Obligate Brood Parasitism: occurs in 90+ species, has evolved 7 times

1. Obligate Brood Parasitism: occurs in 90+ species, has evolved 7 times

2. Brood Parasites and Their Hosts • Old World and New World Cuckoos: host = passerines • Whydahs, Indigo Birds and Parasitic Weaver: host = related finches • Honeyguides: host = other cavity-nesters in same order (woodpeckers, barbets) • Cowbirds: hosts = other passerines • Black-headed Duck: host = coots

3. Obligate brood parasites cannot reproduce without hosts Some cuckoos feed their own fledglings, no other parental care performed by obligate brood parasites

4. Evolutionary Pathways to Obligate Brood Parasitism • Intraspecific to facultative interspecific to obligate interspecific brood parasitism • Intraspecific egg dumping is widespread • Facultative interspecific parasitism is less common, occurs cuckoos and ducks • Nest parasitism to obligate brood parasitism • Nest parasitism occurs cowbirds, weavers

5. Factors Affecting Payoff of Brood Parasitism Option • Specialized foods may make parasitic species poor parents • Toxic caterpillars cuckoos • Beeswax honeyguides • Nest predation may favor spreading eggs among several nests • Host behavior not a barrier, parasites exploit strong host response to own eggs, young

6. Brood parasitism is an antagonistic evolutionary interaction between species Parasites benefit, hosts suffer

7. Adaptations of Parasites • Develop faster than host • Shorter incubation, hatch before host young • Grow faster after hatching • Hold eggs in oviduct before laying to allow embryo to develop • Also enables parasites to hatch first

8. End result: parasites outcompete host young for food (cuckoos, cowbirds) Weavers replace host egg with one of own, host raises parasite plus (one less of) own young

9. Cuckoo chicks flip host eggs, young out nest • Honeyguide chicks (and a few cuckoos) stab host young to death using hook on bill

10. Additional Points • Lack of relatedness of parasite chick to nestmates promotes harmful behaviors • Brood parasites lay eggs in clutches • Lay more total eggs than “normal” birds, success of eggs less because host defenses sometimes work

11. Defensive Adaptations of Hosts & Counter-adaptations of Parasites • Aggressive to female parasites near nest • Scout nests, sneak into nests, lay quickly • Few cuckoos mimic hawks, male induces mobbing while female sneaks into nest • Egg recognition – reject parasite eggs • Not normally present in birds, evolves in response to parasitism

12. Respond to parasite egg by removing the egg, abandoning the nest, or covering over clutch

13. Counter-adaptation is egg mimicry

14. Recognize young – refuse to feed parasitic young • Mimicry of young

15. Whydahs mimic mouth markings of finch hosts, some cuckoos mimic general appearance of hosts

16. Adverse Impacts of Brood Parasites on Host Populations • Brown-headed Cowbirds have decimated populations of Kirtland’s Warbler, Wood Thrushes & other species in eastern US • Cowbird population explosion due to increased food increases impact of parasite

17. Shiny Cowbirds threaten Yellow-shouldered Blackbird populations in Puerto Rico • Host has no evolutionary history with, defenses against, recent invader

18. Normally Hosts and Parasites Coexist • Parasites are uncommon compared to hosts, numbers regulated by food supply • Long history of interaction with parasite leads to host defenses that protect host • Parasites and hosts coevolve, become more tightly linked over time • Each exerts evolutionary pressure on the other, neither “wins”

19. Example of Coevolution • Finches evolve recognition of young, whydahs evolve mimicry of finch young • Whydah species can only mimic one finch species, causing host specialization • Each whydah has only one finch host, each finch has only one whydah parasite

20. Are cases where different races of the same parasite species specialize on different hosts:(1) indigo bird races mimic young of different finch houses (sympatric speciation?)(2) Common Cuckoo races mimic eggs of different passerine hosts

21. Cowbirds illustrate that more recently evolved parasites exhibit less coevolution • Brown-headed Cowbird most recent species, has 200+ hosts whose defenses range from none to egg recognition • The oldest parasitic species, the Screaming Cowbird, has only 1 host, another (non-parasitic) cowbird

23. Coevolution of Giant Cowbirds and Chestnut-headed Oropendolas • Oropendolas tolerate cowbirds when parasitism improves reproductive success • Cowbird chicks remove botfly larvae from oropendola chicks • Oropendolas attack cowbirds, reject their eggs when parasitism reduces reproductive success • Nest near wasps, wasps protect from botflies • Cowbirds mimic oropendola eggs

24. Black-headed Duck • Commensalism rather than parasitism • Neutral for coots, + for duck • Ducklings leave nest after hatching, require no parental care • Trivial cost to coots (incubating duck eggs) • No host defenses have evolved in coots

25. Mating Systems of Brood Parasites • Some are monogamous, defend territories that include host nests (cuckoos, cowbirds) • Some exhibit resource defense polygyny • Little value to male parental care • Honeyguides defend bee nests • Some exhibit female defense polygyny (whydahs) • Females have no nest, travel in groups