Au­thor Do­minic Couzens re­veals some fas­ci­nat­ing facts about the tiny Sedge War­bler.

Weigh­ing no more than four sugar cubes, this tiny war­bler is able to travel many hun­dreds of miles non-stop on mi­gra­tion – it re­ally is a true ‘su­per-bird’!

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That’s it, now. The great sum­mer bird party is over. The trav­ellers came to visit, had a ball and are now on their way back. Tick off the av­er­age au­tumn de­par­ture dates of your favourite mi­grants one by one, as they trickle away. It’s been a party, al­right. All the guests bring their own unique im­print, just as we recog­nise hu­man char­ac­ters at gather­ings. There’s the creepy one, a char­ac­ter you never quite trust, the Cuckoo. There’s the loud one, whose pres­ence dom­i­nates the early evening karaoke un­til their part­ner gets them to be quiet – the Nightin­gale. There’s the one who seems to get drunk early and is soon out of con­trol, all over the place, like a Whitethroat. There’s the no­to­ri­ous love-rat and wom­an­iser, the Pied Fly­catcher, mix­ing with the im­pos­si­bly el­e­gant beauty who leaves a dodgy set of re­la­tion­ships in her wake, the Swal­low. From the one who ar­rives early but never says any­thing in­ter­est­ing (Chif­fchaff) to the one who stays for such a short time that it is al­most rude (Swifts), ev­ery per­son­al­ity is there. At al­most ev­ery sum­mer so­cial there is also the ‘life and soul’ char­ac­ter. This is the one who lifts and en­livens pro­ceed­ings as soon as they ar­rive. They are a force of na­ture, and when they fi­nally say good­bye, you look at each other, raise your eye­brows and go “wow!”. And no bird fits that pro­file bet­ter than the ir­re­press­ible, mys­te­ri­ous, ex­cep­tional, un­usual su­per-bird, the Sedge War­bler.

Con­tin­u­ous fly­ing

You think I am jok­ing about it be­ing a ‘su­per-bird’? Well, take this in. The ev­i­dence sug­gests that, on au­tumn mi­gra­tion, many Sedge War­blers cover most of their jour­ney to trop­i­cal Africa in a sin­gle or a few spurts, some po­ten­tially en­com­pass­ing more than 1,800 miles of con­tin­u­ous fly­ing – and this from a bird that only weighs up to 13g. Fur­ther­more, it seems that this ex­tra­or­di­nary feat is fu­elled by a spe­cial su­per-food, an ul­tra­sug­ary invertebrate called the plum-reed aphid. Af­ter breed­ing in ter­ri­to­ries that lie to the edge of reedbeds, in drier, more di­verse veg­e­ta­tion, Sedge War­blers move in late sum­mer to large, pure reedbeds that abound with this par­tic­u­lar bug. For sev­eral weeks they eat these snacks, put on plenty of weight and then, on an early Septem­ber night, take off into the dark­ness to face their long jour­ney. What a thought this is. The in­domitable midget rises into the air, jet-pro­pelled by sugar all the way to warmer climes. In­deed, thou­sands of them are do­ing it at once, per­haps at the mo­ment you are read­ing this. No­body has ever seen it hap­pen, and per­haps no-one ever will. But while Reed War­blers have a slower mi­gra­tion strat­egy, fat­ten up at many stages as they move south at leisure, Sedge War­blers are fly­ing a marathon.

Putting on the style

Mi­grat­ing isn’t the only thing that the Sedge War­bler does in style. It also has a fan­tas­ti­cally zest­ful song, which fizzes along with great gusto and in­ten­sity, with un­pre­dictable changes of pace, un­usual riffs and bursts of mimicry. The phrases are longer than for most other birds (but not Reed War­blers), some­times belt­ing along for a minute or more, and when they do they can in­clude 300 or more syl­la­bles. The in­ven­tive­ness and con­tent could be com­pared to a speeded-up mono­logue by the late Robin Wil­liams (him­self, no doubt, a mem­o­rable party guest). Those that study these things have found that the Sedge War­bler’s is among the most elab­o­rate bird songs in the world. Com­pared to the Reed War­bler’s song, which starts and con­tin­ues at a pacey but con­stant rhythm, the Sedge War­bler’s seems to get faster and faster, so much that you half-ex­pect to see steam com­ing out of the bird’s mouth. The phrase cer­tainly be­comes more com­pli­cated as the bird goes along, and no song is ever quite the same. To add some ex­tra drama, a Sedge War­bler of­ten starts its song slightly hid­den in the veg­e­ta­tion, but then climbs up­wards un­til it is in full view – a real per­former. Some­times the ex­cite­ment boils over and it takes off on a very short

song-flight, up and down and ap­pear­ing un­steady, as if tipsy on its own ef­fer­ves­cence. The Sedge War­bler was one of the first species in which it was proven that a male’s reper­toire is cru­cial in at­tract­ing fe­males. Sci­en­tists played recorded songs to fe­males in cages and recorded their re­sponses, find­ing that, the more com­plex a male’s reper­toire, the greater the in­duced sex­ual dis­plays. In fact, a male’s reper­toire changes from year to year, with neigh­bour­ing males adding in sec­tions to match each other’s songs. The older males also have larger reper­toires than their younger ri­vals. The fre­quency of song-flights also sends a mes­sage, with well-per­form­ing males usu­ally pair­ing up ear­lier than their ri­vals. The fre­quency of the song-flights also equates to good health, in that slug­gish per­form­ers tend to have more par­a­sites on their feath­ers. This re­mark­able song sends mul­ti­ple vi­tal mes­sages about male qual­ity to the lis­ten­ing fe­males. Af­ter all the ef­fort that Sedge War­blers put into their song, you’d think that they might put their vo­cal­i­sa­tions into prac­tice, all sum­mer long. But here’s an­other, most un­usual quirk. The mo­ment a male Sedge War­bler pairs up, it goes quiet, un­like the Reed War­bler, which might grum­ble and rum­ble on singing for weeks. Cu­ri­ously, it seems that the Sedge War­bler’s song is purely ut­tered for sex­ual at­trac­tion, and once it’s wo­ven its magic, the male has no use for it. This is very un­usual, be­cause most small birds sing for ter­ri­to­rial rea­sons, as well. But ap­par­ently the Sedge War­bler uses vis­ual dis­plays for this pur­pose.

All-singing breed­ing strat­egy

Of course, the sit­u­a­tion is never quite as sim­ple as this, not in the life of the ‘Edgy Sedgie,’ any­way. This ir­re­press­ible bird is never go­ing to be sat­is­fied with a sim­ple boy-meets-girl-and-stops-singing sce­nario, liv­ing hap­pily ever af­ter. Nat­u­rally, the melt­ing-pot in the rank veg­e­ta­tion throws up sev­eral vari­a­tions. For one thing, a pro­por­tion of males (40%) are polyg­a­mous; these in­di­vid­u­als don’t en­tirely stop singing once they are paired, but in­stead take it up again when their fe­males have be­gun egg-lay­ing or in­cu­ba­tion and are per­haps dis­tracted! The males that do this are of­ten suc­cess­ful in breed­ing with a se­cond fe­male. In­ter­est­ingly, at the same time, a mi­nor­ity (fewer than 10%) of fe­male Sedge War­blers are also polyg­a­mous. Once they have raised one brood, con­ven­tion­ally feed­ing their young, along with the male, to in­de­pen­dence, they ev­i­dently then de­cide that their breed­ing sea­son isn’t done, as it would be for most in­di­vid­u­als. These fe­males seek out an en­tirely dif­fer­ent mate for the pur­pose of their se­cond at­tempt – but how do they find it when the males have stopped singing? It turns out that they lis­ten out for what might be un­char­i­ta­bly de­scribed as the dross, the males that are un­paired and still singing in the hope of their luck chang­ing. These males in­vari­ably have a smaller reper­toire than the fe­male’s orig­i­nal mate. The fe­males are con­tent with a po­ten­tially sub­stan­dard mate the se­cond time around, but the ar­range­ment would seem to suit ev­ery­one. That, I hope you will agree, is a pretty good charge-sheet of in­trigu­ing be­hav­iour. From its re­mark­able mi­gra­tion to its whole­hearted song to the ‘vari­a­tions’, shall we say, in its breed­ing ar­range­ments, the Sedge War­bler is never sim­ple and never dull. Imag­ine the sto­ries it would have to tell. So it’s bye bye, amaz­ing Sedge War­bler, see you next sum­mer. You will be the first on our party guest list.

The Sedge War­bler is one of only a few of our war­blers which have a song flight

Ju­ve­nile Sedge War­bler. Note the broad pale cen­tral crown area and the spot­ted up­per breast, plus the neat (un­worn) feath­ers

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