Survival estimates of Northern Gannets Morus bassanus in Alderney: big data but low confidence

Abstract

Capsule: There has been a linear increase in the survival rates for both adult and juvenile Northern Gannets Morus bassanus breeding on Alderney in the English Channel. However, given large confidence intervals, improved monitoring is required to better evaluate threats to this population. Aims: To estimate the age-specific survival and reporting rate from an internationally important population of Northern Gannets M. bassanus breeding at one of the southernmost colonies for this species. Methods: We use 28 years of ringing and recovery data in order to estimate age-specific survival and reporting rates. Results: Adult and juvenile survival rates differ, and both survival and reporting rates are considerably lower in first-year birds than older birds. Additionally there is an increasing linear trend in survival rates over time, and a decreasing trend in reporting rates. Conclusion: Threats to Gannets from anthropogenic disturbance are likely to be age and population specific. While these rates point towards continued growth of this population, the confidence intervals around our estimates are large, highlighting the need for improved resighting efforts in long-term studies of this nature.