Across their range, Common Ravens (Corvus corax) are quite precise when replying contact calls

doi:10.21203/rs.3.rs-2117479/v1

Download PDF

Research Article

Across their range, Common Ravens (Corvus corax) are quite precise when replying contact calls

https://doi.org/10.21203/rs.3.rs-2117479/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted

You are reading this latest preprint version

An essential signal for social animals is to establish presence or absence of conspecifics. This investigation looked for instances of the Common Raven contact call "aww" [ɒ:] being answered by one or more other ravens at a distance. The data was gleaned from the entirety of raven recordings -- 5435 recordings, mostly audio, but also audio with video -- held by the Cornell Lab of Ornithology’s Macaulay Library, from 1954 Manitoba to 2022 Morocco. The main finding was ravens sent from 1 to 6 “aww”, and replying ravens tended, to a statistically significant degree, to match this number. This precision suggests not only a strong awareness of quantity of sounds, but the raven's awareness of the minds of its fellow ravens.

animal communication

bird vocalizations

citizen science

raven

corvid

Corvus corax.

In the 1959 film Journey to the Center of the Earth, Professor Lindenbrook (played by James Mason), imprisoned in a cellar, hears tapping through a wall. “It's a code,” Lindenbrook cries to his student, “jot it down!” The sender turns out to be domestic duck Gertrude [1], and it sets one wondering about the utility of simple signals -- such as the ubiquitous contact call of ravens.

This call -- “universally diagnostic of the raven” -- is always given loudly, with the sender looking upward (Fig. 1), and can be heard as far as 2 km away [2]. From personal experience -- within a few meters of ravens as they hear a distant flock -- the volume of these contact calls correlate with the distance the message needs to travel. Sometimes the sound is in response to distantly-heard calls, and sometimes the sender initiates the dialog. Sometimes the sender is not reciprocated, as perhaps no other ravens are close enough to hear; sometimes distant calls are heard but a raven seen on video (or in person) does not reply.

This main raven contact call has been transliterated as “craack” [3], “haw” [4], “croake” [5], "awk" [6], and most commonly, “caw” [6, 7, 8, 9] or "kaww" [10]. As with human speech, one hears different accents across the range and even in one locale; still, most transliterations have the [ɒ] vowel sound. But do consonants precede or follow [ɒ]? To decide, we must first inspect spectrograms of the International Phonetic Alphabet (IPA). The Fig. 2 spectrograms were generated from recordings at each phoneme's web page [11, 12], captured using the acoustical software Raven Pro [13], and checked against spectrograms in the seminal textbook "A Course in Phonetics" [14].

To deduce phonemes from spectrograms, consider that vowels have clearly defined formants [15]. As monophthongs vowels have a single unchanging sound quality, at least until closure [16, 17]. Vowels last longer than consonants, and -- if spectrograms are clear enough -- one can see the periodicity of individual waveforms [18]. Finally, vowels can often be identified by the frequencies of their lowest (and main) two formants, F1 and F2 [19]; for example, [u] as in "hoot" has its F1 and F2 at 250 and 595 Hz in a typical human male voice, while [i] as in "free" has a high and prominent F2 at 2400 Hz. The F1 and F2 of [ɒ] are moderately low and close: about 700 and 760 Hz.

In the both the raven and human spectrograms of Fig. 3, formants stretch horizontally across the vocalization; and ravens, being songbirds, have significant higher formants also: F3, F4, and upwards. But all show the medium-low F1 and F2 typical of the prolonged open back rounded vowel [ɒ]. (Its IPA symbol is an inverted version of the vowel dubbed 'Turned script A'; its IPA number is 313, Unicode character U + 0252.)

With [ɒ] established, let us look for the messier formants of consonants. A leading [h] as in "haw" would have a stretch of low-amplitude noise [18] with turbulence in its middle [15] -- but we don't see [h] in Fig. 3. A leading or trailing plosive [k] would show bursts then abrupt losses of energy [17] but these too are absent. A trailing [w] phoneme would show a merged F1/F2 rising slightly at end of the enunciation [14], and this too is missing.

Thus, lacking consonants, this main raven contact call is best transliterated as "aww" [ɒː], with 'ww' to cue non-IPA experts to the pronunciation of the vowel and an IPA "length" suprasegmental to indicate its prolongation. (A colon can represent prolongation, but in an IPA-compatible font it is denoted by two tiny triangles one above the other, pointing at each other. It is Unicode character U + 02D0.)

Naturally, there are variations in pitch, timbre, and duration from different ravens just as with humans. But the sound of ravens' “aww" is much like the vowel in “saw”, and more precisely like the vowel in the UK pronunciation of “ox” [ɒks] according to the Cambridge Dictionary [21], the vowel being sounded more from the back of the vocal tract.

So now, feeling confident in naming this vocalization "aww", the research question is, What patterns are there, if any, in the use of the "aww" contact call by Common Ravens? The null hypothesis would be that there are no consistent patterns. The alternative hypothesis is that there are patterns, and those patterns provide advantages.

All cases of exact, close, or not-so-close “aww" reciprocations are shown in Table 1. Of these, 19 cases were audio-only, and 4 were video with audio. The concentration of more recent recordings arises from more citizen scientists doing more recordings and with better equipment, and because the Macaulay Library is becoming better known.

Table 1

Reciprocation of “aww" sequences whether exact or not.
number sent then replied	recording	region	year
1–1	ML62603	Manitoba	1971
1–1, 1–1, 2–2, 1–1	ML50335	Oregon	1990
3–4	ML56844	Arizona	1991
3–3	ML96242	Morocco	1995
2–3, 2–3	ML201047401	Norway	2007
2–2	ML201222111	Spain	2014
4–4	ML20307291	North Carolina	2015
4 − 3, 3–3	ML37077101	New York	2016
1–1, 1–1	ML100500201	New York	2018
2–2, 2–2, 1–1, 2–2, 1–1, 3 − 2, 3 − 2	ML104490971	British Columbia	2018
1–1	ML104726671	Maryland	2018
3 − 2	ML172538221	Connecticut	2019
1–1, 4 − 3, 4 − 2	ML198807721	Pennsylvania	2019
3–3	ML264021511	New York	2020
6 − 4	ML307742891	Colorado	2021
4–4, 4 − 3	ML313726831	Arizona	2021
4–4, 3–3, 4 − 2	ML327400801	Ontario	2021
2–2	ML332041901	California	2021
3 − 1, 2–3, 3 − 2	ML367216291	Utah	2021
1–1	ML390375311	Pennsylvania	2021
2 − 1, 2 − 1	ML403163241	Mexico	2022
3–3, 4 − 3, 3 − 2, 3–3	ML425980901	New York	2022
4 − 3	ML451421851	New York	2022

To quantify correlation between "aww" sent and replied, a non-parametric test is needed since there are non-normal distributions of both sent and replied "aww" numbers. The proper test appears to be the Spearman's Rank Correlation [22], and so we plug the pairs of numbers into the RStudio statistical package [23] and call on the Spearman:

# Let x be the number of aww sent, y replied

x = c(1, 1, 1, 2, 1, 3, 3, 2, 2, 2, 4, 4, 3, 1, 1, 2, 2, 1, 2, 1, 3, 3, 1, 3, 1, 4, 4, 3, 6, 4, 4, 4, 3, 4, 2, 3, 2, 3, 1, 2, 2, 3, 4, 3, 3, 4)

y = c(1, 1, 1, 2, 1, 4, 3, 3, 3, 2, 4, 3, 3, 1, 1, 2, 2, 1, 2, 1, 2, 2, 1, 2, 1, 3, 2, 3, 4, 4, 3, 4, 3, 2, 2, 1, 3, 2, 1, 1, 1, 3, 3, 2, 3, 3)

result = cor.test(x, y, method = "spearman")

print(result)

The result is R_S = 0.775, p < 0.001. That is, the correspondence between number of “aww” sent and number of “aww” replied shows a very strong correlation, not likely the product of chance.

To avoid pseudoreplication -- e.g. the 1990 Oregon ravens counting as 4 exact reciprocations -- let us reduce each such case to a single averaged pair of "aww" sent and replied. E.g. "1–1, 1–1, 2–2, 1–1" becomes "1.25–1.25"; "3 − 1, 2–3, 3 − 2" becomes "3-1.7" to reflect the average of 1.3 difference in sent and replied “aww", and so on. Though this reduces our sample from 46 to 23 sent-and-replied pairs, the correlation emerges even more strongly, with R_S = 0.852, and p < < 0.001.

Surprised at the strength of the R_s, the calculation was run again with 46 random pairs of numbers in the range 1 to 5 “aww"; this produced R_s 0.086, p = 0.58. (A final test was running 46 pairs of numbers where sent and replied “aww" were equal; naturally, the R_s was a perfect 1.00, p-value 2.2e-16.)

This study has a small sample size, is not evenly representative of the global Corvus corax range, and lacks video documentation of most of the data set. Still, the criteria for data selection was strict, and intended to err on the side of missing true and exact reciprocations.

Ravens most commonly vocalize 1, 2, or 3 "aww", but up to 6 "aww" is not uncommon. There were cases of a dozen or more “aww" -- e.g. 34 "aww" by the Ukraine raven in ML201867091 -- but these longer "aww" packets were always unreciprocated.

Despite all the possible combinations of "aww" sent and replied, there was a high degree of matching. Of the 46 sets of sent-and-replied “aww", 26 were exact matches -- the number replied equaling the number sent. In 16 sets the number replied was only 1 different from the number sent, e.g. “aww aww aww" replied by “aww aww"; the remaining 4 sets had 2 "aww" different.

A skeptic might argue that both ravens are calling “aww” not one to another but in response to some stimulus they detect. This would be impossible to fully disprove since we cannot see the entire surroundings of each raven nor know their inner thoughts or feelings, conscious or subconscious. But it seems simpler to conclude these “aww” are contact calls, as so many have reported. In some cases, the calling and replying ravens were seen or remarked to have united soon after contact calling. Up close I have many times seen ravens suddenly incline their heads upwards then begin calling “aww” loudly as they hear their conspecifics a kilometer or more away. Birds can reciprocate song [24], grooming [25], defensive efforts against predators [26], cooperate with humans to find honey [27], and much more. So we are not surprised they can reciprocate each other's calls.

But what might cause the difference in number of “aww” sent? Urgency would be one hypothesis: Raven A, mildly interested in contact, might call "Aww aww" while Raven B, more desperate to respond or rejoin Raven A, shouts "AWW AWW AWW AWW!" On the other hand, taking care to respond exactly to the sent message might have extra value, emphasizing “I am here, and received what you just sent!” Imagine two humans separated, and one hollers "John, can you hear me? I am by the lake shore!" "Yes Mary I hear, you are by the lake shore!" But match or no-match, simply responding with any number of "aww" might usually be sufficient; e.g. shouting "I am here, where are you?" and hearing a response of "Over here, over here, over here!"

We are know nature selects for traits that increase likelihood of survival – including parental care [28], establishing status and identity [29, 30], recruiting help [31], resolving conflicts without fighting [32], and much more. We must suspect most vocalizations have important purposes, whether instinctive or learned, or driven by the subconscious or conscious mind [33]. The contact call “aww" [ɒ:] of Corvus corvax is one such vocalization, and ravens may learn to give it special value by reciprocating the number of “aww" another raven has sent.

With an effective contact call, a group might avoid the late Act Two predicament of Professor Lindenbrook and his student, deep in the Earth and separated:

"Alec! Alec! Alec! Can you hear me? Can you hear me?"

The main conclusion is that ravens are uncannily precise in their reciprocation of contact calls. This near-exactness suggests not only ravens' attention to quantity, but awareness of the minds of their conspecifics.

The aim of this study was to look for presence or absence of patterns in ravens' use of contact calls.

The data was gleaned from 284 Corvus corax videos and 5151 Corvus corax audio-only recordings from the Cornell Lab of Ornithology’s Macaulay Library (ML) -- all of ML's raven recordings as of mid-August 2022. Every video was watched. For the audios all recordists' notes were read, searching for words and word forms suggestive of "contact", "reply", "answer", "distance" and so on, as well as "haw", "croak", "awk", "caw", and other transliterations.

From these we find 46 sets of "aww" sent and replied, in 23 recordings from 17 regions of the world ranging from British Columbia to New York, Norway to Morocco, where one raven (or sometimes two near each other) are answered within 2 seconds by another raven at a distance of approximately 100 meters or more. Transcripts were made (e.g. "013Ra2x; 021Ra2x,Rb1x") listing how many seconds into the recording which raven (a, b, etc.) vocalized a contact call, and how many times (e.g. 2x means "aww aww"). If it was uncertain which raven was which, the dialog was discounted. Near-simultaneous calls were ruled out too as not being a sending and replying. Also, if an “aww” was heard at the very beginning (or end) of a recording it was disqualified, for we could not tell if how many “aww” were expressed before (or after) the recording.

Occasionally in a video both sending and replying ravens could be seen. But most often one or the other were heard only. In the audio-only recordings, positions of ravens were gauged by the recordist's notes and also difference in volume of the sent-and-responded calls. If a recording were in stereo then faint, distant “aww" were judged to be from different ravens if they emanated from one then the other of the stereo channels.

The desire was to err on the conservative side, thus many probable cases were ruled out which would have supported an exact-match hypothesis. For example:

"028Rb3x,Ra3x cant be sure > 100m from each other ";
"049 sounds Ra1x,Rb1x but recordist doesn't affirm 2 ravens";
"024Rb3x but gap too long to 029Ra3x";
"too faint to distinguish Ra Rb tho sounds like several Ra1x,Rb1x".

Ethics approval and consent to participate

Ethical approval was not required for this study as it did not involve capture or other manipulation of animals.

Consent for publication.

Not applicable.

Availability of data and materials

The recordings and recordists' notes from the Macaulay Library at the Cornell Lab of Ornithology are available at https://www.macaulaylibrary.org.

Competing interests

The author declares the author has no competing interest.

Funding

Not applicable.

Authors' contributions

MK designed the research, analyzed the data, wrote, revised, read, and approved the final manuscript.

Acknowledgments

The Macaulay Library at the Cornell Lab of Ornithology provided sounds and videos. To ornithologist, photographer, and videomaker Pierandrea Brichetti, "Grazie amico mio." Also we must thank the recordists who happened to capture the Corvus corax “aww" reciprocations: William W.H. Gunn, Geoffrey A. Keller, Linda Macaulay, Per Naesje, Eric Francois Roualet, Martin Wall, Celeste Morien, David Badke, Max Wilson, Valerie Heemstra, Alan Buriak, Arthur Shippee, William Kuk, Nate Badger, Blaine Carnes, Paul Fenwick, Ben Filreis, Ted Floyd, Rose Ann Rowlett, Rafael Galvez, and Robert Whitman. Ruslan Mazuryk caught the Ukraine 34x "aww" moment.

Brackett C. 1959. Journey To The Center Of The Earth Script - Dialogue Transcript. http://www.script-o-rama.com/movie_scripts/j/journey-to-the-center-of-the-earth.html. Accessed 29 September 2022.
Boarman WI, Heinrich B. 2020. Common Raven (Corvus corax), version 1.0. In Birds of the World (S.M. Billerman, Editor). Ithaca: Cornell Lab of Ornithology.
Bendire, C. 1895. Life Histories of North American Birds, from the Parrots to the Grackles: With Special Reference to Their Breeding Habits and Eggs (No. 3). Washington: U.S. Government Printing Office.
Harlow RC. 1922. The breeding habits of the northern raven in Pennsylvania. The Auk 39: 399–410. https://doi.org/10.2307/4073437.
Bent AC. 1946. Life Histories Of North American Jays, Crows, And Titmice: Order Passeriformes. Washington: U.S. Government Printing Office.
Conner R. 1985. Vocalizations of Common Ravens in Virginia. The Condor 87: 379–388. https://doi.org/10.2307/1367219.
Ross RC. 1925. Field notes on the raven. The Condor 27: 2307.
Dorn JL. 1972. The Common Raven in Jackson Hole, Wyoming. University of Wyoming: ProQuest Dissertations Publishing, EP15907.
Laiolo P, Rolando A. 2003. The evolution of vocalisations in the genus Corvus: effects of phylogeny, morphology and habitat. Evolutionary Ecology 17: 111–123. https://doi.org/10.1023/A:1023003110969.
Brown RN. 1974. Aspects of vocal behavior of the raven (Corvus corax) in interior Alaska. M.S. thesis, University of Alaska, Fairbanks.
Isotalo P 2005. Voiceless glottal fricative, Voiceless palatal plosive, Voiced labial–velar approximant, Voiceless velar plosive. The sound files for each are available at their respective pages e.g. https://en.wikipedia.org/wiki/ Voiceless_glottal_fricative. Accessed 29 September 2022.
Roach P. 2016. Open back rounded vowel. Sound file available at https://en.wikipedia.org/wiki/Open_back_rounded_vowel. Accessed 29 September 2022.
Raven Pro. 2022. K. Lisa Yang Center for Conservation Bioacoustics. (2019). Raven Pro: Interactive Sound Analysis Software (Version 1.6.4) [Computer software]. Ithaca, NY: The Cornell Lab of Ornithology. https://ravensoundsoftware.com/. Accessed 29 September 2022.
Ladefoged P, Johnson K. 2009. A course in phonetics, 6th edition. Boston: Wadsworth. Their spectrograms are visible at https://linguistics.berkeley.edu/acip/. Accessed 29 September 2022.
Russell, K. 2005. Identifying sounds in spectrograms. https://home.cc.umanitoba.ca/~krussll/phonetics/acoustic/spectrogram-sounds.html. Accessed 29 September 2022.
Ohala D. 2004. LING 314 Course Notes. http://www.u.arizona.edu/~ohalad/Phonetics/notes/. Accessed 29 September 2022.
Hagiwara R. 2009. Monthly Mystery Spectrogram Webzone. https://home.cc.umanitoba.ca/~robh/howto.html. Accessed 29 September 2022.
Kirby JP. 2012. Lecture 10. http://www.lel.ed.ac.uk/~jkirby/hanoi/slides/lecture10-hanoi-4up.pdf. Accessed 29 September 2022.
Catford JC. 1988. A Practical Introduction to Phonetics. Oxford: Oxford University Press.
Russ K. 2018. IPA Reader. http://ipa-reader.xyz/. Accessed 29 September 2022.
Cambridge Dictionary. 2022. https://dictionary.cambridge.org/. Accessed 29 September 2022.
Long D. 2006. Spearman's Rank Correlation Test. https://www.cs.cmu.edu/afs/cs/project/jair/pub/volume20/long03a-html/node66.html. Accessed 29 September 2022.
Core Team R. 2022. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. https://www.R-project.org/. Accessed 29 September 2022.
Watson M. 1969. Significance of antiphonal song in the eastern whipbird, Psophodes olivaceus. Behaviour 35: 157–178. https://doi.org/10.1163/156853970X00187.
Barbour, D B., & DeGange AR. 1982. Reciprocal allopreening in the Brown-headed Nuthatch. The Auk 99: 171. https://doi.org/10.2307/4086037.
Wheatcroft DJ, Price TD. 2008. Reciprocal cooperation in avian mobbing: playing nice pays. Trends in ecology & evolution 23: 416–419. https://doi.org/10.1016/j.tree.2008.04.011.
Spottiswoode CN, Begg KS, Begg CM. 2016. Reciprocal signaling in honeyguide-human mutualism. Science 353: 387–389. https://doi.org/10.1126/science.aaf4885.
Hamilton WD. 1964. The genetical evolution of social behaviour. II. Journal of theoretical biology 7: 17–52. https://doi.org/10.1016/0022-5193(64)90039-6.
Gwinner VE. 1964. Untersuchungen uber das Ausdrucks- und Sozialverhalten des Kolkraben (Corvus corax L.) [An investigation of the expressive, and social behavior of the Raven (Corvus corax L.)]. Zeitschrift für Tierpsychologie 21: 657–748. https://doi.org/10.1111/j.1439-0310.1964.tb01212.x.
Boeckle B, Szipl G, Bugnyar T. 2018. Raven food calls indicate sender’s age and sex. Frontiers in Zoology 15: 1–9. https://doi.org/10.1186/s12983-018-0255-z.
Sierro J, Loretto MC, Szipl G, Massen JJ, Bugnyar T. 2020. Food calling in wild ravens (Corvus corax) revisited: Who is addressed?. Ethology 126: 257–266. https://doi.org/10.1111/eth.12991.
Morton ES. 1977. On the occurrence and significance of motivation-structural rules in some bird and mammal sounds. American Naturalist 111: 855–869. https://www.jstor.org/stable/2460385.
Heinrich B, Marzluff JM, Marzluff CS. 1993. Common ravens are attracted by appeasement calls of food discoverers when attacked. The Auk 110: 247–254. https://doi.org/10.1093/auk/110.2.247.

No competing interests reported.

Download PDF

Version 1

posted

You are reading this latest preprint version

Across their range, Common Ravens (Corvus corax) are quite precise when replying contact calls

Status:

Version 1

Abstract

Figures

Background

Results

Discussion

Conclusions

Methods

Declarations

References

Additional Declarations

Status:

Version 1