When looking to hire independents, organizations have quickly made Apple's Swift programming language one of the most sought-after skills, freelance marketplace Upwork said on Thursday.

Swift claimed second place on a chart of the 20 fastest-growing skills during the December quarter, Upwork said. The platform was outpaced only by a demand for skills in natural-language processing, thanks largely to the Amazon Echo and Google Home, which offer voice control and integration with third-party apps, services, and devices.

All of the top 10 skills saw over 200 percent growth year-over-year, some other examples being Amazon Marketplace Web Services (#4) and Instagram marketing (#6).

The Upwork Skills Index is based on billings made through the service.

Swift's growth was attributed mainly to the holidays, and specifically "record high" sales for the Apple Watch, which runs apps based on the language. Swift can be used to code for other Apple devices however, including iPhones, iPads, Macs, and the Apple TV.

Upwork noted that skills in Bluetooth ranked 19th on the list, owing to a range of new devices including Apple's iPhone 7, which lacks a 3.5-millimeter headphone jack and depends on Bluetooth or Lightning for external audio. Accordingly, Apple launched several new Bluetooth headphones in the fall and winter, namely the AirPods and two Beats products, the Solo3 and Powerbeats3.