AA #17: Mystery Mini-Migrant
The cache, a small, camo-taped plastic pot, is hidden at the side of the lane ascending from the East Beck ford up towards Askwith Lane.
As you pass through Crow Trees farm, you will note a footpath sign on your left. This is for an alternative route to cross the beck during or after very wet conditions when you would otherwise get your feet wet.
Heading towards and through the farm from #16, several of these familiar small birds were buzzing around the farm buildings and surrounding fields foraging for flying insects . . .
The (common, northern or western) house martin (Delichon urbicum) is a common summer visitor to the UK, arriving from sub-Saharan Africa in April and leaving in October and widely found in towns and villages, as well as in agricultural areas.
Its preferred habitat is open country with low vegetation, such as pasture, meadows and farmland, and preferably near water, although it is also found in mountains up to at least 2,200m. It is much more urban than the barn swallow, and will nest even in city centres if the air is clean enough. It is more likely to be found near trees than other Eurasian swallows, since they provide insect food and also roosting sites. It species doesn't normally use the reed-bed roosts favoured by migrating barn swallows.
It is glossy black above, completely white below, and has a distinctive white rump and a short, forked tail. It is much smaller than a barn swallow or swift - 13cm long, weighing 19g, and with a 28cm wingspan. It has a lifespan of 5-14 years and spends much of its time on the wing collecting insect prey. A flexible body gives it agility in the air to evade predators and strong wings enable it to stay airborne for long hours catch insects.
Hunting grounds are typically within 450m of the nest, with a preference for open ground or water, the latter especially in poor weather, but it will also follow the plough or large animals to catch disturbed insects.
It builds mud nests, sometimes in small colonies, under ledges, on cliffs and, as their name suggests, under the eaves of houses. Both males and females help to build the nest, collecting mud from streams and ponds and building up layers with bill-sized pellets. These intricate mud nests take about 10 days to build and are often returned to from African wintering grounds and reused in following years.
However, with the climate crisis and warmer weather in UK, one of the biggest problems facing breeding house martins is lack of mud. Long, dry spells with little or no rain, mean that the pools and puddles where the house martins usually collect mud dry up and without mud, they cannot nest and raise a family.
Conversely, heavy rains make it hard for these little birds to find the flying insects they need to feed their chicks. Yet another example, if one were needed, of how unpredictable and extreme weather conditions are threatening one of our most familiar and best-loved birds.
They are territorial during the breeding season, and male martins engage in fights with other males if there is intrusion into their territory. Such fights are often violent and result in serious injury for one or both combatants, maybe even death for one.
The breeding season martin varies according to the range and they can produce two broods of offspring per year. They may congregate in large colonies consisting of 100s of pairs. Clutch size consists of 4-5 eggs, which are incubated mostly by the female for about 14-16 days.
Chicks are born under-developed, and remain in the nest under the parents’ care for 22-32 days, after which they leave the nest, but they stay a further week with their parents after fledging.
Because of their aerial agility it has few predators, but the Eurasian hobby has been known to prey on this bird successfully.
Come September, most House Martins return to their wintering grounds. But where they go has remained relatively unknown, but it's certainly sub-Sahara Africa. Unlike other migrants, it 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 1km 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.
See here for more info on this remarkable small super-flier and see short videos here (perched & twittering), here (calls and nest site), here and here (nesting), and here (distinguishing swifts, swallows and martins).
