By James Duncan
Learning and Engagement Officer
The delicate House Martin (Delichon urbicum) is a remarkably small bird, roughly the size of a Goldfinch and smaller than a House Sparrow. This diminutive species is one of our most common Palearctic African migrants, a bird that like its close relatives, the Barn Swallow and Sand Martin, belongs to the family Hirundinidae. In fact, the very terms 'swallow' and 'martin' have been largely interchangeable over the centuries - martins typically have shorter tails and swallows longer tails, though there's no effective scientific distinction. Along with the Swallow the tiny House Martin is without doubt one of our most welcomed spring arrivals, having completed a monumental journey from sunny winter skies close to the Equator. Their sublimely iridescent blue-black plumage, snow white underparts, clean white rump and short forked tail make them rather distinctive and no British summer would be complete without their gritty, evocative calls.
By the nineteenth century, the appropriately named House Martins had somewhat abandoned traditional cliff-based nesting sites and chosen to move in with us. Not literally of course, but our ample range of architecture seemed to suit them nicely. They're vastly more urban than our Barn Swallows and have undoubtedly benefitted from an abundance of safe nest-sites underneath eaves and gables. A gregarious species, they'll typically nest colonially showing superb craftsmanship in their nest design. It'll take one to two weeks to construct from scratch, though the martins will show supreme restraint by allowing it to dry properly during the build. The foundation will compose up to a thousand 'mud pellets' collected from wet ground close to ponds and streams, glued into place under a suitable overhang. However, this only really applies to late arrivals, as most take the opportunity to reuse existing nests. These long-established colonies may truly be symbolic of an ancient lineage of House Martins.
House Martin & House Sparrow © Roger Wilmshurst
Should a House Martin manage to requisition a nest built during a previous spring, it'll probably manage two broods, even allowing for potential repairs to a degraded nest. Should it have to build the nest in its entirety it'll be a struggle to manage a second brood in time for peak insect emergence. Demonstrating their social nature, a nest may well house a substantial number of individuals at roost, including the adults and perhaps even both broods. Cooperative breeding is a distinct possibility, where members of the first brood may assist the parents in feeding the second. House Martins have few natural predators, other than nature's ultimate aerial acrobat, the Hobby, but they may well be turfed from their nests by an unlikely source - the House Sparrow. They're remarkably dominant over House Martins and will attempt to evict the construction workers during the build process. Once the nest is complete, it's too late as the entrance hole will be too small for a chunky Sparrow.
Like their close relatives the House Martin also feasts on insects, the typical size of which denotes it as feeding on aerial 'plankton.' Fortunately for us, a large proportion of this prey may be the pesky Mosquito. Come September, the majority of House Martins will be on the move to their wintering grounds. But just where do they go? That's a question that has remained relatively unknown, but it's certainly sub-Sahara. Unlike other migrants, the House Martin has quite literally stayed 'off the radar', perhaps partly owing to its use of cavity roosting in Africa and a propensity for staying high, very high - quite possibly even roosting over a kilometre up, on the wing. They've remained a remarkable mystery of nature, though it's likely a significant area of central and southern Africa serves as their winter abode. Like the Swallow they've seen similar declines since 1970, though these are particularly hard to understand for the House Martin owing to the lack of migration data. Their colonial nesting habits certainly suggest a need for ecological stability, even suggested by Shakespeare, in Macbeth. This also points to the fact that House Martins may have been using man-made nesting structures for a long time indeed. Here's an excerpt from Macbeth: Act I, Scene VI -
"This guest of summer,
The temple-haunting martlet, does approve
By his loved mansionry that the heaven's breath
Smells wooingly here. No jutty, frieze,
Buttress, nor coign of vantage, but this bird
Hath made his pendant bed and procreant cradle;
Where they most breed and haunt, I have observed
The air is delicate."