Chapter 1-5 Notes: Cuckoo Parasitism and Host Arms Race

Introduction / Context

• Continuation of Lecture 3 on parental care, focusing on brood parasitism.

• Core theme: the co-evolutionary “arms race” between cuckoos (obligate brood parasites) and their host bird species.

• Lecture segment picks up after coverage of:

  • Pre-laying host defence behaviours.

  • Manipulation of clutch size.

  • Egg mimicry and pattern complexity.

  • Host rejection thresholds.

Cuckoo–Host Arms Race Overview

• Arms race = reciprocal, escalating adaptations and counter-adaptations.

• Key selective pressures:

  • Cuckoos must successfully exploit host parental care.

  • Hosts must avoid wastage of limited resources on unrelated young.

• Brood parasitism costs host parents in terms of energy, time, and lost genetic fitness.

• Conflict occurs before laying, at egg stage, and throughout chick development.

Egg Stage Adaptations & Counter-adaptations

• Hosts evolve highly variable, intricate eggshell patterns → harder for cuckoos to replicate.

• Common cuckoo (Europe):

  • Parasitises 8 distinct host species.

  • Exhibits plastic egg mimicry: changes colour/pattern expression to match the current host clutch.

• Host-specific examples:

  • Brambling → excellent at detecting foreign eggs; strong selection on cuckoo egg accuracy.

  • Dunnock → poor at detection; weak selection on egg mimicry so cuckoo eggs may look un-matched.

• Prediction: If host detection improves, cuckoo egg morphology should evolve toward finer mimicry (future arms-race steps).

Chick Stage Adaptations: Vocal Mimicry & “Passwords”

• Some cuckoo nestlings mimic host begging calls to trigger adult feeding.

• Superb fairy-wren study (Australia):

  • Species parasitised by bronze cuckoo.

  • Female wrens sing a unique “password” call to embryos while still in the egg.

  • True offspring match the password post-hatch; cuckoo chicks fail → parents reduce/cease provisioning.

  • Trade-off: Increased calling raises risk of nest predation but enhances chick recognition accuracy.

• Demonstrates learning pre-hatching and cross-generational information transfer.

Morphological Chick Mimicry

• Bronze cuckoo complex (shining, Horsfield’s, etc.):

  • Adults look nearly identical across species → minimal selection on adult phenotype.

  • Chicks show dramatic divergence mirroring respective host nestlings.

  • Mimicked traits: skin pigmentation, distribution of natal down tufts, gape colour/pattern, feather tracts.

• Host-specificity vs mimicry precision:

  • Horsfield’s bronze cuckoo: least accurate chick mimicry but least host-specific (multi-host strategy requires phenotypic plasticity rather than perfect fidelity).

• Illustration of developmental plasticity and selective pressure targeted at life-history stage with highest detection risk.

New Zealand & Pacific Examples

• Shining cuckoo (NZ name = pīpī wharauroa; AUS name = shining bronze cuckoo):

  • Migrates to Pacific islands during non-breeding season.

  • Primary NZ host: grey warbler (riroriro).

• Long-tailed cuckoo (NZ):

  • Hosts: whitehead, yellowhead, brown creeper.

  • Highlights geographic variation in host–parasite networks.

Evolutionary Implications & Trade-offs

• Parents possess finite resources (R_{total}), allocated among:

  • Current offspring (\sum R_{current})

  • Future reproductive opportunities (R_{future})

• Parent–offspring conflict:

  • Present within broods (sibling rivalry) and across generations (current vs future broods).

  • Brood parasitism amplifies conflict by adding unrelated competitor.

• Coevolutionary dynamics are frequency-dependent and may oscillate through time (Red Queen scenario).

Ethical, Philosophical & Practical Notes

• Raises questions of genetic vs social parenthood: who “owns” parental investment?

• Demonstrates natural examples of deception, adoption, and selective discrimination.

• Practical relevance to conservation:

  • Understanding host vigilance can inform management where endangered hosts suffer high parasitism.

  • Predicting outcomes as species distributions shift with climate change.

Connections to Broader Course Content

• Links to sexual selection (next lecture) via mate choice costs and reproductive strategies.

• Complements earlier discussions on life-history theory (resource allocation, trade-offs).

• Illustrates principles of signal evolution, sensory biases, and learning.

Further Reading, Media & Activities

• Textbook: Rubenstein & Alcock, Parental Care chapter (Chs. 11 or 12 depending on edition).

• Video resource: David Attenborough’s documentary segment on cuckoos (link in Moodle).

• Upcoming Moodle quiz “Cuckoo vs Host” to consolidate recognition of adaptations.

Take-Home Messages

• Limited resources force strategic allocation; raising non-kin is costly.

• Brood parasitism produces a multi-level arms race (egg, call, morphology).

• Evolution acts on whichever life stage offers most leverage for success or defence.

• Studying these systems illuminates conflict resolution mechanisms across taxa and generations.