I’m publishing this text later than common nevertheless it’s the topics themselves which can be accountable. For the reason that Nineteen 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 different considerably between years and even inside the winter itself. A few years in the past, significantly 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 puzzled 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 unhealthy climate in components of Spain final week, particularly alongside the east coast, I puzzled if which may have had an impact by dashing the arrival of Crag Martins. Recoveries of birds now we 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 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’ve got been finding out the roost for some years now with some very attention-grabbing and thrilling outcomes. It seems that particular person birds occupy specific caves inside the advanced of caves. They’re trustworthy to the cave, returning every night time 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 very best cave, maybe due to specific 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 solely on aerial bugs for meals. Throughout unhealthy climate the birds will keep on the roost and received’t feed. They depend on physique fats to see them by 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 possibly can and lay down fats. Then expend the fats when it turns into inconceivable 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 getting into the roost which I’ve described! White’s observations began a pondering course of that instructed that swallows migrated somewhat 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!
