Redwing

The icy zeeer of Redwing Turdus iliacus is among the commonest nocturnal flight calls that almost every avid European birdwatcher may have noticed at night. Together with calls of Song Thrush T philomelos and other thrushes it can dominate the auditory soundscape, especially on October ‘thrush nights.’  The first thrushes appearing on September nights often herald a change of migration gear, from engaging to spectacular.

Redwing’s nocturnal flight call, here rendered zeeer, is virtually the same one they use in daytime flight (Bulyuk et al 2017). The rapidly modulated nature of this call highlights a limitation of sonagrams: depending on the ‘window size’ setting it will appear either as fine vertical zigzag lines or as several long horizontal lines sandwiched above each other. The former emphasizes visually the duration and modulation of the call, the latter chiefly the frequencies involved. We need to take this into account when measuring calls (see also Introduction). The sonagrams for this page have been made with a window size of 270.

There is a phenomenon in zeeer calls whereby every so often a secondary modulation is superimposed on the already finely modulated call, making the call appear strongly rippled and giving a more ringing timbre. Some variants may in fact partially lack typical fine modulations (eg, p). As such, they can sound similar to the srrri NFC of Common Blackbird T merula or the flight calls of Bohemian Waxwing Bombycilla garrulus.

Other more rarely heard calls at night include a thrush-typical gak, which can be combined into a gak-zeeer. This is arguably a call with a social function as it is common when several Redwings can be heard flying together. You may also hear rattling alarm calls, especially in nights with strong Redwing migration, and probably as a reaction to the presence of many congeners or a predator.

zeeer calls

main features

• high-pitched, with buzzing timbre, long and gently descending
• fine modulation throughout the call gives its buzzing timbre
• duration 214 – 404 ms (90% range; median 302 ms; n = 134)
• ‘fully dressed’, the call may show an upward-pointing ‘foreleg’ and a ‘hindleg’, some calls may be missing those or will show a regular foreleg (g)
• mean max frequency 8.1 kHz (±0.46 kHz; n = 134), across all types and variants (measured at the highest point of a modulation at window size 270; see also Introduction)
• when inverted ‘foreleg’ present, mean max frequency 8.4 kHz (±0.45 kHz; n = 13)
• mean min frequency 6.1 kHz (±0.40 kHz; n = 134), across all types and variants
• when ‘hindleg’ present, frequency curve can go down quite a bit: mean min frequency 5.5 kHz (±0.52 kHz; n = 9)
• modulation rate 155 – 198 Hz (90% range; median 174 Hz; or 5 – 6 ms for one modulation; n = 108)
• some calls have a hoarse undertone and sound ‘distorted’ (e)
• slope (Hz/s) of the frequency curve is typically negative (ie, descending) in a direct line or sometimes slightly concave; in a set of 134 calls, only 6 had a negative slope almost approaching zero (appeared straight in sonagram) and 2 had a just about positive slope (very slightly ascending)

secondary modulation

• may be superimposed on the regular zeeer call to any degree, eg, only one or two undulations at the start of the call, slight undulations throughout the call or a heavy up-and-down from start to end
• secondary modulation rate 15 – 22 Hz (90% range; median 19 Hz; or 46 – 62 ms for one modulation; median 53 ms; n = 34)
• presence of primary modulation nestled within the secondary modulation varies, from clearly present to hardly discernible
• there are all possible transition forms, from a regular zeeer to a strongly rippled, Common Blackbird-like call with secondary modulation
• timbre somehow watery or liquid due to the remaining primary modulations; compared to clear, fine and silvery srrri of Blackbird
• some appear loose in shape, ie, modulation rate varies within a call (i)
• secondary modulation occurs on only a minority of NFCs, roughly on fewer than 25% of zeeer calls, but note that Redwings commonly appear to answer calls with secondary modulation having matching calls (domino effect)

Effects of recording quality

• Good quality recordings may show inverted ‘forelegs’ and ‘hindlegs’ that go much further up or down. This means measured minimum and maximum frequency also depend on recording quality.
• The buzzing fine modulations may be blurred and not audible in poorly recorded calls. Especially in calls with secondary modulations it means that these can apparently lack any fine (primary) modulations and sound even more similar to Blackbird NFCs.

gak calls

• slightly nasal and short gak with a timbre typical for many thrush species
• shape in sonagram a smooth convex arch with two bands and at closer range, harmonics of these
• two bands may differ subtly in shape
• max frequency of lower band 2.2 kHz (±0.09 kHz, n = 17)
• max frequency of upper band roughly 1.5 times higher, 3.4 kHz (±0.18 kHz; n = 13)
• only used one at a time, or in a combination to gak-zeeer (s), or combined with rattling alarm calls (u); combinations typically in very close succession
• duration 42 – 66 ms (90% range; median 53 ms; n = 7)

Effects of recording quality

• Good quality recordings of a gak call are rare and will show more harmonics than the prominent two.
• Most gak calls are very faint and hardly visible in sonagrams. Since either the lower or the upper arch can be louder, and therefore one of the two can be invisible in sonagrams, it is easy to misinterpret which arch you are looking at. Some gak calls may be even audible with good headphones but not visible in sonagram.

alarm calls (trt-trt-trt-trt…)

• impulses of usually two, sometimes more, short broadband tk elements fixed tightly together to make a rattling ‘trt-trt-trt’ sound
• the two tk elements always closer together than succeeding pairs of tk elements
• gap between tk elements varies, roughly 22 ms long
• single notes have a rapidly descending slope and look lightning-like (a mirrored letter ‘N’) in sonagram
• main energy roughly in 2 – 6 kHz range
• this is the same alarm calls Redwings use during daytime at the nest etc. – in night-flight probably used in conspecific interactions

zeeer calls

• Common Blackbird Turdus merula is a confusion risk when a Redwing NFC shows prominent secondary modulations. However, 1) Blackbird NFCs never show more than a tiny hint of any fine rapid modulation that is so typical for Redwing. But note, in faint recordings of Redwing these can be almost invisible. 2) Redwing shows a single frequency band rather than well-separated sidebands, assuming that you use a ‘window size’ where you can see Redwing’s fine primary modulation clearly. 3) Blackbirds often have double-calls (srrri-srri); in Redwings this is extremely rare. 4) Blackbirds have a slightly but audibly higher modulation rate compared to Redwing’s secondary modulations. For this reason, Redwings with secondary modulations typically sound ‘looser’ than normal Blackbirds.
• Bohemian Waxwing Bombycilla garrulus also migrate at night but are rarely if ever recorded. So far, we have not recorded their calls at night. Their calls sound similar to Redwings with secondary modulations but have a clearly different shape in sonagrams. Waxwing calls are composed of several separated elements combined to a trill, each element resembling a letter ‘n’ with a long foreleg and hindleg, without a connection to the next element. Redwings always show a connected undulating frequency line. Waxwing calls are also slightly lower pitched and do not descend in frequency.
• Tree Pipit Anthus trivialis has a spihz NFC that can sound similar to a very short Redwing zeeer NFC. But Redwing calls are notably higher pitched, their fine modulation rate is slightly higher and even the shortest Redwing NFCs should be longer in duration than any Tree Pipits NFC. Tree Pipit NFCs typically show a prominent foreleg and a hindleg, which we have not observed in the very short variants of Redwings.

gak calls

• Fieldfare Turdus pilaris has vaguely similar calls at night, consisting of convex arches with several harmonics. They are, however, higher pitched, often given in pairs and the single notes are not so short and abrupt as Redwing’s gak calls are.

• anywhere: over towns, forests, mountains, offshore
• nocturnal and diurnal: uses same flight calls day and night
• can be expected at any hour: a bias of activity towards second half of night

Although the ‘normal’ NFC of Redwing is fairly distinct among European thrushes and rarely gives reason for confusion there are thrush species in the Eastern Palaearctic that share highly similar NFCs. However, we consider an aberrant Redwing NFC among hundreds of calls far more likely than a Siberian vagrant, and special care should be taken if a vagrant is suspected. Among the eastern thrushes Eyebrowed Thrush T obscurus, Grey-backed Thrush T hortulorum and Japanese Thrush T cardis share similarly high-pitched and buzzing NFCs. They all differ slightly in the inflection of the call being rather straight or, in the case of Eyebrowed Thrush, slightly ascending.

Bulyuk, V N, Bolshakov, C V & Evstigneeva, M 2017. Do flight-calls of Redwings differ during nocturnal and diurnal migration and daytime stopovers? Ornis Fennica 94: 172-179.