© Doug Welch photo images/BTO
House martins have just arrived back in the country – but where have they been?
Increasing numbers of house martins have been arriving here over the last few weeks, much to the delight of householders fortunate enough to have these iconic summer visitors breeding under their eaves.
The first individuals to return tend to be older birds and these quickly occupy those nests that have survived the vagaries of the winter weather.
Birds that arrive late at nesting colonies may be forced to build a new nest from scratch, a process that can take up to two weeks and involve more than 1000 beakfuls of mud.
Arrival of house martins, 2013 – data from BirdTrack.
Despite being a colonial-nesting species, house martins can be surprisingly robust in defending their nest, seeing off rivals and having a go at house sparrows looking to use a nearly completed nest cup for their own breeding attempt.
You might imagine that house martins compete for aerial insects with swallows, swifts and sand martins. To some extent they do, but these species tend to feed on differently sized insects and at different heights, thereby reducing the degree of competition between them.
Information from monitoring schemes, such as the BTO/JNCC/RSPB Breeding Bird Survey, indicate a long-term decline in British house martin populations, something that has seen the species placed on the amber list of birds of conservation concern.
Interestingly, the decline has been most pronounced in South East England, with populations further north fairing rather better.
Somewhat surprisingly for a bird that is so familiar to us, we know virtually nothing about what happens to house martins once they leave our shores.
Recoveries of ringed birds from across France and on into North Africa reveal a migration route south but we have no idea what happens to these birds once they have crossed the Sahara.
In fact, there has only been one record of a ringed house martin from south of the Sahara, despite the many thousands of individuals to be fitted with rings in Britain each year (see map).
House martins are rarely seen in Africa during our winter, which might suggest that they remain on the wing and feed above the canopy of the equatorial rainforest, out of sight of human eyes.
Ring-recoveries of British house martins, from Time to Fly by Jim Flegg.
New technologies may provide an answer, however.
The development of tiny tracking devices, known as geolocators, has revolutionised our understanding of how small birds migrate to and from Africa.
These devices, no bigger than a shirt button, have an inbuilt electronic calendar, a clock and a light sensor that constantly monitors the daylight against the clock and the calendar and stores that information.
Once the bird returns to Britain, scientists can remove the device, download the data and work out where on the planet the geolocator was at any given time and date.
Tracking house martins
Scientists at the BTO have already used these devices to revolutionise our understanding of the movements of nightingale, swift and nightjar. This year they hope to fit geolocators to British house martins (see below).
House martins are one of the last of our summer migrants to depart in the autumn and some breeding pairs may still have young in the nest during September or even October.
The presence of young so late in the year can prompt the question as to their chances of survival.
Assuming that the weather does not deteriorate suddenly, then the young should be able to make their way south, feeding on the wing as they go.
With luck, a small number of the birds heading south this autumn should be carrying geolocators and, when they return to the breeding colonies next spring we should get an answer to the question of where they spend the winter.
The British Trust of Ornithology (BTO) works in partnership with over 40,000 volunteer birdwatchers to chart the fortunes of UK birds.
Among the surveys that we coordinate is our popular Garden BirdWatch, the largest year-round survey of garden birds in the world.
Each month we highlight a bird for you to look out for in your garden.
For more information about Garden BirdWatch or to speak to the Garden Ecology Team please email firstname.lastname@example.org
Find out more about the BTO’s work to fit geolocators to British house martins.
Read previous BTO Garden Bird of the Month blogs.