A-Z of na­ture:

M for martins with our ex­pert Dr Ben Ald­iss

EDP Norfolk - - INSIDE -

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

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