I’m publishing this text later than ordinary however it’s the topics themselves which are accountable. Because the Seventies I’ve been monitoring a wintering roost of Crag Martins (Ptyonoprogne rupestris) at Gorham’s and Vanguard Caves, off the east aspect of Gibraltar. The typical wintering inhabitants is round 2,000-3,000 birds however numbers have diversified considerably between years and even inside the winter itself. A few years in the past, notably chilly circumstances additional north appear to have been the reason for an unprecedented inflow, once we had as much as 27,000 birds on the roost!
When going as much as rely the entry of birds tonight, I questioned what outcomes we might get. Final week there have been 800 birds on the roost, about proper for the time of yr, with extra resulting from come. With the dangerous climate in elements of Spain final week, particularly alongside the east coast, I questioned if which may have had an impact by rushing the arrival of Crag Martins. Recoveries of birds we’ve got ringed on the caves have proven us that these birds come from so far as the Italian Alps and an east coast route for them appears logical, supported by a restoration in Castellón.
Nicely, from the outset of the rely, it was clear that it was going to be massive. Crag Martins began to reach an hour-and-a-half earlier than sundown, quickly swarming over the vegetated slopes above the roost as they caught the final bugs of the day. In the long run we counted simply over 5,500 birds! Final yr was a poor yr with below 3,000 birds on common, so it’s nice to see numbers reaching above-average figures already, with extra birds resulting from arrive throughout November.
We now have been finding out the roost for some years now with some very fascinating and thrilling outcomes. It seems that particular person birds occupy explicit caves inside the complicated of caves. They’re devoted to the cave, returning every night time all through the winter and even from one winter to the following. There’s virtually no intermixing of birds between the caves, that are very shut to one another.
Gorham’s Cave seems to be the perfect cave, maybe due to explicit microclimatic circumstances. The birds listed here are at all times in higher situation, with increased physique mass than these within the different caves. Their survival charges are additionally increased. Many of those birds are adults, skilled birds at choosing the right ledges contained in the caves.
All that is extremely essential for birds that rely completely on aerial bugs for meals. Throughout dangerous climate the birds will keep on the roost and received’t feed. They depend on physique fats to see them via these lean intervals. If these are lengthy, many birds will perish. So, the success of the Crag Martin, the one aerial insectivore able to surviving the European winter, is to behave like a migratory hen in transit. Throughout heat and sunny spells feed on as many bugs as you may and lay down fats. Then deplete the fats when it turns into unimaginable to catch bugs.
These “brown swallows” had been reported by the Reverend John White, stationed at Gibraltar, to his brother Gilbert White of Selborne in 1771. He noticed them exactly coming into the roost which I’ve described! White’s observations began a pondering course of that steered that swallows migrated reasonably than bury themselves within the mud in ponds for the winter.
There’s one additional twist to my story. After we began excavating in these caves, websites that had been occupied by Neanderthals 100,000 years in the past, we discovered the stays of Crag Martins! Sure, right here was proof of a roost that had been lively for tens of hundreds of years. Once I see the birds on the roost, I can not assist however suppose that I’m seeing what a Neanderthal as soon as noticed. Now that’s heritage!
