Tim has been involved in studying Guillemots on Skomer for 40 years - and this has been a conclusion of 40 years work. A lot of the statistics for the paper were produce by Jess Meade - who was a Field Assistant on Skomer for two years.
The gist of it is that the expansion of Guillemots on Skomer (Reported here) can be explained by an increase in the population by breeding success and adult survival of Skomer birds alone and that immigration from other colonies (maybe due to poor breeding success) is not necessary to generate the observed growth.
|Guillemot population on Skomer Island 1960-2012|
Full paper here.
Meade, J., Hatchwell, B. J., Blanchard, J. L. and Birkhead, T. R. (2013), The population increase of common guillemots Uria aalge on Skomer Island is explained by intrinsic demographic properties. Journal of Avian Biology, 44: 055–061. doi: 10.1111/j.1600-048X.2012.05742.x