We implemented four different statistical methods for apportioning birds to breeding colonies. The four methods include the existing approach that is currently used in practice (the “SNH tool”) and three novel approaches based upon statistical modelling of GPS data.

The first of these novel approaches (“WAKE”) derived the apportioning percentages associated with a statistical model (Wakefield et al., 2017) which describes the utilisation distribution of birds from a particular colony in terms of variables relating to accessibility, competition and environmental effects, and which can be used to predict the utilisation distribution of birds originating from each breeding colony in the British Isles.

Wakefield et al. (2017) used colony size data derived from the Seabird 2000 census. The second novel approach (“UCC”) is similar to the first, but revised the calculations to use more recent colony size data, where available (and to impute more recent colony sizes in situations where data were not available).

Wakefield et al. (2017) only considered breeding birds. The third novel approach (“BNB”) extends this, by using spatial survey data (both at-sea and aerial) to estimate the distribution of non-breeding as well as breeding birds, and thereby to calculate the apportioning percentages associated with all birds (whether breeding or non-breeding).

The BNB model for kittiwake and razorbill estimated the distribution of breeding and non-breeding birds to be identical. This may either suggest that the distributions are genuinely similar, or that the data are insufficiently informative to be able to detect differences between the distributions. Therefore, the WAKE and BNB approaches only provide different results for guillemot.