Why ‘crit­i­cal mass’ is crit­i­cal

Bird Watching (UK) - - Conservation Bird Population -

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.

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