Monday, 7 January 2013

Guillemot growth on Skomer not just from immigration

Tim Birkhead who has been studying guillemots on Skomer has recently published an interesting paper on the guillemot situation on Skomer. 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
The population of common guillemots Uria aalge on Skomer Island, Wales has been monitored since 1963, and in the last 30 yr has increased at an almost constant rate of 5% yr-1. A previous attempt to model the population based on intrinsic demographic parameters estimated over just five years failed to explain the observed population increase, probably  because the estimate of juvenile survival was too low. This raised the possibility that immigration fuelled the population increase. Here we use 30 yr of detailed field observations to re-estimate key population parameters (productivity, adult survival and juvenile survival) in order to model the population. We show that the observed rate of increase can be explained by these intrinsic parameters, and we therefore conclude that immigration is not necessary to generate the observed population growth.

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

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