Tslew is another call type with a descending overall shape, commonly used both day and night. It differs fromtewmainly in that it sounds clearly disyllabic. The first syllable is around 5 kHz, and sonagrams show that it usually has a slightly ascending shape. The second syllable lies between 3-4.5 kHz and is much more variable. It can descend from the first syllable like atewcall, or it can be completely separated from it and arch-shaped. Very often it actually overlaps in time with the first syllable, indicating that each was made with a different syrinx (birds have two vocal organs, one for each lung). In the following recording of a bird migrating on an autumn morning there are severaltslewcalls among a rich medley of other call types. Below it is a sonagram and corresponding sound file showing six variations oftslewcalls recorded by day.

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v. Ortolan Bunting Emberiza hortulana, Sagres, Vila do Bispo, Portugal, 08:26, 17 September 2008 (Magnus Robb). A rich medley of calls, including tslew from a migrant passing during the day.

w. Ortolan Bunting Emberiza hortulana. Six variations of tslew calls recorded during the day. 1) Sagres, Vila do Bispo, Portugal, 08:26, 17 September 2008 (Magnus Robb) 2) Chokpak, South Kazakhstan, Kazakhstan, 2 May 2000 (Magnus Robb) 3) IJmuiden, Noord-Holland, Netherlands, 17 September 2003 (Magnus Robb) 4) Cabo Espichel, Setúbal, Portugal, 09:33, 3 September 2010 (Magnus Robb) 5) Sagres, Vila do Bispo, Portugal, 08:24, 17 September 2008 (Magnus Robb) 6) Berlenga, Leiria, Portugal, 11:19, 28 September 2011 (Magnus Robb)

At night there were tslew calls in 27 of our 141 recordings. When we record them they are often given in sequences. For example, there were six consecutive tslew calls in the following recording from Portland Bill; another from Poole Old Town also had six. Below this is a sonagram and corresponding sound file showing six variations of tslew calls recorded by night.

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x. Ortolan Bunting Emberiza hortulana, Portland Bill, Dorset, England, 22:55, 25 August 2016 (Nick Hopper). Several tslew calls of a nocturnal migrant on a night of intense Tree Pipit Anthus trivialis migration.

y. Ortolan Bunting Emberiza hortulana. Six variations of tslew calls recorded during the night. 1) Portland Bill, Dorset, England, 22:55, 25 August 2016 (Nick Hopper) 2) Cabriz, Sintra, Portugal, 02:02, 17 September 2012 (Magnus Robb) 3) Cabriz, Sintra, Portugal, 03:02, 26 September 2011 (Magnus Robb) 4) Poole Old Town, Dorset, England, 02:00, 24 August 2016 (Paul Morton) 5) Poole Old Town, Dorset, England, 02:46, 7 September 2016 (Paul Morton) 6) Cabriz, Sintra, Portugal, 05:49, 18 September 2015 (Magnus Robb)

Very few other species have calls similar to the tslew of an Ortolan Bunting. One that can sound a little similar at times is Eurasian Siskin Spinus spinus. We have only heard it once as a nocturnal migrant and that was less than 100% certain.

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z. Eurasian Siskin Spinus spinus, IJmuiden, Noord-Holland, Netherlands, 09:01, 18 November 2007 (Magnus Robb). Social calls of a solitary migrant passing on autumn migration. Siskin will usually mix other calls in its flight sequences, making misidentification as an Ortolan Bunting highly unlikely.

Tsrp is the last of the calls commonly heard both by day and night. This is perhaps the most difficult Ortolan Bunting call to identify. Tsrp calls are so varied in shape that future studies may show this ‘type’ to be several. What all have in common, however, is that they are relatively high-pitched, monosyllabic and delivered on what sounds to the human ear like a single pitch: their inflections, which we can see on sonagrams, are not obvious to the ear. The whole call fits within a fairly narrow frequency range, typically centred around 5 kHz. Most have a slight huskiness, reminiscent of a Dunnock, and this is what sets them apart from plik calls. Listen to an example of diurnal tsrp calls in flight. Below it is a sonagram and corresponding sound file showing six variations of tslew calls recorded by day.

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aa. Ortolan Bunting Emberiza hortulana, Sagres, Vila do Bispo, Portugal, 08:21, 17 September 2008 (Magnus Robb). A medley of calls including many tsrp from a migrant in flight during the day.

bb. Ortolan Bunting Emberiza hortulana. Six variations of tsrp calls recorded during the day. 1) & 2) & 4) Sagres, Vila do Bispo, Portugal, 08:21 & 08:24 & 08:26, 17 September 2008 (Magnus Robb) 3) Breskens, Zeeland, Netherlands, 08:02, 13 May 2007 (Magnus Robb) 5) Sagres, Vila do Bispo, Portugal, 10:23, 10 September 2010 (Magnus Robb)6)Vedi, Ararat, Armenia, 15:37, 14 May 2011 (Magnus Robb)

At night there were tsrp calls in 44 out of 141 recordings of passing migrants. Usually tsrp is given in the presence of other call types, and if plik is among them the mystery is soon solved. When plik is missing, it can be more difficult to identify. The following recording had MR stumped for a long time, and were it not for the tslew call also in this sequence, he might still be guessing. Below it is a sonagram and corresponding sound file showing six variations of tsrp calls recorded by night.

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cc. Ortolan Bunting Emberiza hortulana, Barão de São João, Lagos, Portugal, 21:14, 5 October 2009 (Magnus Robb). A series of tsrp calls with one tslew from a nocturnal migrant.

dd. Ortolan Bunting Emberiza hortulana. Six variations of tsrp calls recorded during the night. 1) Sagres, Vila do Bispo, Portugal, 21:14, 5 October 2009 (Magnus Robb) 2) Cabriz, Sintra, Portugal, 01:38, 13 September 2013 (Magnus Robb) 3) Poole Old Town, Dorset, England, 03:34, 30 August 2016 (Paul Morton) 4) Portland Bill, Dorset, England, 22:44, 5 September 2016 (Nick Hopper) 5) Cabriz, Sintra, Portugal, 05:38, 18 September 2015 (Magnus Robb) 6) Portland Bill, Dorset, England, 21:55, 25 August 2016 (Nick Hopper)

Possible confusion species for this call-type include Tree Pipit A trivialis, Dunnock and Yellowhammer. All are regular nocturnal migrants.

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ee. Tree Pipit Anthus trivialis, Shizzafon Kibbutz fields, Arava Valley, Israel, 08:00, 2 December 2001 (Killian Mullarney).Tip calls of a wintering individual. These are similar in pitch and duration to the tsrp of Ortolan Bunting, but more simple and pure-sounding.

ff. Dunnock Prunella modularis, De Cocksdorp, Texel, Netherlands, 06:25, 6 September 2016 (Magnus Robb). Calls of a migrant at dawn, at first perched then flying off. Individual calls longer than tsrp of Ortolan Bunting, and often doubled in a way that excludes Ortolan.

gg. YellowhammerEmberiza citrinella, IJmuiden, Noord-Holland, Netherlands, 31 October 2002 (Magnus Robb).Dzheucalls of a passing migrant. Much coarser (more heavily modulated) than most Ortolan Bunting calls, although one visually confimed Ortolan did give a single call that was almost identical.

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