Earlier quoted context omitted.
Microsoft could decide to terminate their access to Bing APIs at any time resulting in DDG’s immediately losing most of their users. DDG does not have their own crawler or index.
DuckDuckGo do have their own crawler (DuckDuckBot), and they augment their results with other search providers other than Bing, including Yandex.
They seem to only use their crawler for Instant Answer type results while all their "traditional links" come from "a variety of partners, including Verizon Media (formerly Yahoo) and Bing".