To hear a mockingbird

Subtle transitions between adjacent song phrases

northern mockingbird composes its song carefully

The northern mockingbird has an extremely lengthy, variegated and complex song. Tina Roeske and colleagues explain why we like this song so much.

For minutes on end, the northern mockingbird, Mimus polyglottos, can sing its song. The song is well known in North America, where the songbird is common in gardens and parks. It makes phrases in which it repeats a syllable (consisting of one or a few sounds) several times, like the European song thrush does, and strings the phrases up to a varied and complex whole.

A beautiful musical whole, in our opinion. Tina Roeske and colleagues know why we enjoy it: the bird orders its phrases carefully.

Car alarm

A singing mockingbird has a repertoire of a few hundred phrase types. These include its own tunes, but it also mimics calls and songs of many other birds. In addition, it mimics sounds of other animals and it can even imitate unnatural sounds, such as a car alarm.

And yet, its singing is not a hodgepodge. The researchers – a neuroscientist, a biologist, and a music philosopher – show that adjacent phrases usually are selected in such a way that they have similar acoustic properties; they sound like repeats that are a bit transformed or morphed. The researchers distinguish four modes of morphing that we can clearly perceive.

Coherence

Two adjacent phrases often have the same pitch and rhythm but differ in timbre (tone quality). For example, the first phrase has clear tones, the next one has some noise in it.

In other cases, a phrase is a repeat of the previous one, but at a different pitch. Or it has a faster or slower pace. Sometimes a combination of morphing can be heard, for instance a phrase that is faster and higher in pitch than its predecessor. And sometimes, a contrasting element is inserted between two similar phrases.

Because successive phrases are usually acoustically related, transitions are subtle, making the song coherent. Composers use the same strategies to add variety to their music. They use instruments with different timbres (for instance flute and violin), repeat a motif at a different pitch (the well-known beginning of Beethoven’s Fifth Symphony) or they change the tempo.

Both mockingbird males and females sing. But males sing more often and more exuberantly; they do it to enchant females. Undoubtedly, they are successful, but exactly what a mockingbird female appreciates in a male’s singing is not known.

Willy van Strien

Photo: AidenD (Wikimedia Commons, Creative Commons CC BY-SA 4.0)

Hear a mockingbird singing and learn how the researchers analyse the song

Another musician bird: pied butcherbird

Source:
Roeske, T.C., D. Rothenberg & D.E. Gammon, 2021. Mockingbird morphing music: structured transitions in a complex bird song. Frontiers in Psychology, online May 4. Doi: 10.3389/fpsyg.2021.630115

Backlit messages

Humboldt squid is like an e-reader

Humboldt squid creates colour patterns with backlight

It is difficult to communicate visually in the dark of the ocean, but Humboldt squid can do it, as Benjamin Burford and Bruce Robison show. It makes pigmentation patterns on its body visible by switching on backlight.

Humboldt squid or jumbo squid, Dosidicus gigas, is a social animal: individuals form groups to hunt prey, for instance lantern fish. Joint hunt requires good coordination, so that the whole group will swim in the same direction and decelerate synchronously to catch prey. And the squid manages to do so, without the animals bumping against each other or attacking each other, Ben Burford and Bruce Robison observed. Apparently, the animals communicate effectively.

Dark

This is remarkable, as the squid mainly lives in the dark. It spends the day hundreds of meters below surface and ascends to the surface only at night. So how do the animals communicate, the researchers wondered.

Squid species that live in light conditions are known to exchange messages with colour patterns on their body. Their skin contains chromatophores, small elastic bags filled with pigment that can be expanded. It was already known that Humboldt squid has chromatophores in one colour, reddish-brown. This enables it to display white-red patterns. But how can the animals show these patterns to each other in the dark?

By turning on backlight, as it turns out.

Glowing body

Burford and Robison studied the behaviour of the animals by filming during daytime at great depth with a camera mounted on a remotely operated vehicle and analyzing the footage.

In addition to chromatophores, Humboldt squid has so-called light organs, with cells that can produce light; this is called bioluminescence. Many deep-sea inhabitants have light organs in the skin, usually located at certain places, and convey messages by changing light intensity. For example, they show a pattern of spots indicating what species they are, they give a light show when courting, they flash to scare off an enemy or they lure prey with a lantern.

Humboldt squid uses bioluminescence in a different way. Its light organs are not embedded in, but located underneath the skin. And they are not located in certain places, but spread all over the body. By making its entire body glow yellow-green, Burford and Robison assume, Humboldt squid creates a backlight that reveals the white-and-red pattern of the chromatophores in the skin. It functions like an e-reader.

Deciphering the Humboldt squid

The squid has a whole repertoire of pigmentation patterns, as was already known. It can flash and flicker. It can make its caudal fins contrast with mantle, head and arms, or make the edge of the fins stand out; it can show stripes along the side of the mantle or on the arms, or create a stain between the eyes. Certain patterns are displayed only when the squid is hunting in a group, and some patterns appear in a fixed order. So, the system seems to enable complex, advanced communication.

The next challenge is to decipher that language. The camera used was not light-sensitive enough to read the patterns in detail, and it is still unknown how the animals respond to each other’s messages.

Willy van Strien

Photo: A Humboldt squid shows its colours in the lights of a remotely operated vehicle 300 meters below the surface of Monterey Bay. ©2010 MBARI

Researchers telling about their work on YouTube

Learn also about Humboldt squid mating behaviour

Sources:
Burford, B.P. & B.H. Robison, 2020. Bioluminescent backlighting illuminates the complex visual signals of a social squid in the deep sea. Proceedings of the National Academy of Sciences 117: 8524-8531. Doi: 10.1073/pnas.1920875117
Trueblood, L.A., S. Zylinski, B.H. Robison & B.A. Seibel, 2015. An ethogram of the Humboldt squid Dosidicus gigas Orbigny (1835) as observed from remotely operated vehicles. Behaviour 152: 1911-1932. Doi: 10.1163/1568539X-00003324
Rosen, H., W. Gilly, L. Bell, K. Abernathy & G. Marshall, 2015. Chromogenic behaviors of the Humboldt squid (Dosidicus gigas) studied in situ with an animal-borne video package. The Journal of Experimental Biology 218: 265-275. Doi:10.1242/jeb.114157
Benoit-Bird, K.J. & W.F. Gilly, 2012. Coordinated nocturnal behavior of foraging jumbo squid Dosidicus gigas. Marine Ecology Progress Series 455: 211-228. Doi: 10.3354/meps09664

Joint forces against brood parasite

When yellow warbler is warning, red-winged blackbird will attack

Red-winged blackbird eavesdrops on yellow warbler's alarm call

The yellow warbler utters a specific alarm call when a brood parasite is nearby. The red-winged blackbird picks up the signal and attacks, as Shelby Lawson and colleagues write. Together, the birds protect their nests.

Brown-headed cowird parasites on nests of songbirdsA bird’s nest with eggs or young is vulnerable. One of the dangers is that a heterospecific bird will lay an egg in it and charge the parents with the care of a foster young, like the cuckoo does. The red-winged blackbird, which breeds in wet areas in North and Central America, runs such risk. Here the brown-headed cowbird is the ‘cuckoo’, the brood parasite.

Although a young cowbird, unlike a cuckoo chick, does not eject its foster brothers and sisters out of the nest, its presence is to their detriment. The foreign chick demands so much attention that the legitimate young will suffer and starve or fledge in a bad condition.

So, the red-winged blackbird must keep the cowbird out of its nest. It takes advantage of the vigilance of the yellow warbler, another passerine bird that is visited by the cowbird, Shelby Lawson and colleagues show. The yellow warbler, in turn, takes advantage of the aggression of the redwing.

Defense

Yellow warbler utters specific alarm call when brood parasite is presentWhen yellow warblers detect a brown-headed cowbird, they utter a specific alarm signal, a ‘seet’ call. Upon hearing that call, all females respond appropriately: they immediately return to their nest (if they were not already there), repeat the seet and sit tightly on their clutch. As a consequence, a cowbird has no access.

Yellow warblers utter the seet call only in response to the brood parasite and only during the breeding period. To warn of predators, they have a different signal, and upon hearing that call, females will change perches and remain alert, but they won’t return to the nest. The combination of the specific alarm signal for brood parasites and the appropriate response of females is unique.

The researchers wondered whether red-winged blackbirds eavesdrop on that specific signal and take advantage of it. They play backed different sounds nearby redwings’ nests and observed their responses.

Both redwing males and females became aggressive upon hearing the seet of yellow warblers and attacked the speaker. They reacted as heated as in response to the chatter of brown-headed cowbirds. Also the call of a blue jay, a nest predator, aroused their aggression. Apparently, the response to the seet call is a general defence against various dangers that threaten a nest. The birds neglected the song of an innocent songbird.

Chatter of other redwings elicited the strongest defence response; the birds seem to consider conspecifics that invade their territory to be the greatest risk.

Together

The yellow warblers’ signal to warn of brood parasites is picked up by red-winged blackbirds, which respond by approaching the danger. This is to the benefit of yellow warblers: previous research had shown that their nests suffer less from parasitism by cowbirds if they breed in the neighbourhood of red-winged blackbirds. Redwings and yellow warblers often nest in loose aggregations; together they are able to resist the brood parasite.

So far, the red-winged blackbird appears to be the only bird species that understands and responds to yellow warblers’ warning of brood parasites.

Willy van Strien

Photos:
Large: Red-winged blackbird. Brian Gratwicke. (Wikimedia Commons, Creative Commons CC BY 2.0)
Small, upper: Female brown-headed cowbird. Ryan Hodnett (Wikimedia Commons, Creative Commons CC BY-SA 4.0)
Small, lower: Male yellow warbler. Mykola Swarnyk (Wikimedia Commons, Creative Commons CC BY-SA 3.0)
https://commons.wikimedia.org/wiki/File:Yellow_Warbler_Setophaga_aestiva_m_Toronto1.jpg

Researchers tell about their work on YouTube

Sources:
Lawson, S.L., J.K. Enos, N.C. Mendes, S.A. Gill & M.E. Hauber, 2020. Heterospecific eavesdropping on an anti-parasitic referential alarm call. Communications Biology 3: 143 . Doi: 10.1038/s42003-020-0875-7
Gill, S.A. & S.G. Sealy, 2004. Functional reference in an alarm signal given during nest defence: seet calls of yellow warblers denote brood-parasitic brown-headed cowbirds. Behavioral Ecology and Sociobiology 5671-80. Doi: 10.1007/s00265-003-0736-7
Clark, K.L. & R.J Robertson, 1979. Spatial and temporal multi-species nesting aggregations in birds as anti-parasite and anti-predator defenses. Behavioral Ecology and Sociobiology 5: 359-371. Doi: 10.1007/BF00292524

From reliable source?

Nuthatch transmits indirect information only partially

red-breasted nuthatch eavesdrops on black-capped chickadee

The red-breasted nuthatch understands the alarm call of black-capped chickadees perfectly. But it doesn’t propagate all the information that it contains in its own call, as Nora Carlson and colleagues show.

An owl that is perched on a tree branch during daytime does not pose an immediate threat to songbirds. Yet, they would rather not have it in their neighbourhood. By making a lot of fuss with a group, which is called mobbing, they try to bully the predator away.

This behaviour is also exhibited by the red-breasted nuthatch from North America. If the bird is aware of an owl being around, it will recruit conspecifics to participate in mobbing. In its mobbing call, it encodes how dangerous the owl is that has to be chased away, as Nora Carslon and colleagues write. At least: if the nuthatch itself observed the enemy.

Aroused

That is because not all owls pose similar threats. The great horned owl, a large bird about half a meter in length, is not agile enough to easily catch a songbird; it is therefore not very threatening. The small, agile northern pygmy owl is much more dangerous.

Accordingly, nuthatches react differently to hearing either great horned owl or pygmy owl, as appeared from playback experiments in which the researchers exposed the songbirds to the calls of both predators. Upon hearing a pygmy owl, the mobbing call of nuthatches consists of shorter, higher-pitched calls that are uttered at higher rate than after hearing a great horned owl. Their conspecifics then are more aroused and exhibit mobbing behaviour for longer and more intensively – in this case against the speakers that were used by the researchers.

Consequently, the songbirds spend their time and energy mainly in chasing away the most dangerous enemies.

Eavesdropping

black-capped chickadee encodes threat level in its alarm callNuthatches not only rely on their own ears; they also make use of the vigilance of other songbirds and eavesdrop on their alarm calls.

The researchers had shown previously how they respond appropriately to mobbing calls of black-capped chickadees, which also encode whether they face a less dangerous great horned owl or a more dangerous northern pygmy owl. When nuthatches hear chickadees calling in response to pygmy owl, they make more fuss and they will also produce more mobbing calls than when they hear chickadees’ response to great horned owl. So, they understand the message of chickadees very well.

But despite that understanding, nuthatches don’t propagate in their own mobbing call the level of danger according to chickadees, like they do after observing the enemy themselves. If the information is from chickadees, they will not indicate how dangerous the enemy is; their mobbing call is intermediate in call length, pitch and rate at high and low risk.

Less reliable

And perhaps, this is not so bad. Although nuthatches and chickadees share many predators, they are not equally vulnerable to those enemies, due to their different lifestyles. How chickadees perceive and communicate the threat of different enemies can differ from how nuthatches would estimate the level of danger, making the information obtained from chickadees a bit less reliable.

Willy van Strien

Photos
Large: red-breasted nuthatch. Cephas (Wikimedia Commons, Creative Commons CC BY-SA 3.0)
Small: black-capped chickadee. Shanthanu Bhardwaj (Wikimedia Commons, Creative Commons CC BY-SA 2.0)

Sources:
Carlson, N.V., E. Greene & C.N. Templeton, 2020. Nuthatches vary their alarm calls based upon the source of the eavesdropped signals. Nature Communications 11: 526. Doi: 10.1038/s41467-020-14414-w
Templeton, C.N. & E. Greene, 2007. Nuthatches eavesdrop on variations in heterospecific chickadee mobbing alarm calls. PNAS 104: 5479-5482. Doi: 10.1073_pnas.0605183104
Templeton, C.N., E. Greene & K. Davis, 2005. Allometry of alarm calls: black-capped chickadees encode information about predator size. Science 308: 1934-1937. Doi: 10.1126/science.1108841

Hidden beauty

Chameleons are characterized by glowing bony tubercles

Calumma crypticum possesses glowing bony tubercles

Many chameleon species are discovered to possess bony bumps that emit blue light through the skin. The glowing bumps enable the animals to recognize conspecifics and males probably show them off to females. We don’t perceive them in natural light, but the animals do, David Prötzel and colleagues think.

There are many ways in which an animal can make itself stand out, for instance with scent, colour, song, dance, decorative feathers or eyes on stalks. But here is a new one: chameleons appear to have small bumps on their skull that emit blue light, David Prötzel and colleagues report.

The glowing tubercles of Calumma crypticum become visible under UV lightThat may sound like horror, but there is nothing ghostly about it. Bone tissue is fluorescent: if it is hit by ultraviolet light, it is excited and will emit blue light. Chameleons use this natural phenomenon. The bumps, or tubercles, on their skull protrude through most of the skin and are covered only by a thin, transparent layer of cells, which is like a window. If ultraviolet passes that window and falls on the tubercles, they will glow blue.

Blue light

Normally we don’t see it, as the amount of ultraviolet (UV) in natural light is too small. Therefore, the phenomenon was only discovered when the researchers illuminated heads of chameleons with a UV lamp. But natural light does contain enough ultraviolet to make the glowing bumps visible to the chameleons themselves, the researchers suspect, as their eyes are more sensitive to blue light than ours. Many chameleon species in Madagascar and in Africa have such glowing bumps, especially species that inhabit humid forests, where the component of ambient UV light is relatively high; the emitted blue light contrasts well with the dark background.

Seduce

The pattern of bony bumps is species specific; most bumps are seen around and behind the eyes, but some species have such blue glowing tubercles not only on the skull, but on the entire body. Chameleons will recognize their conspecifics by the distinctive pattern of tubercles, which is a stable signal for these animals with their colour changing behaviour.

As in the nearly all species males posses on average more tubercles than females, the researchers assume that males display them to seduce a female.

So, chameleons possess glowing bone bumps as ornamentations used for recognition and display; it is possible that other lizards and snakes will be discovered to show fluorescencent bony bumps.

Willy van Strien

Photos:
Large: Calumma crypticum, male. Axel Strauβ (Wikimedia Commons, Creative Commons CC BY-SA 3.0)
Small: head of Calumma crypticum, preserved specimen from the Zoologische Staatssammlung München (Germany), under ultraviolet light. Copied from David Prötzel et al. (Creative Commons, CC BY 4.0) and mirrored, so that head is oriented in the same direction as in the large picture

On this YouTube video, the researchers illuminate Furcifer pardalis and two Brookesia species to elicit the blue glow. And on this video you see the knobby skull of Calumma globifer.

Source:
Prötzel, D., M. Heß, M.D. Scherz, M. Schwager, A. vant Padje & F. Glaw, 2018. Widespread bone-based fluorescence in chameleons. Scientific Reports 8: 698. Doi: 10.1038 / s41598-017-19070-7

Percussion

Palm cockatoo drums with self-fashioned drumstick

Palm cockatoo makes a drumstick

With a female listening, palm cockatoo males may repeatedly strike a hollow branch or trunk with a stick. Robert Heinsohn and colleagues heard that the birds have good rhythm and that every male has his individual drumming style.

A palm cockatoo male from North Australia can produce different sounds while erecting its crest. That is impressive, but there is something that really stands out: it may start drumming.

Regular pulse

When a male is going to perform, it breaks off a twig, removes the leaves, trims it to approximately 20 centimetres, grasps it in one of both foots and starts beating repeatedly on a hollow branch or trunk. Instead of a stick, it may use a seed pod of a particular tree (Grevillea glauca, the bushman’s clothes peg) after adjusting the shape with its beak. It may continue drumming for a while, producing a sequence of up to 90 taps.

It is remarkable that the intervals between the taps don’t occur at random intervals; instead, the cockatoos produce a regular pulse, as Robert Heinsohn and colleagues assessed. They also noticed that each male has its individual, consistent style; some males have slow drumming rates, whereas others drum at a faster rate, or insert short sequences of faster drumming in the performance occasionally.

Solo

It is not known yet which function the performance might have. Palm cockatoos form monogamous pairs which occupy a large territory. The sound does not travel far enough to be heard by the neighbours, so a male cannot communicate with them by drumming; he always is playing solo. As most performances are attended by the female, the music probably is meant for her, and it may be a male’s way to inform its partner about its condition or age; the birds may live more than 50 years. We don’t know whether the females like the percussion and what rhythm they prefer.

Willy van Strien

Photo: Christoph Lorse (Via Flickr. Creative Commons CC BY-NC-SA 2.0)

The researchers explain their work on You Tube;
short fragment of a drumming cockatoo

Source:
Heinsohn, R., C.N. Zdenek, R.B. Cunningham, J.A. Endler & N.E. Langmore, 2017. Tool-assisted rhythmic drumming in palm cockatoos shares key elements of human instrumental music. Science Advances 3: e1602399. Doi: 10.1126/sciadv.1602399

Surprising and familiar

The pied butcherbird and the art of composing

pied butcherbird, male. V. Nunn

A clever composer is able to grip the audience with variations, but without presenting music that is a confusing chaos. The pied butcherbird masters that art too, as Eathan Janney and colleagues report.

A piece of music with more variety in it is more pleasing to listen to. But it should not be too surprising: the piece must remain recognizable as a unit. In order to maintain consistency, a composer will repeat parts of the music and take care that themes can be heard several times.

In this respect, the beautiful singing pied butcherbird can compete with a good composer, according to research done by Eathan Janney and colleagues.

The black and white bird, slightly smaller than a magpie, lives in Australia where it is the most accomplished song bird with a very complex song. The bird may sound like a flute, a cornet or organ; hence the name. Males can sing continuously for hours at night. Their song consists of hundreds of clear phrases which take about two and a half second. After each phrase they wait a few seconds before they proceed.

Motifs

Janney wondered if they, just as composers, keep a balance between novelty and repetition. That would be important to prevent habituation in female listeners while at the same time the bird remains identifiable as an individual. He studied the nocturnal solo songs of 17 birds. He divided the phrases of each bird in types and investigated how often and in what order he sang each type. Also, he discerned motifs; a motif is a single tone or a group of a few tones (syllable) that often recurs. Several types of phrases may share a same motif. Finally, for each bird he investigated how he arranged his phrases and motifs. Was there any structure in the temporal patterning?

The singing of the birds is well organized, the analysis shows. Types of phrases and particularly motifs were regularly spaced in time. That regularity arises, as the researchers show, because a performing bird orders the different types of phrases in such a way, that each motif is heard at constant time intervals.

Large repertoire

The birds differ greatly in the amount of variation in their song. Some birds have more types of phrases and more different motifs in their repertoire than others. The more variety, the greater the risk that the song as a whole will be incoherent. But, as it turns out, the birds with the most different phrases and motifs organized their song more strictly. The larger the repertoire is, the stronger the temporal regularity with which the motifs are repeated. The birds seem to actively maintain the balance between variety and regularity – just like a good composer.

Willy van Strien

Photograph: Pied butcherbird male. Vicki Nunn (Wikimedia Commons, Creative Commons CC BY-SA 4.0)

An accomplished performer can be heard on this video of the researchers
Hear another record of the song

Source:
Janney, E., H. Taylor, C. Scharff, D. Rothenberg, L.C. Parra & O. Tchernichovski, 2016. Temporal regularity increases with repertoire complexity in the Australian pied butcherbird’s song. Royal Society Open Science 3: 160357. Doi: 10.1098/rsos.160357