Monday, 14 September 2009

House Martins remain a bird of mystery

The soft, dry-sounding, ‘priit’ calls of House Martins provide the soundscape to my weekend, as dozens of birds wheel and hawk for insects in the air above me. These robust little birds have a delicate appearance, less showy than the Swallows with which they share the skies and certainly not as exuberant as the laddish parties of immature Swifts that have, by now, mostly left the country. As the martins twist and turn, taking insects above the acres of pasture over which I am walking, so their large white rumps can be seen, foreshortening the bird’s appearance even further. I’d imagine that these individuals have finished breeding and are now in the first stages of an autumn migration that will take them south over Europe and the Mediterranean, before entering North Africa to face a crossing of the Sahara.

Autumn migration begins in late August but, with some birds still feeding young in late nests, it can extend through into October. Records from our east coast bird observatories show a peak in autumn passage during September. It is amazing to think that our House Martins are part of a much bigger European population, with an estimated 90 million birds crossing the Sahara over a period of just a few weeks. This ‘super population’ of House Martins moves on a broad front, with birds initially crossing the Mediterranean from Gibraltar east to Israel. Those from Britain migrate down a route towards the western end of this wave of birds, ultimately heading for wintering grounds in West Africa.

The exact location of these wintering grounds remains something of a mystery. One of the reasons for this is that, despite the sheer number of birds involved, House Martins are not seen regularly in big numbers in Africa. This may suggest that they winter over the vast belt of tropical forest that sits across the region, hawking for insects above the canopy and out of view of researchers and birdwatchers. This lack of knowledge is in contrast with what we know about the Swallow, vast numbers of which roost communally in African reedbeds. Here the birds can be caught and ringed by researchers, who not infrequently find individuals bearing British or European rings. Fortunately, the efforts of bird ringers have at least revealed the autumn migration of our House Martins south across Europe, providing us with a better understanding of their movements than was the case back in the time of Gilbert White. White was fascinated by the autumn ‘departure’ of our swallows and martins, but was of the opinion that the birds spent the winter here in a state of torpor. Now we know different, but we still don’t know everything about House Martins.

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