A-Z of nature:
M for martins with our expert Dr Ben Aldiss
Martins are the subject of this month’s look at nature and I’ll compare them to their relatives the swallows and to some similar-looking, but totally unrelated birds – the swifts.
Last year we had 23 house martin nests attached under the west-facing eaves of our house – more than I ever remember seeing. The 46 adults and their numerous young finally departed for their winter quarters in sub-Saharan Africa around mid-October – a sure sign of the approach of winter.
We always miss their cheerful chattering calls and swooping flight, so it’s a great day when they return the following spring, as they did on April 15 – exactly the same date as they appeared the year before.
Apart from the house martin, we have two other closely related species in Norfolk; the sand martin and the swallow. We also have the similar-looking swift.
All four are migratory and are wonderfully adapted for feeding on winged insects and other creatures they find floating in the air, such as spiders on their silken strands of gossamer.
As an ecologist, I frequently marvel at the complexity of ecosystems and the food-webs that under-pin them. It especially interests me when two or more different animal species have similar feeding habits and yet seem to coexist without robbing each other of their food-supply.
At first glance it would appear that these four birds are in danger of doing just that, especially on those grey summer days with a north-easterly sea fret when any self-respecting insect with wings will remain firmly under cover. The Russian ecologist Gause was the first to suggest that no two species can occupy the same niche without one eventually usurping the other.
He called it his Competitive Exclusion Principle. Essentially what this means is that if two different organisms inhabit the same space and eat the same food, one will have to go.
So, how does this apply to our
four insect-eating birds? Well, if you observe them carefully, you’ll see that they all exploit flying insects in different ways and in different habitats, so they can coexist without conflict.
One strategy they use is a temporal one – they stagger their arrival and departure dates, resulting in less overlap at times of fewer insects. The first to arrive from Africa is the sand martin, reaching Norfolk around mid-March. Then comes the swallow a week or so later, followed by the house martin in mid-April. The swift is a very late arrival; we have three pairs nesting in our roof this summer and they weren’t spotted until May 12.
Each species has different nesting habits too. As its name suggests, the sand martin chooses sandy river banks or other soft vertical surfaces such as quarry faces or cliffs in which to excavate its tunnel nests. Like the house
martin, it’s a sociable bird and always lives in colonies.
I well remember as a boy seeing these birds swooping low over the water of the River Stiffkey near the secret hideout my brother and I had created in the water meadows.
House martins build their hemispherical nests of mud, lined with feathers and with an oval entry hole where the top meets the eaves. They usually choose the wide eaves of old houses, invariably on a south or westfacing side.
Swallows are less inclined to live socially, but also use mud like the house martin. Their nests are open-topped cups placed on flat surfaces such as joists in farmyard barns.
Amazingly, swifts never deliberately touch the ground, yet they manage to make a nest of sorts out of feathers, gossamer and other material they find floating in the air. Like their tropical relatives whose nests are collected to make birds’ nest soup, swifts have comparatively huge salivary glands and glue this airborne debris together with their sticky saliva.
The nest is usually placed on a flat surface under a loose tile or other cavity relatively high off the ground. If you’ve ever witnessed a swift returning to its nest, you’ll have been as impressed as I am with their speed of approach and their ability to close their wings and enter the narrow entrance with hardly a pause.
But it’s their choice of airspace for catching insects that is the main reason all these birds can live together so amicably. The sand martin tends to specialise in river insects or those found close by, but the other three are much more cosmopolitan.
Swallows are often found close to livestock and are a familiar sight on summer evenings as they dip and glide by water meadows and pastures, snapping up the flies and other insects disturbed by cows and horses, or which are a product of their dung. House martins prefer to fly higher and can be seen in their hundreds in suitable habitats swooping and soaring together up to 100ft or so.
Swifts are the true high-fliers, though, catching insects and wind-blown spiders at breakneck speed as they hurtle through the air sometimes almost out of sight. I’m going to finish with a look at why swifts are so different from the swallow and martins and talk briefly about the migration of all four. Although it looks similar to the other three, the swift is completely unrelated and only bears a resemblance due to something biologists call convergent evolution.
This is where two unrelated organisms have evolved to exploit similar habitats in similar ways. In the case of these four birds, the best shape to be if you’re going to be an aerial feeder is streamlined with scythe-shaped wings.
Apart from that, the swift couldn’t be more different. Martins and swallows can frequently be seen on the ground – when collecting mud for instance – but incredibly the swift never lands, except on its nest platform or on rough vertical surfaces to roost on rare occasions. Its legs are so useless in fact that, if accidentally grounded, its chances of taking off again are slim. No wonder the name of the group the swift belongs to is called Apodidae – literally meaning the family of birds with no legs!
When it comes to flying back to Africa for the winter, swifts are the first to leave – in August, followed by the other three who stay until October. It wasn’t until relatively recently that the mystery of the disappearance of birds in the winter was explained.
The great English naturalist Gilbert White published his Natural History of Selborne in 1789 and was the first person to realise that the chiffchaff and willow warbler were separate species. However, when it came to the perplexing problem of where swallows went in the winter, even he was convinced that they hibernated in the mud of reedbeds.
It was only when storks were spotted returning to Europe from Africa with the remains of hunters’ spears embedded in them, that biologists began to appreciate the amazing truth.
Dr Ben Aldiss is a wildlife journalist and broadcaster, deputy managing editor of World Agriculture and an adviser in biodiversity and education on farms. waspsandwildlife.co.uk
‘Amazingly, swifts never deliberately touch the ground’
ABOVE: The common house martin RIGHT: The swallow in mid-flight
Dr Ben Aldiss wildlife journalist and broadcaster, deputy managing editor of World Agriculture