The history of mammals suggests three broad overlapping phases: natural colonisation after the ice age, human-caused extinctions, and introduction by humans of non-native species. The greater mobility of birds makes such generalisations hard to substantiate in their case. Modern humans have done great damage to bird species, especially the raptors, but natural variations in populations are complex. For example, northern fulmars were present at Skara Brae during the Neolithic period, but in medieval times their breeding range was restricted to St Kilda. Since then they have spread throughout the British Isles.