Mega flock of 5 million Brambling in Slovenia
Estimating the size of bird flocks is hard enough, as British and Irish birders know when watching a large Starling murmuration such as those at Ham Wall (Somerset), Gretna (Dumfries and Galloway) or Nobber (Co. Meath), all of which have featured prominently on social media in recent weeks.
So spare a thought for Tomaž Mihelič and the other poor Slovene ornithologists of the country’s BirdLife partner, DOPPS attempting to count a roost of Bramblings as it develops in Beech woods along the River Sava, a feeder river of the Danube in Central Europe. A minimum of 2 million birds, but probably as many as 5 million, have descended on an area of about 100 square kilometres, but every night they pack into a patch of trees covering just 5 hectares, a circle about 250 metres in diameter. The exact site of the roost is not being revealed to prevent disturbance but is drawing crowds of locals to watch each evening.
Why have they arrived in Slovenia? Their main food in winter is usually tree seeds, often Beech or Hornbeam. In most years in mid-October the vast populations of Brambling in the forests of the taiga west of the Urals spread out across the forests of central and eastern Europe with relatively small numbers (in most years) reaching Britain and Ireland, perhaps from Scandinavia. This year, as many of you will have seen on the news, damp, northerly wind systems have brought chaos and more than a metre of snow to much of central and eastern Europe, north of the Alps. This has covered much of the Bramblings’ food supplies and they have irrupted south beyond the Alps to escape. Much of the Alps enjoyed a long, hot summer and the production of wild food was good. The mountains have also created a precipitation shadow for the recent weather systems and indeed the areas in the inner southern Alps (northern Italy) are suffering drought and serious forest fires. So Slovenia is relatively snow-free. As the Bramblings arrive from the north they focus on areas where numbers of their own species have already found good feeding and beneath various stands of Beech there may be a carpet of seeds (‘beechmast’) 2-3 centimetres thick. This is the case in the Zasavje area in 2018/2019. Bramblings draw in Bramblings and rapidly a substantial chunk of the European population accumulates in a single area.
Why the single big roost? The main reasons for these roosts are protection from predators (by swamping the local Sparrowhawks, Buzzards and various owl species) and (perhaps) exchange of information. A bird that has fed poorly one day may notice a well fed group arrive, roost with them and leave in search of food the following morning.
Giant Brambling flocks are a fairly frequent feature in Slovenia with more than half the country covered in forest, much of it Beech, but the last on this scale in the country was in the winter of 2004/2005 in the area of Bohorje while in the winter of 2008/2009, a flock of about a million birds gathered in the forest of Trnovo close to the border with Italy. The largest flock of birds ever recorded in recent times (at least since the extinction of the Passenger Pigeon, another species feeding on irregular ‘mast’ crops) was of 70 million Bramblings in Switzerland in 1951-52.
How long will the birds stay? It is hard to say. Heavy snow (some is forecast for Wednesday 23/01) covering the food will see the birds move on, either down into the Balkans or into Italy but is unlikely to stick together as a single unit. A flock of this size requires 25-75 tonnes of food per DAY to keep it going and the larger the flock the more quickly it eats itself out of house and home, clearing the food from a given radius around the roost. In the meantime, if there are further developments Rare Bird Alert will keep you up to date.
Paul Tout
21 January
Paul Tout (@adriawildlife) is a naturalist, wildlife tour-guide and translator living on the border between Italy and Slovenia since 1989. He also contributes regularly to Domen Stanic’s bird-heavy wildlife blog from the area, Carniolicum* and Domen’s website containing lots of information about wildlife watching in Slovenia, www.wild-slovenia.com
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