Eurasian Bittern

When it comes to species as elusive and secretive as Eurasian Bittern Botaurus stellaris, spending most of their time well hidden in dense reed beds, nocmig really comes into its own. This is a great way for you to enhance the understanding of this species’ movements, and perhaps even get it on your ‘garden list’.

During nocturnal flights Eurasian Bitterns are fairly vocal, although the pace of calling can be quite slow. You may only hear three calls in two minutes as the bird passes by.

As soon as you hear one at night it will grab your attention, especially when calling over a location as unexpected as the illuminated centre of a megacity. Being one of the largest species encountered through nocmig, it also has one of the deepest NFCs. As a result, distant calls can be obscured by traffic noise, or dismissed as some other low-pitched sound of the night, including those of humans or dogs (the latter sometimes being extremely similar to Eurasian Bittern’s graow).

graow

• very low-pitched
• mean peak frequency 1.1 kHz (±0.1 kHz; n = 55)
• frequency range 0.6 – 1.4 kHz (mean min 0.9 kHz; mean max 1.2 kHz; n = 46)
• shallow smooth arch; major part of the call descending in frequency, thus sounding downwards overall
• duration 171 – 288 ms (90% range; median = 227 ms; n = 46); ie, roughly twice as long as Little Bittern Ixobrychus minutus
• usually only one frequency band visible; closer calls show more frequency bands above and below (a & b)
• a few have additional incomplete second frequency band above (ie, only beginning and end of arch visible), which has equal power to main band (e)
• mnemonic: it rhymes with ‘cow’

Effects of recording quality

• Closer calls show several frequency bands above and below the peak frequency band. Closer NFCs can also have a croaky or rasping timbre to them, which is barely audible in distant calls.
• Distant calls show only the peak frequency band and may appear shorter in duration. They show only a smooth shallow arch in sonagrams.

• Black-crowned Night Heron Nycticorax nycticorax NFC qwar is usually higher pitched and ascends in pitch overall (Eurasian Bittern descends). Sonagrams show a prominent rising part and often one or two kinks in the frequency band.
• Grey Heron Ardea cinerea is more hoarse and strident, showing a broad block of frequencies in sonagrams. However, faint calls may show only the power carrying frequency band, which is more similar to Eurasian Bittern’s graow in shape. In Grey Heron it is much higher pitched, still retains some of its shrill timbre even at a distance, and every so often has additional frequency bands above and below with similar power, also visible in faint recordings.
• Great Black-backed Gull Larus marinus and other large gulls have a call they sometimes use in flight, which sounds similar to Eurasian Bittern’s graow as it incorporates the same diphthong aʊ. It is sometimes even lower pitched (eg, at 700 Hz) and more notably shows very dense harmonics of more even power, ie, several frequency bands close to each other with many having similar power to the main band (Eurasian Bittern usually shows only one powerful frequency band).

• anywhere: over towns, forests, mountains, offshore; but more likely near rivers or lakes
• mainly nocturnal: but may start or finish migration in twilight
• typically in the middle of the night: but recorded any time between dusk and dawn