The familiar house martin, with its blue-black upperparts, crisp white underparts and white rump, has had something of a mixed summer. The extended period of drought will have made it difficult for individuals to find the mud needed to build or repair nests but, at the same time, the summer warmth has helped to support an abundance of aerial prey. Many house martin pairs will be on, or have finished, their second broods and it will not be long before some begin to depart our shores. Others may be further behind and a few, well ahead of the game, will have managed to squeeze in a third brood. The recent run of cold wet weather will have limited chick growth rates, extending the development time well beyond the usual 16-22 days. This means that some youngsters may remain in the nest through into the middle of October. The presence of such lingerers often prompts phone calls to my office from worried homeowners, fearful that “their” house martins will be unable to make the journey south. Such fears are usually unfounded unless the weather deteriorates and food becomes scarce. I suppose that this highlights our attachment to these delightful birds that have chosen to nest alongside us. Although the majority of nests are now on man-made structures, especially under the eaves of houses or under bridges, house martins would have once nested in cliff faces, like those at Hunstanton.
Not everyone welcomes nesting house martins and the mess that accumulates below the nest, but it is worth remembering just how far these transequatorial migrants have come and the amount of effort they have put into building their nests. Each nest takes about 10 days to build and is made up of over 1,000 beak-sized pellets of mud. On occasion, having just completed their nest, a pair will find themselves evicted by house sparrows. These more dominant birds muscle in and take over what is, for them, an ideal nest site.
Despite the apparent familiarity, we actually know surprisingly little about house martins. Although some individuals roost within the nest during the breeding period, it is thought that most roost at altitude on the wing (like swifts). We know even less about what happens to these birds during the winter months. Even though some 300,000 have been ringed in Britain, only one has been recovered in Africa, south of the Sahara, although local reports suggest that this is where they winter. Researchers believe that house martins spend the winter months feeding on the aerial insects that gather above the rainforest canopy. Since very few people live in such areas, perhaps it is no surprise that more individuals have not been found.