Unprecedented increase in crag martin winter roost at the Gorham’s Cave Complex UNESCO World Heritage Site

Researchers studying the wintering population of crag martins (Ptyonoprogne rupestris) at the Gorham’s Cave Complex UNESCO World Heritage Site have been surprised by an unprecedented arrival of these birds during the early part of the month of November. During one of two weekly counts of the birds entering the caves in the evening, the count exceeded 5,700 birds. This figure could represent up to 1% of the European post-breeding population, which is remarkable, and makes this the largest known roost of this species in the world.

Previous counts undertaken in recent years had reached a maximum of 3,600 birds which was in keeping with counts in the 1970s (done by some of the researchers of the current project) that placed the winter roost at 2,000-3,000 birds. It had been thought that the wintering numbers had since dropped significantly but no accurate counts had been done until the current project resumed three years ago. It is known that the birds wintering in the caves come from as far as the French Pyrenees and the Italian Alps.

The project research team is made up of scientists and co-workers from the Gibraltar National Museum, the Gibraltar Botanic Gardens and the Gibraltar Ornithological & Natural History Society, working jointly as a University of Gibraltar project. There may be a number of reasons for the observed increase: these birds may have had an exceptionally good breeding season in Europe or a recent spell of bad weather in the Iberian Peninsula may have pushed greater numbers than usual south. Another explanation, in part at least, may well be the protection offered by the Gorham’s Cave Complex since it became a UNESCO World Heritage Site in 2016. These birds are able to rest and sleep undisturbed as the roost is nowadays carefully protected.

The crag martin roost was first noticed as far back as the late eighteenth century when the Reverend John White reported their presence to his brother, the renowned naturalist Gilbert White of Selborne. These observations contributed to the understanding of the phenomenon of bird migration at that time. The crag martin roost is an ancient one, the remains of these birds having been found in Neanderthal contexts dating to over 100 thousand years ago.