# 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", *s*_{0}, 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 *s*_{0}, will not be exactly equal to *s*_{0}. That is because *s*_{0} 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 *s*_{0} 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 *m _{ij}* = 0 it follows immediately from Equation 4 that

*p*=

_{j}*s*

_{0j}; otherwise, however,

*p*will not in general be equal to

_{j}*s*

_{0j}. It follows that the "baseline mean survival" value

*s*

_{0j}(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

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