A new electronic unit that broadcasts crow distress and alarm calls efficiently repels the birds and reduces crop damage in almond orchards, reports the UC Davis research team that developed the device.

"One of these units, which covers four acres, costs about $125 to build and would likely be less to produce commercially," said agricultural engineer Michael Delwiche. "We estimate the units could result in savings of between about $30 and $60 per acre."

He noted that in California almond orchards, crows can cause damage ranging from $130 to $1,000 per acre in a single season.

Broadcast distress calls have been used for more than a decade to chase crows from urban and agricultural areas. Delwiche and colleagues set out to develop an electronic system that would save power and prevent the birds from growing accustomed to the calls over time. They taped two calls from birds in the wild and made use of two other calls that had been taped at the National Wildlife Research Center in Fort Collins, Colo.

The unit they designed, to be hung from tree branches, features a stainless steel plate on which the speaker and circuitry are mounted. It plays four different 25-second-long distress calls, pausing for 12 minutes between each call and automatically switching off at dusk and back on at dawn to save power.

The researchers tested 36 of the broadcast units in commercial almond orchards for 10 weeks during the second year of the study and compared the damage levels with first-year results when the units were not used. When the units were activated, large flocks of crows approached the source of the calls, circled overhead while calling and then flew away.

A decrease in the crow population the second year of the study accounted for some decrease in crop damage, but the broadcast units appeared to be responsible for a further decrease in crop damage. The researchers found that the crows did not grow accustomed to the recorded calls until near the end of the study, and they are optimistic that the period of effectiveness can be stretched to span the eight-to-10-week period when almonds are vulnerable to crow damage.