Little Bittern

Little Bittern Ixobrychus minutus is arguably one of the most underrated nocturnal migrants. Until fairly recently, it was simply not on the radar of birders living anywhere away from key habitats. Now we know it is possible to hear Little Bittern’s typical ker at night almost anywhere. In fact, we have recorded Little Bitterns over habitats as diverse as vast spruce forests on remote mountain ranges, sandy deserts with no oases in sight, remote offshore islands and over the centre of large cities. So despite the fact that they are usually water-bound, Little Bitterns appear to be much less picky about habitat when it comes to migration.

The dominant type of NFC varies a lot in shape but not in pitch. Migrating flocks in particular sometimes use a different variant as a short-distance contact call, probably to maintain contact with the other birds in the flock (examples i, j & k).

Little Bitterns usually migrate singly or in small flocks. If you want to maximize your chances to get them, the last ten days of August turned out to be the best time for them in Central Europe.

ker calls

• highly variable in shape, duration and degree of hoarseness
• nasal timbre
• high-pitched; has highest pitch of all heron and bittern NFCs in Europe
• falling pitch; sonagrams show descending frequency over more than half of the call
• mean peak frequency 2.2 kHz (±0.14 kHz; n = 76)
• frequency range 1.3 – 2.8 kHz (mean min 1.8 kHz; mean max 2.4 kHz; n = 76)
• second band above main band typically incomplete, looking like a broken bridge in sonagrams; this is a diagnostic feature (d, f & l)
• duration 101 – 175 ms (90% range; median 138 ms; n = 76); many ker calls on breeding grounds are much shorter than this; the given range refers to ker calls on prolonged migration flights only
• calls used for short-distance contact (i, j & k) are usually quieter and typically show frequency bands in a much denser array than normal, louder calls
• mnemonic: it rhymes with the colloquial ‘eh!’ (imagine it spoken by a dwarf)

Effects of recording quality

• Distant and quiet calls may lose any additional frequency bands above and below the peak frequency band in the sonagram and croaky versions in particular will lack the cloudy surroundings (or fuzzy smudge usually blurring the bands), even when sounding slightly croaky through headphones.
• In close calls, additional faint frequency bands will appear above and below the peak frequency band (cf example a, b, c & d).

• Striated Heron Butorides striata has a similar NFC, kyur, which is similarly high-pitched. As a difference, it sounds more hoarse (more blurred in sonagrams) and more importantly it descends in frequency more rapidly and much further down (it sounds lightning-like!). In sonagrams it often looks like a backslash, ie, descending frequency over almost entire length of the call.
• Purple Heron Ardea purpurea is only likely to be a problem in comparison to short-distance contact calls of Little Bittern (eg, k). However, Purple sounds even drier and only shows a diffuse cloud of a call with no clear peak frequency band.
• Little Egret  Egretta garzetta has a similarly croaky NFC but is lower-pitched, sounds like argh and typically shows a fine zig-zag line on the frequency bands.
• Black-crowned Night Heron Nycticorax nycticorax has a NFC that is remotely similar. The first thing to look for is peak frequency. Little Bitterns are distinctly higher-pitched whereas Night Herons have a mean peak frequency of 1.6 kHz. Apart from that, Night Heron’s NFC qwar often sounds questioning due to a general rise in pitch, whereas Little Bitterns with their slightly descending ker resemble a miniature, or high-pitched, Eurasian Bittern Botaurus stellaris.

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

At current knowledge we cannot safely separate NFCs of Little Bittern from any other Ixobrychus species. We already have recorded NFCs of Von Schrenck’s Bittern I eurhythmus and possibly Yellow Bittern I sinensis of the Far East (the latter also occurring as close as the Red Sea coast) but are not able to tell them apart as yet. Please consider this when recording ker calls at night in the Middle East or parts of Asia where other species of the genus Ixobrychus may occur. However, within Europe Little Bittern is the only likely candidate.