Sedge Warbler

This article was first published in the September 2018 issue of Bird Watching magazine…

Words: Dominic Couzens

 Damian Waters/Alamy

Damian Waters/Alamy

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

 imageBROKER/Alamy

imageBROKER/Alamy

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 ultra-sugary 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

 David tipling/Alamy

David tipling/Alamy

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.

Sedge Warbler factfile

Scientific name: Acrocephalus schoenobaenus
Length: 11.5-13cm
Wingspan: 17-21cm
UK numbers: 260,000 breeding territories
Habitat: Reedbed, damp wetland
Diet: Insects, autumn berries

Where to see Sedge Warbler

Found across most of the UK, look for Sedge Warblers in reedbeds or damp wetland areas, particularly around dawn or dusk. Singing birds could be seen on the outside of a bush.