Renowned bird author
Spare a thought for the poor old Sand Martin that has to build a nest from scratch after travelling more than 2,000 miles from Africa – and that’s before the hard work of parenthood begins…
Dominic Couzens
explores the Sand Martins’ complex love life and the effort they put in as parents.
There comes a time in every male Sand Martin’s breeding season when all he wants is an overweight female. Slim, toned Sand Martins are of limited interest. They can fly past, light and petite, and excite not a flicker of desire.
If, however, a female exits a nest hole and takes just that little longer to reach ascending flight, suggesting that her extra bulk is having an aerodynamic effect – like a mini jumbo jet taking a long run-up to get airborne – the watching male will notice the effect, immediately.
The female’s fulsomeness gets the hormones raging, and the male, or several males, will immediately chase her in flight, attempting to get close.
It’s mid-may, and the Sand Martins’ season has reached fever-pitch. It has been a long, exhausting effort to get this far. A few short weeks ago, all the Sand Martins were in the Sahel belt of Africa, between Senegal and Mali.
Although their migratory flight is shorter than that of Swallows or House Martins, it is still about 2,170 miles, under their own steam. The birds arrived and quickly settled at a suitable nest site, usually a vertical side of a river with suitable sandy banks. Here, the birds, or at least the males, launched into their first great task of the breeding season.
We are all casually aware that Sand Martins nest in burrows, but have you ever wondered how much work this involves? The Sand Martin housing market is, to put
it mildly – fluid: nest sites that existed last year are often eroded away by powerful winter storms. In artificial sites, such as gravel pits and sand workings, the site can disappear altogether.
The upshot is that, once it has found a suitable vertical sand or gravel face, an arriving Sand Martin often has to build its own burrow from scratch.
Burrow building
The task falls to the males, at least at first. And construction is literally from scratch. The bird clings to a projection on the vertical side of the bank and slashes away with its bill until there is a shallow cavity in front of it. Once it has made a rim, enough to perch upon, it can then begin to scratch the sand away with its feet. Over the course of many hours, the male creates a tunnel in the sand, burrowing straight in and parallel to the ground or water surface. After a few sessions the bird disappears into the tunnel, and the only evidence of its work comes from the regular puffs of sand kicked out of the entrance by the bird’s feet.
Tunnel-building must seem strange to an aerial bird that spends much of its time whisked along by the fresh outdoor breezes. Yet in the stultifying confinement of the burrow it uses its long, pointed wings to shift the substrate, along with the kicking actions of its feet and the digging of its bill. The Sand Martin’s eyes are adapted more fully to the dark than those of a Swallow or House Martin.
On some days, a Sand Martin has been known to dig 13cm of burrow, the same length as the bird itself. Once the male has dug to 30cm, it has a foot in the breeding door and its progress is necessarily interrupted – the tunnel will eventually be extended to double, or even triple the length. Thirty centimetres, though, seems to be the threshold at which it is appropriate to invite a potential mate across the threshold. The males switch their attention to flying around the entrance to their burrow and singing their fizzing, flatulent songs.
This attracts the females and, if they show interest, a male will try to fly close and ‘escort’ them towards the burrow.
If all goes well, they pair up and the female will attempt a little desultory excavation, leaving pretty much everything to their mate, who completes the job by excavating a spanking interior nestchamber 4-6cm in diameter. The female builds a small nest inside, a cup of grass, feathers and twigs.
You might think that a male Sand Martin can now sit back and reflect on a job well done, but in fact, its second great task of the breeding season is just beginning – protecting its own paternity. While the females are completing nest-building and laying their clutch of four-five eggs, they are at their most fertile and are vulnerable to interest from other males. During this period, which lasts just over a week, the male has to guard against any rivals inseminating his mate. This means that, every time the female flies out of the burrow, to collect nest material for example, the male must follow her closely.
The male never strays more than a metre away from her, even if she is out of the burrow for as long as half an hour. On a short flight a male will often try to ‘escort’ his mate, roughly if necessary, back to the nest.
In one study, a female made more than 100 excursions away from the burrow in a single day, every time closely, and perhaps stiflingly, pursued by her mate, as well as varying numbers of opportunistic males. The sheer effort and concentration this
must take from the mate is considerable, but the rewards of success are intense.
It must be a relief when the last egg is laid, though, and the female can no longer be successfully cuckolded. But the male Sand Martin cannot relax for long; its last great effort of the season will soon be underway – feeding the chicks.
Insect buffet
After the two-week incubation, the chicks hatch and at first the male is responsible for providing almost all the food. This involves catching a lot of small flying insects. The need gets greater as the young grow, but the female quickly joins in. For three more weeks the burden is exhausting. On average, the adults bring in food nearly 120 times a day, with each visit delivering some 60 insects, all mashed together into a food parcel known as a bolus – that’s more than 7,000 insects snapped up in flight every single day; every insect is caught individually, by deft flying, every capture requiring concentration.
The work carries on for a few days after the young fledge, too. Only then is the burden lifted – unless, of course, the parents attempt a second brood, which happens, sometimes. Amid all this immense and demanding effort, you could probably sympathise with a hard-pressed male Sand Martin. You might also forgive it a small indulgence. And that’s where the plumper females come in. A female that is particularly heavy, much heavier than any male, is normally one that is carrying partly or fully-formed eggs.
It is at that stage when, should its own mate lose it for a moment in the melee of the colony, an alert male has the chance to inseminate it, and thereby steal paternity. Most of the time, chasing plump, eggproducing females is unsuccessful, but about 14% of chicks result from extra-pair liaisons. A most unusual twist is that sometimes, when a female is inseminated by a ‘strange’ male, it will lay its next egg in his nest, ensuring that the male’s own social mate incubates an egg that isn’t hers. This is known as quasi-parasitism.
The male Sand Martin has a long and bruising breeding season, full of toil, concentration and commitment.
We could perhaps forgive it for, just occasionally, stealing an idle moment and plumping for some easy paternity when the weight is over.
FOR THREE MORE WEEKS THE BURDEN IS EXHAUSTING. ON AVERAGE, THE ADULTS BRING IN FOOD NEARLY 120 TIMES A DAY, WITH EACH VISIT DELIVERING SOME 60 INSECTS