I’m publishing this text later than ordinary nevertheless it’s the topics themselves which might be accountable. For the reason that Seventies I’ve been monitoring a wintering roost of Crag Martins (Ptyonoprogne rupestris) at Gorham’s and Vanguard Caves, off the east facet of Gibraltar. The typical wintering inhabitants is round 2,000-3,000 birds however numbers have diverse 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 depend the entry of birds tonight, I questioned what outcomes we’d get. Final week there have been 800 birds on the roost, about proper for the time of 12 months, with extra as a result of come. With the dangerous climate in components 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 now have 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 depend, it was clear that it was going to be huge. 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 12 months was a poor 12 months with underneath 3,000 birds on common, so it’s nice to see numbers reaching above-average figures already, with extra birds as a result of 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 advanced of caves. They’re trustworthy to the cave, returning every evening all through the winter and even from one winter to the subsequent. There may be 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 below 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 vital 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 durations. 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 chicken in transit. Throughout heat and sunny spells feed on as many bugs as you’ll be able to and lay down fats. Then expend the fats when it turns into unimaginable to catch bugs.
These “brown swallows” have been reported by the Reverend John White, stationed at Gibraltar, to his brother Gilbert White of Selborne in 1771. He noticed them exactly getting into the roost which I’ve described! White’s observations began a pondering course of that advised that swallows migrated quite than bury themselves within the mud in ponds for the winter.
There may be one additional twist to my story. Once 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 energetic for tens of 1000’s of years. After I see the birds on the roost, I can’t assist however assume that I’m seeing what a Neanderthal as soon as noticed. Now that’s heritage!
