Why ‘critical mass’ is critical
In 1939, the RSPB bought North Warren in Suffolk for its breeding bird communities. They included Red-backed Shrikes, at that time on the cusp of a severe plummet in fortunes, after already a century of decline. It was a smart move and one that’s proven the salvation of many birds, including Bitterns, Roseate Terns and, perhaps most of all, the Marsh Harrier (right), once down to a single pair at carefully-protected Minsmere. Yet, by 1960, there were just 27 pairs of shrike left along this section of coastline and, by 1989, shrikes bred regularly in England for the last time. So why wasn’t North Warren the saving grace for shrikes? Why, with optimal habitat and food, were North Warren and other heathland sites unable to hang on to their five, 10, 15 or 20 pairs of Red-backed Shrike? Why did the Minsmere approach work for harriers and Bitterns – but not for shrikes? This might sound a bit academic, but this question has to be asked now if we want to reverse or freeze declines in our Turtle Doves
You can have habitat, and you can have food; but without connection in a population, extinction will always follow. Isolation is extinction.
and Cuckoos, Wood Warblers and flycatchers. In Somerset, a few months ago, an irate lady demanded to know why her garden was no longer good enough for Spotted Flycatchers. On explaining that other gardens were also needed, I saw not only that I was explaining things badly, but also that I’d made an implacable enemy for life. There is no anger on Earth like that kindled by impugning someone’s bee plants. So, here’s my chance to try the connection argument again. Polish ornithologist, Viktoria Tacaks, has done lots of work on Red-backed Shrikes, where they are still common. Her work brings to the forefront one of the most important questions facing conservationists – how many birds do we need in a population to prevent extinction in the long term? Shrikes are particularly important because their numbers in Britain, in any one place, were self-evidently too
low to prevent their demise. We need to learn why this was the case, in order to stop that happening to other species, especially other summer migrants. Viktoria’s paper “Predictions of changes in population size of the Red-backed Shrike (Lanius collurio) in Poland: Population Viability Analysis” is important to understand why we’ve lost birds we care about. In her study, Viktoria looks at a stable population of Red-backed Shrikes in east Polish farmland. She asks a simple question: how many pairs are needed to keep things as they are? What if the food and habitat remain prime? What if we factor in a bad summer, which, in Poland, happens every four and a half years? What if we start with 30 pairs? What if we start with 200 pairs? How likely is it that, in 50 years’ time, this population of shrikes will still exist? Viktoria puts these questions to VORTEX. When I heard this term, I pictured a cosmic black hole, but it’s actually a piece of software for Population Viability Analysis (PVA). It’s used to estimate extinction probability in any single population. In Poland, the team knew things like the percentage of shrikes laying between one and five eggs, the breeding age of females (they’re ready at the tender age of one, but past it by the age of nine), and the mortality of young and adult birds. Bear in mind that 60% of hatchling shrikes do not last their first year. Like many migrants, just one third will make it back from Africa the following spring. From the VORTEX results, Viktoria calculated approximately how many shrikes you need in a single population to stand the test of time.
How many birds make a future?
The study revealed that for a shrike population to have a 95% chance of surviving 50 years – in favourable conditions – you’d need around 80 to 90 pairs in that population, or 160 to 170 individual shrikes. That puts a massive demand on the size of your landscape, and the food resources within it. This conveys, perhaps, quite how robust a population of summer migrants needs to be in order to stand the test of time. Those single pairs of Spotted Flycatcher, Swallow or Wood Warbler near you are already extinct. It doesn’t matter if the habitat is prime, or nothing has changed. It’s not the habitat that’s vanished. It’s the connection.