HIDDEN SOUNDS

The species guides

Each of the species we have written up in this guide follows the same format, so that you can read, compare and understand the nocturnal sounds you might here. We explain this format below, so that you can get the most out of your nocmig experience.

Key species

Herons and bitterns

Waders

Thrushes

Chats and old world flycatchers

Wagtails and pipits

Buntings

Reading the species guides

The identification section provides relevant features for the identification of the species’ NFC. First, it presents mean values for measurements of duration and frequency. The standard deviation and sample size are given in parentheses. Sample size gives number of both individuals and calls, which are the same: from any given recording, we have chosen one call, the clearest one available for measurement.

Next is a block of typically 8-12 sonagrams of typical NFCs of the relevant species, covering the variation of the call. The number of NFCs shown will depend on the degree of variation in the call. Note that sonagrams in this section will be at a different scale from the one at the top of the page (see the scale bar at the bottom left). Occasionally, for particularly low-pitched calls, we may show a different frequency scale from 0 – 10 kHz. Each single call in these sonagrams has a letter above it. The letter refers to the longer recording and its caption below, which gives information about location, date, and source. The letters also appear in the descriptions at the start of the identification section, where they indicate typical examples of a feature being described. The call shown is usually the first and loudest in the recording. When this is not so, we tell you at what point the call occurs.

Calls of birds at either very close or distant range may appear slightly or strongly different to those presented. In addition, the acoustics of your location (e.g., presence of walls or cliffs causing echoes) and the quality of your recording equipment will have consequences for how the sound changes. Under Effects of recording quality, we highlight those aspects of the call that may be subject to variation.

Similar NFCs

Many times, NFCs of different species can be confused, e.g., Common Moorhen Gallinula chloropus and Bar-tailed Godwit Limosa lapponica, Redwing Turdus iliacus and Common Blackbird T merula, or Spotted Flycatcher Muscicapa striata and European Robin Erithacus rubecula. It may take some time to work out the correct species, so we have included some common confusions (from experience). 

Where and when?

The three points refer to:

Where? Migration is thought to happen mostly over a broad front. However, at certain points migration can be canalised along rivers, coastlines and peninsulas or deviated by mountain ranges, large lakes or oceans. At night, these effects can be different from daytime, and species-dependent. For example, waterbirds are more likely to be recorded over or near water bodies. We will note if we have recorded a species over all kinds of habitats or mainly over certain habitats.

How nocturnal? Some species use more or less the same flight-call during nocturnal migration as during their daytime flights, while others have a different flight-call at night.

What time of night? Some species rarely appear except during the last couple of hours before dawn (e.g., Dunnock Prunella modularis). Others like Common Scoter Melanita nigra may peak at a particular hour at your location, depending on how long it takes to arrive from the coast, or some other migration stopover.

Phenology graphs

The phenology graphs are based purely on nocmig data and are intended to give an idea of the seasonal occurrence of nocturnal flight calls in two different areas within the Western Palearctic. As there are birds that travel mainly via an eastern route to their wintering grounds in Africa, and others that use a more westerly route, the phenology can differ markedly from place to place. For this guide we decided to use central Europe and south-western
Europe. Phenology for countries like France or the UK is likely to be intermediate between central Europe and south-western Europe. The data behind the graphs was mainly obtained from Germany (especially Potsdam, Brandenburg) for central Europe and from Portugal (especially Cabriz, Sintra and Sagres, Vila do Bispo) for south-western Europe. We also consulted additional data from other locations.

The graphs show maximum estimates of individuals per night for ten-day periods in three steps, which can be read as a low, medium or high level of nocturnal flight call activity.

As nocmig is still in its infancy, the phenology data is only provisional and likely to be adjusted in the future.

Note of caution

For NFCs, it is important to be realistic about how much we still don’t know. This section draws attention to challenges yet to be tackled. These often concern less well-known species we may need to exclude from our identifications, geographic variation still to be studied in more detail, or matters concerning the flight behaviour of certain species at night.

Further reading

We list publications referred to or which are a useful reference for the species.

Defining ‘night’ and Nocturnal Flight Calls

We define night as the period between civil dusk and civil dawn, when the sun is 6° or more below the horizon. This is a fairly generous definition of night, and for a short period at the start and end of it some species will be using their daytime call repertoire. However, virtually all observation of migration during this period will have to be done by ear. Any examples recorded outside this period will be marked clearly as such, even if they miss it only by a minute or two.

The term nocturnal flight call or NFC is, for the purposes of this guide, mainly used for calls given during migration flights. We also include calls in flights between roosting and feeding areas (e.g., Jack Snipe Lymnocryptes minimus), mainly because these are indistinguishable from migration calls. We include vocalisations of some species that may form part of nocturnal song flights, i.e., prospecting for breeding partners and/or unoccupied territories, where there is evidence that these may also be used during migration flight. Examples include Common Quail Coturnix coturnix, Little Grebe Tachybaptus ruficollis, Common Moorhen Gallinula chloropus and Great Spotted Cuckoo Clamator glandarius. Occasionally, even sandpipers Scolopacidae may give snatches of song while migrating, and where we have examples that were clearly nowhere near breeding habitat we may include them. We will not include nocturnal songs that are normally only given on breeding territories, such as those of Eurasian Woodcock Scolopax rusticola or larks Alaudidae. Note that for some species, wingbeats can be more useful than calls for identification (e.g., Mute Swan Cygnus olor or Common Goldeneye Bucephala clangula) and we will also mention them.

Bioacoustic terms

Pitch and frequency are not synonyms, although they are related. Frequency is an objective measurement of sound waves per second, whereas pitch is a subjective sensation of how ‘high’ or ‘low’ a sound may be. Pitch is limited by our human abilities, and we are often unable to resolve details of frequency change at fine temporal scales. In general, our impression of the pitch of a sound is based on the features with the longest duration or the loudest component. So, for example, we may hear the wit of a European Pied Flycatcher Ficedula hypoleuca as simply rising in pitch, whereas at the end of the call the frequency actually often descends to a point nearly as low as the starting point, doing so in such a short time that we cannot hear this.

You can find out more about reading sonagrams here.

Reading sonagrams

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