. The descendants of those unsung heroes are still at work. A dirty job, but the beetles are happy to do it.