Urban Oasis

I last saw the California Street barn owls on the Fourth of July. On a previous visit, three young owls were clamoring for food in the Atlas cedar near Presentation Mini-Park. Now there were only two. They were taking longer flights, around the cedar and back and forth between it and the fan palm where they had hatched. If they seemed agitated, I couldn’t blame them; Berkeley sounded like Baghdad. I had to wonder how their acute sense of hearing registered the fireworks barrage.

Behind their ghostly faces is one of the most elaborate sound-detection systems in nature, a model for research that led to the human cochlear implant. Channels in the facial ruff direct sounds to asymmetrically-placed ears: the opening of the right ear is tilted upward, the left ear downward. With different paths for high-frequency sounds from above and below, the owl can triangulate on the elevation and horizontal direction of the source. Sounds register in an aural map of three-dimensional space in the bird’s midbrain, allowing it to target its prey in total darkness.

That would mean mostly house mice and rats for these urban owls. California studies show small mammals composing 95 to 99 per cent of a barn owl’s diet, but the mix varies: in rural areas, voles (67 per cent in one Alameda County sample), pocket gophers, deer mice, and shrews predominate. You can tell what owls have been eating by dissecting the pellets, containing the hard parts of their prey, that they regurgitate at roost sites. North Bay naturalist Stan Moore says there’s a commercial market for barn owl pellets for science classes: “They’re graded like chicken eggs.”

Barn owl nestlings can consume their own weight in rodents each night. Bruce Colvin, who studied the birds in Ohio and New Jersey, has done the math: he calculated a brood of six would eat 600 voles during the ten-week period from hatching through fledging. And that’s not counting the parents’ own intake.

Six owlets may seem like a crowd, but barn owl broods may be as large as 12. Their reproductive strategy is unusual for a raptor. Some organisms have a few offspring over a long lifespan and invest heavily in their care: humans, elephants, condors. Others are short-lived but prolific, like codfish with their millions of eggs. Barn owls are more like codfish than condors. Their life expectancy is low; although they can survive up to 34 years in the wild, 75 per cent may die during their first year. Traffic takes a heavy toll—Carl Marti, then at Weber State University in Utah, counted 35 dead owls in a single day along an Idaho highway—and some fall prey to their larger relative, the great horned owl. Marti found that most Utah owls had only one breeding season in their lifetimes. So they made the most of it, with large clutches and occasional second broods. Barn owl populations may track rising and falling numbers in their prey base.

How many owlets survive to fledging depends mainly on food supply. Unlike young eagles, owlets don’t engage in fratricidal struggles; Marti has even seen older nestlings feeding their siblings. But they’ll cannibalize any nestmates that die. And some young owls are casualties of poor site selection.

Cavity-dwellers by nature, these adaptable birds will nest “any place that gives them a bit of privacy and elevation,” says Stan Moore: construction scaffolding, highway bridges, derelict chicken-farm water towers, haystacks. If you set up nest boxes, they’ll move right in. In cities, though, they have a predilection for palm trees: “a giant attraction, but a really bad nesting site,” according to Susan Heckly, director of wildlife rehabilitation at the Lindsay Wildlife Museum in Walnut Creek. She says, “A lot of our barn owls are babies that have fallen out of nests in the fronds.” The museum receives more barn owls than any other raptor species—65 in 2000 alone.

What are they like to work with? “They only weigh about a pound,” says Heckly. “That’s a surprise when you pick them up; they’re all muscle and feathers. They’re the most vocal birds we have, with an ear-splitting scream.” Older nestlings are housed in a barn where they learn to hunt live food. Very young owlets are fed cut-up mice by handlers wearing sheets, to prevent the birds from imprinting on humans. Later they’re moved to a hack box, a halfway house near appropriate habitat—in many cases, a vineyard or orchard in need of rodent control—where they can adjust to freedom. Young owls may disperse as far as 1200 miles before settling down, although 30 is more typical.

Not everyone wants barn owls next door, and every now and then some idiot takes a shot at one; the Lindsay Museum treated an owl blinded and deafened by a slingshot. And two Berkeley pairs recently lost their nest sites when their palm trees were cut down. But the California Street owls seemed to have an appreciative audience; on my first visit, a half-dozen people were lined up along the sidewalk, marveling at the neighborhood raptors whose screeches filled the summer night.

Joe Eaton writes about wildlife for the Berkeley Daily Planet, the San Francisco Chronicle, and California Wild.

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