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
• “Lay” in own nest and nests of
• A common trait, especially in colonial
and cavity-nesting species (E.g.,
starlings, snow geese, cliff swallows)
• 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
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
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
• 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
• 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
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
– 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
– Host re-nests
– Parasite then lays in new nesting attempt
P. Arcese et al. Proc Nat. Acad. Sci. USA93:4608-4611 (1996)
• 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
• Long history of coexistence with many hosts
• 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?
• 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
• Under some circumstances can be a severe
conservation threat