Nocturnal flight calls of Black Redstart: an unexpected discovery

Two recent observations and a well-stocked archive of ‘UFOs’ led to a surprising discovery earlier this month. Black Redstart Phoenicurus ochruros has a nocturnal flight call that we were unfamiliar with until now, which so far appears to apply to Western Black Redstart P o gibraltariensis and Caucasian Black Redstarts P o ochrurus. This was completely unexpected and leaves us quite humbled. How many other common species have nocturnal flight calls that we are still unable to identify?

The key observation that led to this discovery was when Daniel López-Velasco watched a Black Redstart coming in off the sea at Cabo Peñas in Asturias, northern Spain on 7 November 2020, flying high and giving a call he had never heard before. This left him puzzled, but quite certain that the call had come from that bird. Fortunately, he recorded it, and sent it to Magnus Robb the next day, knowing Magnus would be interested to hear an unfamiliar call of a Black Redstart in active migration.

Hearing the recording, Magnus realised that he had in fact heard it before, and indeed it was quite fresh in his memory. On 5 November he had recorded an identical-sounding call while birding at Sagres, Vila de Bispo, Portugal. At the time he had been puzzled and slightly disappointed to find that this exotic-sounding call was ‘only’ coming from a Black Redstart. In this case the bird was perched on some rocks, not flying, so Magnus had not immediately made any connection with flight calls.

Now the grey matter clunked into gear and Magnus began to wonder if this unfamiliar call might actually prove to be a normal flight call of Black Redstart at night. The Sound Approach has a large archive of mystery calls or ‘UFOs’ so he started searching through it for anything similar. Since the ‘new’ call somewhat resembles the alarm call of a Common Starling Sturnus vulgaris, he also checked for nocturnal flight calls previously filed as starlings and any other species he could think of that were remotely similar. Sure enough, within a short time he found three convincing examples, given by migrants passing at night. Without further ado, here is the clearest one, recorded in Azerbaijan on 9 November 2018. So far, all examples have been recorded between 22 October and 9 November, fitting neatly with the peak migration period for this species (eg, de Juana & Garcia 2015).

Daniel reported our joint conclusion to a large nocmig discussion group via WhatsApp, and within a week three ‘nocmiggers’ in Catalunya – Oriol Baena Crespo, Lluis Brotons and Seán Ronayne – came forward with examples of Black Redstart NFCs. At the time of writing the total currently stands at six examples of this call in the context of nocturnal migration and no doubt, now that we are onto it, many others will follow.

A few days later Lukas Pelikan also contributed another diurnal recording of the same call-type. While visiting Brocken in the Harz National Park, Germany, in June 2017, he had recorded two calls of a flying migrant, one of several migrants at the time but the only one that gave this call. Brocken is one of the best stopover sites for Black Redstart in northern Germany, with up to 100 there at times. The first post-breeding migrants, which are probably dispersing young birds from the lowlands, appear as early as June (Nicolai & Hellmann 2008).

Here follows a provisional description in order to share how we are currently identifying this call-type. The sample size is of course still tiny, and the description draws on both confirmed diurnal and unconfirmed nocturnal examples, taking measurements from the clearest call in each recording. It is important to note that 1) all nocturnal examples should be considered provisionally identified and 2) the average values will shift as the sample grows and any misidentified calls are removed.

Provisional criteria for identification

• short, rising, slightly husky bit
• usually a single-note call, but one recording also contains a rapidly doubled call (f)
• duration 40 – 70 ms (median 50 ms; n = 8)
• at least two frequency bands normally visible; these are not harmonically related to one another; additional bands may appear that are harmonically related to the two main ones, ie, the same form but at a multiple of the frequency.
• upper band starts with a ‘foreleg’ followed by a less steeply sloping part that is either straight (f, h), slightly convex (a, c, g) or arched, usually with the highest point towards the end (b, e); total frequency range 3.2 – 6.7 kHz; mean peak frequency 6 kHz (±0.3 kHz; n = 8)
• lower band starts with a ‘foreleg’ followed by a slope that differs in shape from that of the upper band; it can be anything from a concave U-shape (a, f) to an arch with the highest point towards the end (h); total frequency range 1.7 – 5.8 kHz; mean peak frequency 4.3 kHz (±0.5 kHz; n = 8)

Other calls of Black Redstart at night

Black Redstart is an early riser, and it is not unusual to hear two better-known call-types of this species before civil dawn. At times, a bird giving tek calls in particular may seem to move over a longer distance, as in the recording below. At other times it is a short rising whistle that one hears in the dark. It is not clear whether passing migrants can give these calls, or only birds that are already in situ, either residents, resting migrants or wintering individuals. We recommend that for the meantime, in nocmig counts only birds giving the ‘new’ bit call should be counted as passing migrants. Time will tell whether Black Redstarts in fact use more than one type of flight call at night, for example bit and tek in combination. So far we have no evidence of this.

Black Redstart now joins a very small group of European chats and flycatchers Muscicapidae for which nocturnal flight calls are known: Spotted Flycatcher Muscicapa striata, European Robin Erithacus rubecula, Bluethroat Luscinia svecica, Collared Flycatcher Ficedula albicollis and European Pied Flycatcher F hypoleuca. Other Ficedula and Muscicapa species presumably have flight calls that we still need to clarify, and indeed there are already some strong hints. Now this discovery leaves us with several burning questions to solve. The hottest one is this: does Common Redstart P phoenicurus also have a nocturnal flight call and if so, what have we been mistaking it for?

Acknowledgments:

Many thanks to Lluis Brotons, Oriol Baena Crespo, Lukas Pelikan and Seán Ronayne for sharing their recordings with us. Thanks also to Lukas Pelikan and Mark Pearson for commenting on a draft of this article, and to Geoffrey Morrison and the Besh Barmag team, especially Micha Heiss, for helping to collect recordings from Sagres, Portugal and Besh Barmag, Azerbaijan respectively.

Literature:

de Juana, E & Garcia, E 2015. The birds of the Iberian peninsula. London.

Nicolai, B & Hellmann, M 2008. Der Hausrotschwanz Phoenicurus ochruros im Harz und sein Vorkommen auf der Brockenkuppe. Vogelkundliche Berichte aus Niedersachsen 40: 269-275.