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Thank you and welcome – a wonderful honour to give this #BOU2022 #AlfredNewtonLecture talk. A sight for a curious naturalist: why is this warbler apparently so silly? Or, how does the cuckoo get away with this?
#ornithology
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The talk celebrates Alfred Newton, founder of the BOU in 1858, the year of Darwin and Wallace’s papers proposing the theory of natural selection. Newton was an immediate early fan.
#ornithology
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Newton’s paper in 1869 suggested that egg-colour variation in different host-races of Common Cuckoos was a result of natural selection. He coined the term “gentes” (singular “gens”) for these host-races
Newton’s Nature paper: nature.com/articles/00107…
#ornithology
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Common Cuckoos are our harbinger of spring. But, the poet Ted Hughes reminds us that their arrival heralds potential doom for thousands of small birds
🔊 volume up ⬇️
Photo: Artur Stankiewicz
#ornithology #cuckoo
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It has been known since ancient times that Common Cuckoos lay in the nests of other birds and that the young #Cuckoo ejects the host’s eggs and young.
#ornithology
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The first detailed description of the #Cuckoo chick’s ejection behaviour was by Edward Jenner in 1788. Though sadly, Jenner’s wonderful and accurate account was first met with widespread disbelief.
#ornithology
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Amazingly, the ejection often takes place while the host parent looks on, yet it does nothing to interfere. Watch what happens next…
#ornithology #cuckoo
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Having taken sole command of the nest, now three weeks old and about to fledge, the #Cuckoo chick is seven times the mass of the Reed Warbler foster parent. Early naturalists, such as John Ray and Gilbert White, were perplexed, even outraged.
#ornithology
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They wondered if Cuckoos were badly designed by the Creator, either through faulty anatomy or faulty migration behaviour. Perhaps the hosts were only too pleased to give the poor Cuckoo a helping hand?
#ornithology #cuckoo
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Darwin dismissed these quaint views. He pointed out that, freed from parental duties, brood parasites could lay more eggs. So, selection should favour the evolution of brood parasitism from parental ancestry.
#ornithology #evolution #cuckoo #broodparasitism
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We now know that brood parasitism (shaded genera) has evolved independently three times from parental ancestry within the cuckoo family tree
#ornithology #evolution #cuckoo #broodparasitism
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Darwin proposed that hosts accept cuckoos through a mistaken instinct – they get tricked… So in theory there should be a ding-dong evolutionary battle as each party evolves in response to the other.
#ornithology #evolution #cuckoo
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Yet, the sight of a host feeding a #Cuckoo chick begs the question: do hosts really have defences?
#ornithology
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We need to discover how the #Cuckoo lays her egg. The credit goes to Edgar Chance who, a hundred years ago, studied Common Cuckoos parasitizing Meadow Pipits on a heathland in central England.
#ornithology
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He studied 1 female (“Cuckoo A”) over 5 summers. [down arrow] here are his discoveries, which enabled him to film the laying of the egg by a #Cuckoo for the first time…
#ornithology
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Edgar Chance published two books. The second no longer had ‘MBOU” after his name. Collecting eggs of common species (which included Cuckoos) was not illegal back then. But Chance also collected eggs of rare species. He was forced to resign from the BOU.
#ornithology
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Chance’s brilliant field observations formed the basis for our field experiments, begun in 1985. We studied Common Cuckoos parasitizing Reed Warblers on Wicken Fen.
Paper: sciencedirect.com/science/articl…
#ornithology #cuckoo
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We discovered that Reed Warblers rejected 20% of the Cuckoo eggs. By what cues did they detect a foreign egg? ⬇️
#ornithology #cuckoo
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We tested this experimentally with model #cuckoo eggs of various colours and patterns and played the part of the cuckoo ourselves…
#ornithology
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Our nest searching attracted attention! Here, Mike is explaining to the policeman that we are not collecting eggs – rather putting extra eggs into nests.
#ornithology
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1⃣ why egg mimicry? The two beards had different ideas...
Darwin ➡️ to fool hosts, who would reject eggs unlike their own.
Wallace ➡️ convergent evolution - host and cuckoo eggs have independently evolved a similar cryptic pattern to reduce predation.
#ornithology
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Our experiments supported Darwin. Reed Warblers were more likely to reject model eggs unlike their own than mimetic eggs. ⬇️Arnon Lotem showed that hosts imprint on their first clutch and then reject eggs that differ from this learnt set.
sciencedirect.com/science/articl…
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Why does the female #Cuckoo remove a host egg? And why not remove all the host eggs? Again, experiments provide the answers ⬇️
Our ‘Cuckoos versus Reed Warblers’ paper: sciencedirect.com/science/articl…
#ornithology
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Why such rapid laying? Reed #Warblers were more likely to reject eggs if they saw a #Cuckoo at their nest. Why, then, the need to be alerted?
#ornithology
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Reed Warblers make recognition errors. An over-fussy host would suffer unnecessary costs, as most nests are not parasitized.
#ornithology #cuckoo #nestparasitism
26 #BOU2022 So hosts seek additional cues for likely parasitism – both personal information (observing a cuckoo at their own nest) and social information about local cuckoo activity (from neighbour mobbing) 🔊
1: science.org/doi/10.1126/sc…
2: science.org/doi/10.1126/sc…
#ornithology
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Since 1980 on Wicken Fen, Reed #Warbler egg rejection has 📉 as have #Cuckoo numbers 📉 = a behavioural response to reduced parasitism risk. Cuckoo mobbing has also [down arrow] as this is potentially costly, as Cuckoos resemble hawks.
onlinelibrary.wiley.com/doi/10.1111/ev…
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The female #Cuckoo’s chuckle (“bubble”) call, often given as she leaves the host nest, might enhance hawk resemblance.
#ornithology #evolution
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Playback experiments – ♀️ #Cuckoo’s hawk-like chuckle diverts host defences from the nest to self-protection. Reed Warblers are alerted by hawk & ♀️ Cuckoo calls (not by controls) and are less likely to reject model eggs
@Jenny_E_York nature.com/articles/s4155…
@Jenny_E_York 30 #BOU2022
Hosts have many defences & Common Cuckoos have evolved specialist host-races. Cuckoo host choice likely develops by imprinting - as shown for brood-parasitic #indigobirds in Africa. #Cuckoo egg-type is likely determined by the W chromosome … (??)
#ornithology
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Indirect evidence suggests only female Common #Cuckoo have distinct host-races. Hiroshi Nakamura’s brilliant field work in Japan combined tracking Common Cuckoos with parentage analysis of Cuckoo chicks
Marchetti et al. 1998 : science.org/doi/10.1126/sc…
#ornithology
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He found that ♀️ Cuckoos are host-specific while ♂️ Cuckoos often cross mate with several female types. Genetic analysis supports the view of female host-races within the one species – #Cuckoo host races differ in mtDNA but not nDNA
Paper: nature.com/articles/35025…
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Egg mimicry must be assessed through bird’s eyes, which have four colour cones, including UV. Colours can be plotted as clouds of points in tetrahedral colour space
Mary Stoddard & @SensoryEcology paper: royalsocietypublishing.org/doi/full/10.10…
#ornithology
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Mary Stoddard & @SensoryEcology found that the more discriminating the hosts (assessed by experiments with model eggs), the better the egg mimicry by their respective host-race of #Cuckoo
#ornithology
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Hosts don’t only evolve egg discrimination. Their egg patterns evolve too. The poet John Clare likened egg markings to “writing scrawls”. But this is no poetic whimsy…
#ornithology
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A hundred years ago, Charles Swynnerton proposed that host eggs evolved distinctive signatures to facilitate easier detection of cuckoo eggs, and cuckoos then evolved forgeries. Another arms race
(described in a 1918 @IBIS_journal paper!)
#ornithology #evolution
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The ideal signature should be:
➡️ individually distinct (different females have different egg patterns)
➡️ replicable (little variation within a female’s clutch).
✅Both features are characteristic of species that are #cuckoo hosts
#ornithology #evolution
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naturepatternmatch.org quantified egg markings (size, shape, orientation) and scored the number of matches between a given egg and other eggs ➡️ more individually distinct signatures produce better matching between eggs of the same clutch
#ornithology
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Comparing across host species, as Common #Cuckoo host-races evolve better host-egg mimicry, their hosts evolve better signatures (their eggs are more likely assigned by the computer program to the correct clutch)
➡️nature.com/articles/ncomm…
#ornithology #evolution
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The most remarkable signatures belong to the Tawny-flanked Prinia, a host of the #Cuckoo Finch, studied by Claire Spottiswoode et al. Each egg ⬇️is from a different ♀️ within one study site. In theory, distinct signatures would fade if parasitism ceased…
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#Weaverbirds were introduced in 1780s from a parasitized population to cuckoo-free Hispaniola in the Caribbean. Their signature diversity has indeed 📉. Not simply the Founder Effect as intra-clutch variation 📈= poorer signatures
Paper: pnas.org/doi/10.1073/pn…
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So, now a puzzle ⁉️. Why accept the Common #Cuckoo chick?
Three cues Reed Warblers could potentially use (as they do to discriminate Cuckoo eggs)
1⃣size
2⃣colour
3⃣spotting
#ornithology #nestparasitism
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Four ideas for why chick rejection might not evolve…
1⃣ Can’t compare chicks
2⃣ Charming chicks
3⃣ Hard to recognise
4⃣ Reject early instead – “strategy blocking”
#ornithology #nestparasitism
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An elegant idea by Arnon Lotem - “accept all chicks” is a better strategy. Unlucky hosts of Common #Cuckoos, parasitized in their first clutch, could imprint only on a Cuckoo chick and so reject their own young from later broods
nature.com/articles/36274…
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But this can’t be a general constraint, because in Australia hosts of Bronze-cuckoos do sometimes reject cuckoo chicks. Either by abandonment or by ejection
abandonment paper: nature.com/articles/natur…
ejection paper: royalsocietypublishing.org/doi/full/10.10…
#ornithology
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In response, cuckoos have evolved host-chick mimicry. Hosts use several cues: visual & acoustic (chicks unlike their own more likely to be rejected) and perceived parasitism risk (more rejection if adult cuckoos present)
Paper: nature.com/articles/natur…
#ornithology
47 #BOU2022 #AlfredNewtonLecture
Might the domed nests of these hosts make egg defences harder, and hence favour chick rejection instead? Perhaps the dark eggs of bronze-cuckoos escape host detection?
#ornithology #nestparasitism
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Surprisingly, hosts don’t reject even conspicuous foreign eggs. But cuckoos are selective when they remove an egg before they lay. Cryptic eggs are an effective defence against other #cuckoos ⬇️
➡️sciencedirect.com/science/articl…
➡️royalsocietypublishing.org/doi/full/10.10…
#ornithology
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If parasitism by ♀️ #cuckoo is high, it pays hosts to accept cuckoo eggs as the chance of own-egg removal is then diluted. This leads to the #evolution of cryptic #cuckoo eggs that aren't removed by other cuckoos
Paper: bioone.org/journals/ornit…
#ornithology
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Examples when cost to reject > accept
1⃣ #cuckoo doesn’t eject host eggs
2⃣ cuckoo egg (white) is too large and thick-shelled to eject
3⃣ Desertion is costly (low chance to re-nest). So better to accept & raise some own young
royalsocietypublishing.org/doi/full/10.10…
#ornithology
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Coots reject conspecific parasitic eggs but accept a large Cuckoo Duck egg! Why?
❌ Costly to eject (crack own eggs)
✔️ Hatched cuckoo duckling requires no care
So cost of #cuckoo duck egg < a foreign coot chick
Paper: nature.com/articles/natur…
#ornithology
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Finally, a case where a non-ejecting cuckoo chick brings a net benefit ‼️ Its faecal secretions deter both avian and mammalian predators
Paper: science.org/doi/10.1126/sc…
#ornithology
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So…not all cases of #nestparasitism lead to an arms race with the #evolution of host-egg mimicry
✅Many selection pressures are involved
🪶 Only a few cuckoo species have been studied
⁉️ Many surprises surely await future curious naturalists
#ornithology
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To echo Alfred Newton: how lucky we are to be inspired by evolutionary ideas. We are just four handshakes from Darwin- perhaps Newton met Darwin’s granddaughter Gwen Raverat who lived in Cambridge, so close to our #cuckoos…
#ornithology
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If you’re fascinated by the extraordinary behaviours & stories about cuckoos, see my book for more “Cuckoo – Cheating by Nature”. Thank you for reading – Nick Davies
Thanks to @Nat_B_Zielonka for wonderful help with this Twitter presentation
#ornithology
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If you have any questions for Prof Nick Davies, please post them as comments to this thread and we’ll pass them to Nick ☺️
Thanks for joining us!
A this new #ornithology paper links perfectly with yesterday evening’s #AlfredNewtonLecture by Nick Davies | #BOU2022
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