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Brood Parasitism: An Alternative Life History N. B. Davies. 2000. Cuckoos, cowbirds and other cheats. Cambridge Univ. Press.

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Presentation on theme: "Brood Parasitism: An Alternative Life History N. B. Davies. 2000. Cuckoos, cowbirds and other cheats. Cambridge Univ. Press."— Presentation transcript:

1 Brood Parasitism: An Alternative Life History N. B. Davies Cuckoos, cowbirds and other cheats. Cambridge Univ. Press

2 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 A few birds lack any “normal” parental care Parental care can be stolen Dominion Power

3 Brood Parasitism The laying, or physical transport, of eggs of one species into the nest of a second species, where they receive parental care

4 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

5 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

6 Cuckoos 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

7 Honeyguides Old World Related to woodpeckers Eat wax (bees nests) Hatchlings kill host chicks with hooked beak Guide honey badgers/people to bees Walter Weber Ian Jackson

8 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

9 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

10 Black-headed Duck Family Anatidae Southern South America Parasitizes other waterfowl, e.g. coots Ducklings hatch before hosts Independent at hatching Ducklings need no parental care

11 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?

12 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

13 Adaptations of Brood Parasites - 1 Rapid laying (often when host absent ) Short incubation period –Brown-headed cowbird eggs hatch in days vs 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: (1999) Predatory behaviours by – Adult great-spotted cuckoos, cowbirds – Nestling common cuckoos, honeyguides

14 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: (1993)

15 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

16 Why Prey on Host Nests? The “Mafia” hypothesis Parasite “punishes” rejecting hosts: –Experimental evidence in magpies J. Soler et al. Evolution 49: (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: (1996)

17 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 days = 60 eggs –Common cuckoo: 8 eggs –Yellow-billed cuckoo: 4-5 eggs A two to six-fold fecundity advantage?

18 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: (1994) Study biology of reproduction –Counts of ovulated follicles in ovaries: eggs D. Scott & C. Ankney, Auk 199: (1983) –Captive brown-headed cowbirds can lay 72 eggs –K. Holford & D. Roby, Condor 95: (1993) Use genetic parentage analysis –Genotype female parents and all their offspring: 2.3 eggs/female B. Woolfenden et al., Animal Behaviour 66: (2003)

19 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

20 Our Local Parasite: the Brown- headed Cowbird

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22 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

23 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

24 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

25 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: (2002) Managed more easily than other threats Remain attractive “targets’ for managers

26 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?

27 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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