Having expressed my interest in Dr Newell’s talk about Swifts in November someone very kindly lent me a book titled ‘Guests of Summer’ by animal ecologist Theunis Piersma. Originally written in his native Frisian language it has been translated into English and published by the British Trust of Ornithology. It is fascinating reading, at times humorous, and there are no boring charts or graphs in what is, essentially, a scientific publication.
Theunis lives in Gaast, a small Dutch village in the province of Friesland. He has been studying House Martins since 2002 when he moved there. Perhaps the most interesting fact to emerge from his years of study is that he and his fellow scientists still haven’t discovered how they spend their winters in Africa. It’s known where, they winter in Cameroon and Congo, but they don’t know any more than that. Swallows and Swifts also winter there. The former can be monitored relatively easily as they roost in reed beds and tall grasses. Scientists have fitted the latter with tiny geo-locator tracking devices and established that they spend the winter airborne. However, House Martins do not come out of the sky to roost at night and they are too small to have tracking devices fitted so what they do remains a mystery. Theunis concludes, but hasn’t been able to prove, that because it is their nature to fly high like Swifts they also remain permanently on the wing.
House Martins live on tiny aerial insects such as flies, flying ants, mosquitoes and aphids which they forage for at high altitudes. By detailed analysis of droppings, blood and DNA samples research has shown what species they consume and where they are found. Samples taken from the tips of primary wing feathers confirm that House Martins feed on insects from tropical rainforests. Some are host to malarial parasites in their blood proving they have been in contact with sub Saharan mosquitoes.
House Martins normally arrive in late April, find a mate, often a different one each year, and start to renovate or build their mud cup-shaped nests under the eaves, which they then line with feathers or grass. They do not favour artificial nestboxes. Four or five eggs are laid and chicks leave the nest after about five weeks. It is common for there to be a second brood although bad weather affects breeding success. House Martins sleep in their nests at night.
In summer, not that many years ago, after a long drive you’d need to clear dead insects off the windscreen – not any more. Across the arable counties of England House Martin numbers have plummeted in recent years. They need insects to eat and insects need flowers and livestock. Coincidently in these areas the numbers of insects, wild flowers and livestock have also drastically declined. Vast acreages of cereals, sugar beet and rye grass are now routinely treated with effective pesticides and herbicides which are not conducive to providing the conditions House Martins require to thrive.