New population estimates for the Balearic shearwater: Should we let down our guard?

Recently the media and social networks have echoed good news for our most endangered bird species, the Balearic shearwater Puffinus mauretanicus.

Indeed, a study published this year in the scientific journal Bird Conservation International points to a global population of about 25,000 birds, roughly doubling the previous estimates. The study is based on data from the Migres Programme, and consists on the census of Balearic shearwaters crossing the Strait of Gibraltar in their outward movement towards the Atlantic, after breeding.

The information is consistent with another study of SEO/BirdLife, which followed a different approach (the census of birds in waters off the Iberian Mediterranean in late autumn, when the birds are back from the Atlantic. In fact, both approaches had been already published together, although the new paper provides more detail on the methodology used in the Strait of Gibraltar. The issue is also considered in the International Plan of Action for the conservation of the species. But given the resurgence of the subject, it seems timely to discuss it.


The new estimate, indeed, is good news. But we must avoid the easy, optimistic interpretation reflected in some media messages: if the population is larger than previously thought, its status might be far better too. Unfortunately, the reality is more complicated, and the situation is still delicate for the Balearic shearwater. Two issues to highlight in this regard are:

  1. The Balearic shearwater population is still in decline. The new estimate is not the result of a population increase, but just of a better knowledge.
  2. The new estimates directly assess the global population, while previous estimates were focused on the breeding population (and, from there, inferred the global figure). But the relationship between global and breeding populations is not straightforward, and inferring one from the other is subject to potentially important errors.

Declining population

Extending on the first issue, the breeding population suffers a sharp decline at a rate of over 7% per year (i.e. the population becomes 7% smaller every year). This was revealed by a study of the Mediterranean Institute for Advanced Studies (IMEDEA-CSIC/UIB) in 2004, which also indicated that at this declining rate the species would become extinct in an average of just over 40 years (less than three generations). This was the main reason to classify this shearwater as Critically Endangered, and nothing has changed to justify its downlisting.

In 2012, given new estimates for the breeding population (from 2,000-3,200 pairs), as well as for the global population (25,000 birds), this “death sentence” was preliminarily revised. Slight changes in the demographic parameters (from updated information) and the model assumptions were made, but the main change was related to the population estimates. As the number of breeding birds did not seem consistent with the overall population (see below), two different scenarios were considered: one with 3,200 breeding pairs (the estimated breeding population), and another with 7,000 pairs, more consistent with the new global figure. The average probability of extinction ranged from 75 to 85 years, depending on the scenario. That is, even if the breeding population was twice as large as previously believed, the species would just gain 10 years before facing its final verdict:  extinction.

The key parameter in the model, therefore, is not the population size (although, of course, the larger the population, the longer it’ll take to reach extinction). The main problem is the population trend, which in turn is heavily influenced by the survival rate of adults. In long-lived species like shearwaters, annual survival is usually above 90% (i.e. at least 9 out of 10 adults survive from one year to the next). In the case of the Balearic Shearwater the estimated survival rate is below 80%, and this would explain the decline of the population (as there die more birds than the population can “restock” from new births, and that translates into a shrinking population).

The weakest point of the model is that adult survival estimates are based on relatively old data, from more than 10 years ago. But the same IMEDEA team is working on updating this information, and preliminary results suggest that the picture is similar nowadays. The reason seems to lie at sea, where many birds are “disappearing”, and the prime suspect is bycatch. Of course other threats must also play a role, both at sea (pollution, overfishing, climate change, etc.) and in the colonies (introduced predators, coastal development). If these pressures are not mitigated, the population trend won’t be reversed, and the species will reach its extinction sooner or later.

Discrepancies in the estimates

We must distinguish between global and breeding population. Generally bird populations are estimated based on the latter, so that the number of breeding pairs is presented. Often the global population of a particular species is inferred from the number of breeding pairs, applying the (simple but fallacious) rule that for each pair are 3 individuals (i.e. one immature bird per each breeding pair, in average). This relationship can be refined if the demographic parameters of the species in question are well known, but inferring the global from the breeding population, or the other way around, is always subject to potential biases.

In the case of the Balearic Shearwater, the global population had usually been inferred from the breeding estimate, and was roughly estimated around 10,000-15,000 birds, taking into account the available information on the species’ life history traits. Reversing the equation, to explain a total of 25,000 individuals, the breeding population should approach the 7,000 pairs. But there is much uncertainty in this equation, and so far we have to bear in mind that the direct estimate of breeding pairs is more conservative, about 3,200 pairs. This figure might be certainly underestimated, as Balearic shearwater nests are very difficult to count, and most often numbers of breeding pairs are inferred using indirect census methods. This is so because the shearwaters breed in crevices and caves of marine cliffs and islets of difficult access, which visit at night, and therefore the direct count of nests is often unfeasible. Thus, some colonies might have more breeding pairs than currently thought, and other colonies might even remain undiscovered. But we still have an endemic species to the Balearic Islands (at least as far as the current evidence amounts), which accuses a sharp decline, and has a relatively small population (either 3,000 or 7,000 breeding pairs). In summary, the situation is still alarming for this species, and we connot let our guard down.


What should we do?

To better understand the dynamics of the Balearic shearwater population, it is essential to settle a good monitoring program in the breeding areas. This would allow to provide a better estimate for the breeding population and, more important, to properly assess the population trend of the species. At the same time, it is essential to make a good assessment of the impact of different threats: presence and impact of introduced predators in the breeding colonies, occurrence and impact of bycatch at sea, etc.

And beyond properly knowing the population and understanding its problems, it is necessary to take measures to reverse the decline, seeking the cooperation of all stakeholders involved. At present, the key seems to lie in bycatch, so efforts should be directed to seek effective mitigation measures, working with fishermen to solve a common problem. It’s a challenge, but there are successful experiences in other parts of the world to show us the way to go. On the other hand, we must not forget other threats, including introduced predators in the breeding grounds, such as cats and rats.


Pep Arcos, Marine Programme Coordinator, SEO/BirdLife

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