In previous years, such in situ deaths had occurred only in association with heavy mosquito parasitism (Gaston et al., 2002; A.J. Gaston, unpubl. data). A minimum of 25 additional deaths were observed elsewhere on the colony, but the total number of deaths was probably higher: our observations covered only a small part of the mosquito-affected zone, as large parts of the colony were not visible from the cliff top.
What evidence suggests that mosquito parasitism was the primary cause of adult murre deaths in 2011, and how might limited visibility of the colony have affected the accuracy of mortality estimates?