Northern Fulmars spend considerable periods of time at their breeding sites in the months before egg laying. In this study the function of pre-laying breeding site attendance was investigated. Three potential benefits of prolonged pre-laying colony attendance were considered: (1) site defence; (2) opportunities for engaging in either pair or extra-pair copulations (EPCs); and (3) mate defence. There was no evidence that the function of pre-laying colony attendance was site defence: sites were left unguarded for long periods during the pre-laying period and were not subsequently lost to conspecifics. Neither did it appear that individuals maintained a high frequency of attendance to engage in pair copulations. The available evidence is consistent with the idea that females attended the colony to engage in EPCs; females present at the colony on a high proportion of days during the pre-laying period were more likely to be involved in EPC attempts. Although the possibility that females attended the colony for an as yet unidentified reason and engage in EPCs simply because they were there could not be ruled out. It appeared that males attended the colony in an attempt to prevent their partners from engaging in EPCs. Males maintained a higher level of attendance than females throughout the pre-laying period and seldom left their female partners unaccompanied during the presumed fertile period.