Gastornis (and Presbyornis) lived in the Arctic

Adam Khor

http://www.sci-news.com/paleontology/giant-flightless-birds-arctic-03639.html

Linnaeus

I had always associated Gastornis mainly with the Messel Shales: fossils of this flightless bird have been found in Germany, and are thought (probably mistakenly) to have hunted small primitive horses such as Propalaeotherium. At least, in the fanciful BBC production "Walking with Beasts", we see a female Gastornis crushing the limp body of a fox-sized horseling in her nasty beak.

That beak, as the article indicates, was much more likely to have crushed nuts and seeds, not horses.

Finding Gastornis so far north is rather strange. The vegetation he was used to in Germany was probably different in the Arctic. He may not have adapted very well in that less clement zone, and his presence may never have been extensive there.

Diatryma, a very similar flightless bird if not actually identical to Diatryma, lived at the same time in America. Its prey or predators would likely have been quite different from those of Gastornis in Germany; but maybe some convergent species were found in both zones.