Brood Parasitism: An
Alternative Life History
N. B. Davies. 2000. Cuckoos, cowbirds and other cheats. Cambridge Univ. Press
Strong Parental Care: The
Typical Avian Life History
• Most birds live in pairs
• Exhibit bi-parental care
• Sex roles variable, but
often near equal
• Some species have uni-parental care
Dominion Power
• A few birds lack any “normal” parental care
• Parental care can be stolen
Brood Parasitism
The laying, or physical transport,
of eggs of one species into the
nest of a second species, where
they receive parental care
Two Kinds of Avian Brood
Parasites
Facultative
• “Lay” in own nest and nests of
conspecifics
• A common trait, especially in colonial
and cavity-nesting species (E.g.,
starlings, snow geese, cliff swallows)
Obligate
• Lay only in nests of other species
• 99 species in 5 families worldwide
• 1% of all bird species
Obligate Avian Brood
Parasites: 5 Taxa: 99 Species
• Cuckoos - 57 of 130 Species
– Probably evolved twice in the order
• Honeyguides – all 17 species
• Cowbirds – all 5 species
• African parasitic finches – all 19 species
• Black-necked Duck - 1 of 150 species
Cuckoos
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Old and New World
Small hosts
Persistent, simple calls
Eat fruit and large hairy insects
In most species, cuckoo chick ejects
host eggs/chicks
• Often lay mimetic eggs
Honeyguides
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Old World
Related to woodpeckers
Eat wax (bees nests)
Walter Weber
Hatchlings kill host chicks with
hooked beak
• Guide honey badgers/people to bees
Ian Jackson
African
Parasitic
Finches
• Genus Vidua
• Hosts in the related family Estrildidae
• Specific host relations (mainly 1
host: 1 parasite)
• Parasites reared with host chicks
• Parasite chicks mimic host nestlings’
mouth markings, begging behaviour
Cowbirds
• American blackbirds
• Three North American Spp.
• These 3 species are host generalists,
prefer smaller hosts
• Two specialize on other blackbirds
• Parasites reared with host chicks
Black-headed
Duck
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Family Anatidae
Southern South America
Parasitizes other waterfowl, e.g. coots
Ducklings hatch before hosts
Independent at hatching
Ducklings need no parental care
Questions about the Brood
Parasitic Life History
• How did it evolve?
• What trade-offs are involved?
• What adaptations make brood
parasites successful?
• How can brood parasitism be
countered by hosts?
How Did Obligate Parasitism Evolve?
• From facultative brood parasitism?
• Selection pressure created by time consuming
feeding habits (like eating wax, hairy bugs)
• In species that steal nests from other species?
– E.g., Bay-winged “Cowbird”
• Via communal nesting
– E.g., Anis, Guira Cuckoos
• During relaxed food limitation
– Yellow-billed Cuckoos, periodical cicadas
Adaptations of Brood Parasites - 1
• Rapid laying (often when host absent)
• Short incubation period
– Brown-headed cowbird eggs hatch in 10-11 days
vs. 12-14 d in hosts
• Noisy begging behaviour
– One cuckoo chick makes as much noise as a
whole brood of reed warblers
R. Kilner et al. Nature 397:667-672 (1999)
• Predatory behaviours by
– Adult great-spotted cuckoos, cowbirds
http://biology.easternct.edu/People/elliott/htm
– Nestling common cuckoos, honeyguides
Adaptations of Brood Parasites - 2
• Thick-shelled eggs
– Resist puncture ejection
• Egg mimicry
– Foils rejecter hosts
– Sharpens their egg discrimination
• Removing eggs from host clutches while
host is laying
– May enhance parasite hatching success,
especially in nests of large hosts
• Superior spatial memory? (in females)
D.F. Sherry et al. Proc. Nat. Acad, Sci. USA 90:7839-7843 (1993)
Defences by Hosts
• Egg recognition and rejection
– Inherited (American Robin)
– Learned (Gray Catbird)
– Costly – can reject own eggs
• Desertion or burial of parasitized clutches
– Yellow warblers build new nest cup
• Aggression directed at parasite
• Discrimination of foreign chicks
– Little evidence of this
except in estrildids
Why Prey on Host Nests?
• The “Mafia” hypothesis
Parasite “punishes” rejecting hosts:
– Experimental evidence in magpies
J. Soler et al. Evolution 49:770-775 (1995)
• To synchronize host/parasite reproduction
– Parasite benefits by “killing” unusable host
clutches/broods
– Host re-nests
– Parasite then lays in new nesting attempt
P. Arcese et al. Proc Nat. Acad. Sci. USA93:4608-4611 (1996)
Trade-offs?
• Brood parasites: no control over kids’ fates
– Eggs fail to hatch
– Kids neglected by host parents
• Maybe can lay more eggs?
– Song Sparrow: 2.5 clutches /year averaging 3.5
eggs = 9 eggs/year
– Brown-headed cowbird: an egg a day for 40-80
days = 60 eggs
– Common cuckoo: 8 eggs
– Yellow-billed cuckoo: 4-5 eggs
• A two to six-fold fecundity advantage?
Tests of the Trade-off Hypothesis
in Brown-headed Cowbirds
• Find all host nests and count
parasites/parasite eggs: to 17 eggs in 21d
J. Smith & P. Arcese, Condor 96:916-934 (1994)
• Study biology of reproduction
– Counts of ovulated follicles in ovaries: 40-70 eggs
D. Scott & C. Ankney, Auk 199:583-592 (1983)
– Captive brown-headed cowbirds can lay 72 eggs
– K. Holford & D. Roby, Condor 95:536-545 (1993)
• Use genetic parentage analysis
– Genotype female parents and all their offspring:
2.3 eggs/female
B. Woolfenden et al., Animal Behaviour 66:95-106 (2003)
Status of Trade-off Hypothesis
is Uncertain
• According to the Woolfenden study, and
a similar study by C. Hahn, brood
parasites may lay fewer eggs than
parental species
• Biases in each type of study?
• Need for further work
Our Local Parasite: the Brownheaded Cowbird
The Ultimate Host Generalist
• 221 Host species overall
• 170 Successful at raising cowbird
• About 10% of hosts are rejecters
• Many other host species desert
parasitized clutches
• Seen as conservation villain
because of spectacular range
expansion since 1800
People’s Attitudes to Cowbirds
• General public dislikes parasitic lifestyle
• Birders dislike cowbirds, because
they see them as harming other birds
• Biologists see the species
as a conservation villain
because of spectacular range
expansion since 1800 and
exposure to many new hosts
Cowbirds and Conservation
• Range has doubled
since 1750
• Prefers wooded habitats,
avoids large forests
• Can depress rare hosts,
maintain numbers on common ones
• Blamed for endangering four hosts
• Kirtland’s Warbler
• Black-capped and Least Bell’s Vireos
• SW Willow Flycatcher
• Suspected of negative
effects on other hosts
Are Cowbirds Villains?
• No hosts extirpated in range expansion
• Most endangered hosts suffer from habitat
loss/degradation
• Long history of coexistence with many hosts
HOWEVER,
• Effect of cowbirds on host numbers are poorly
known, with only one or two reliable estimates
J. Smith et al. Ecology 83:3037-3047 (2002)
• Managed more easily than other threats
• Remain attractive “targets’ for managers
Unsolved Puzzles
• How did obligate parasitism evolve?
• Why are there so few obligate parasites?
– 1% of birds
– 2 % of ants
– 1 Fish (Synodontis multipunctatus)
High extinction rates in parasitic species?
• Why is egg rejection so slow to evolve in
cowbird hosts?
Summary
• Obligately brood parasitic birds have a
highly distinctive life history
• Co-evolutionary “arms race” with hosts
• Origin of obligate parasitism is uncertain
• The local brood parasite, the Brown-headed
cowbird, has a negative public profile with
humans
• Under some circumstances can be a severe
conservation threat
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