Trends in seabird numbers are the results of what their populations have been subjected to, but not a very accurate indicator of their present status. Researchers have now developed and presented a new indicator for population status based on data for their productivity. The approach may be the start of a more extensive and precise method for assessing the status of seabirds and the marine ecosystems they depend on.

Seabirds as indicators

Vertebrate populations are often monitored as part of broader assessments of ecosystem status, where they are expected to provide information on the ability of the ecosystem to support higher-level predators. However, because many vertebrates are long-lived and often only subsets of their populations can be monitored, abundance may not be sufficiently responsive to ecosystem status to provide early warnings of impending changes. Marine birds are often used as indicators of ecosystem status, but, due to their long lifespan and delayed recruitment to the breeding population, changes in abundance are generally slow and often difficult to interpret. Their breeding productivity is, however, also widely monitored and much more responsive to ecosystem status, but the relevance of variation in productivity may be difficult to assess.

Integrating breeding productivity

The researchers behind a new study published in the journal Ibis propose a model-based indicator that integrates monitoring of breeding productivity with published estimates of adult survival rates through demographic matrix models. The metric of the proposed indicator is the expected population growth rate, given the observed level of breeding productivity and unchanged survival. This expected growth rate is then compared with a threshold derived from the criteria employed for red-listing of threatened species by the International Union for the Conservation of Nature. The researchers demonstrate the suggested approach using data from black-legged kittiwakes (Rissa tridactyla) in the Greater North Sea region, NW Europe. Their proposed indicator shows that the current level of breeding productivity is expected to lead to a population decline of 3–4% per year, which is equivalent to a red-list status as Endangered for the black-legged kittiwakes in this region.

Already implemented

The indicator approach is used in OSPAR’s Quality Status Report 2023 and is expected to be used by EU member states for further reporting under the Marine Strategy Framework Directive. It represents a major step forward in assessing the status of marine bird populations. According to the authors behind the article, the ideal next step would be to develop a coherent Integrated Population Modelling (IPM) framework that would allow inclusion of all data on population abundance and demography collected across the large and diverse marine ecosystems involved.

Read the article:

Two banded, adult black-legged kittiwakes with a one-year-old. Photo © Tycho Anker-Nilssen
Black-legged kittiwakes don’t mind nesting in inhabited areas if conditions are favourable, like here on Røst in Northern Norway. This photo shows two adult, ringed individuals alongside a one-year-old youngster.
Photo © Tycho Anker-Nilssen
Black-legged kittiwakes with chicks in nests. Photo © Hanne Pilskog
The black-legged kittiwake may lay a clutch of up to three eggs, but it is rare that all the chicks survive to fledging age.
Photo © Hanne Pilskog

Contact person: Tycho Anker-Nilssen, NINA