A Flickr API developer uses Flickr's Application Programming Interface (API) to build, test, and deploy applications that interact with Flickr's photo sharing service. They utilize the API to access various functionalities such as uploading photos, searching the database, and retrieving user information. They also ensure the applications they develop are secure, efficient, and user-friendly. Their work allows users to interact with Flickr's vast resources in a variety of ways that are not necessarily provided by Flickr's own interface. They may also be responsible for troubleshooting and resolving any issues related to the application's interaction with the Flickr API.