The Zipline second-generation drones have a 10-foot wingspan.

Zipline technicians repair a drone wing. The wing is modular and can be easily replaced.

A Zipline drone drops a test package over a test site on a ranch in Northern California. Here you can see the dual propellers atop the drone.

A Zipline drone delivery package descends by parachute to the ground. Zipline can land it on a patch about the size of two parking places.

Zipline's system flicks a cord up to catch a hook at the back of a passing drone to snatch it out of the air. The cord pays out to decelerate it then winds back in to lower the drone like a caught fish.

Two technicians place a 45-pound Zipline drone on its launch system. Zipline launches dozens of tests flights every day.

A Zipline drone flies over the hills and trees of Yolo County, California.

An electric motor winds a cable that slings a Zipline second-generation drone into the air in Yolo County, California.

Zipline drones are designed to fly at night and in the rain. They integrate with air traffic control systems and are equipped with blinking lights like ordinary aircraft.

After each flight, a Zipline drone's battery and electronics package is placed into a rack for recharging. It takes about five battery packs to operate two drones continuously.

A Zipline drone battery and electronics package. They use the same battery cells as a Tesla car.

Zipline's second-generation drones have a compartment for easy installation of a new battery and electronics module before each flight.

The Zipline drones have a compartment that accepts a payload about the size of a cake box and weighing up to about 4 pounds.

Zipline flies its drones in looping test flights over a quiet ranch in the Central Valley foothills.

The Zipline drone fuselage is sculpted into a teardrop shape from lightweight styrofoam.

Dan Czerwonka, head of global operations for Zipline, communicates with the startup's base station by walkie-talkie.

Zipline's test site in Yolo County, California, consists of a storage container, a couple of temporary buildings on a graded gravel patch, and the aluminum struts of the launch and capture systems.

Drones aren't the only flying objects at its Northern California ranch site. Here a turkey vulture soars over the hills and fields.

A Zipline drone drops a test package over a test site on a ranch in Northern California.

A 10-foot Zipline drone wing goes into the shop for maintenance.

Dual propellers move the drone at speeds up to about 80 mph. If one motor fails, the drone will still fly on a single one, and during cruising only one is used.

The rolling hills around Zipline's test site is surrounded by oak trees filled with acorn woodpeckers and fields filled with western meadowlarks. In summer after winter rains stop, much of the green fades to brown and yellow.

Zipline's test site is equipped with four towers topped with a fuzz of wires to dissipate electrical charge and reduce lightning risks.

A view of a Zipline drone from the rear.

It's a 10-minute drive on dirt roads to reach the site where Zipline's drones drop test packages.

Zipline drones have QR codes; a smartphone app scans them in a process that begins a drone self-test of its different systems.

Zipline tries out new software at its Central Valley 's test site, and drones don't always snag the cord that's supposed to stop their flight. If they miss, they buzz around in a circle for another attempt.

Zipline stores old drones in a white shipping container.