A lot of information on threatened and endangered species is collected by government agencies as well as nonprofits focused on conservation. While the data they collect can usually be considered credible, it is also important to be aware of the sources behind that data, whether government agency or nonprofit, just as it is important to consider the credibility of all information you may come across.
Image: "Spectacled eider (Somateria fischeri)" by Laura L. Whitehouse, USFWS (public domain, via the U.S. Fish & Wildlife Service)