# Feasibility of extending SeabORD to the entire breeding season: study

SeabORD is a method that can assess displacement and barrier effects from offshore renewables on seabirds, but is currently limited to four species during the chick-rearing season. This review examined ways to improve the SeabORD model including extending to the entire breeding season.

## Appendix B: Interpretation of "baseline mean survival"

SeabORD currently estimates the intercept in the mass-survival model by using the "baseline mean survival rate", s0, whose value is based (e.g. Table 3-3 of Searle et al., 2015, for the Forth-Tay SPAs) upon expert judgement or published literature.

In general, however, the mean baseline survival rate generated by SeabORD, using an intercept whose values has been estimated using s0, will not be exactly equal to s0. That is because s0 actually corresponds to the survival rate associated with any bird having mean baseline mass, rather than to mean survival rate across birds with a distribution of masses calculated under baseline conditions. To illustrate this, assume that the baseline mean survival for the population of interest is 95% - within the current implementation of the mass-survival relationship within SeabORD this would be the survival rate for a bird whose mass is equal to the baseline mean mass. Across the entire colony, however, there would actually be more birds with 91% survival probability than with 99%, because of the properties of the logit function, so the average survival probability across the colony would be a bit less than 95%. It follows that, in order for the mean survival rate simulated by SeabORD to actually equal 95%, it would be necessary for the value used for s0 to be a little bit higher than 95%.

To see this in more mathematical terms, we can re-arrange Equation 1 to obtain:

(Equation 3)

Combining Equations 1 and 3 then implies that:

(Equation 4)

If all mij = 0 it follows immediately from Equation 4 that pj = s0j; otherwise, however, pj will not in general be equal to s0j. It follows that the "baseline mean survival" value s0j (e.g. as specified in Table 3-3 of Searle et al., 2015) is the survival corresponding to mean (baseline) mass, not the mean baseline survival.

### Contact

Email: ScotMER@gov.scot