Infanticide and Brood Infanticide and Brood Reduction in BirdsReduction in Birds

By: Michael Sweeney

Ecol 484

2008

Infanticide?

• Infanticide very common in birds.

• Unlike mammals, birds employ behavior to their advantage.

• Equally common among related birds and unrelated birds.

In Birds: Intentionally causing the death of an egg or nestling

Questions:

• What forms of infanticide exist in birds?

• What motives drive infanticide?

• How does infanticide compare across taxa?

• What ultimate reasons explain evolution of infanticide?

Forms of Infanticide

Parental vs. Non-parental infanticide

– Egg vs. Hatchling destruction– Passive vs. Active infanticide

www.sialis.org

Questions:

• What forms of infanticide exist in birds?

• What motives drive infanticide?

• How does infanticide compare across taxa?

• What ultimate reasons explain evolution of infanticide?

Motives for Avian Infanticide: (Proximate Approach)

• Paternity Influenced Infanticide

• Resource Influenced Infanticide

• Sexually Selective Infanticide

• Brood Reduction

Paternity Influenced InfanticidePaternity Influenced Infanticide

A parent may kill young of different paternity, or even of questionable paternity.

– Harem species• Bronze-winged Jacana ♀

– Monogamous species• Tropical House Wren• Palestine Sunbird

Mike Danzenbaker Robert DeCandido

Resource Influenced Infanticide

The struggle for resources and opportunities are frequently motives for killing unrelated young .

• Nest-site Limitations• Common; Tree Swallows, Starlings, Dippers

• Mating Opportunity• Humboldt Penguins, Guira Cuckoos

• Group Dominance• Mexican Jays, Acorn Woodpeckers

www.ejphoto.comwww.freewebs.com

www.animalpicturesarchive.com

Sexually Selective Infanticide

Some parents will kill young in order to preferentially sire male or female offspring.

Eclectus Parrot (Heinsohn, et al 1997).

Encyclopedia Britannica Online

Brood Reduction (David Lack, 1947)

Parents may selectively starve young to reduce size of brood depending on environmental conditions.

Core vs. Marginal offspring– Marginal offspring provide brood “insurance”

• Allows equitable nest fitness if something happens to Core offspring

Optimistic vs. Pessimistic Parenting?

David Faintich

www.holdenby.com

www.avianweb.com

Facultative vs. Obligate Brood Reduction

Brood Reduction (cont’d)Parental Mechanisms of Brood Reduction

Active:• Evicting/Killing marginal young• Starving marginal young through nepotism of larger and/or older young• Nest Desertion due to loss of core eggs/young

Passive:• Hatch order asynchronicity • Facilitating siblicide, or intentionally turning a ‘blind eye’

Hatch order asynchronicity

Questions:

• What forms of infanticide exist in birds?

• What motives drive infanticide?

• How does infanticide (Brood Reduction) compare across taxa?

• What ultimate reasons explain evolution of infanticide?

Brood Reduction Across Avian Orders

Incidence of Brood Reduction Types Across Avian OrdersFacultative Obligate Passive Active

Pelicaniformes X X X XCharadriiformes X X XAnseriformes X X XPodicepiformes X X XSphenisciformes X X X XGalliformes X XCiconiiformes X X XFalconiformes X X X XGruiformes X X X XColumbiformes X XCuculiformes X X XStrigiformes X XApodiformes X XPiciformes X X XPasseriformes X X X

Trends in Brood Reduction

• Members of perhaps every bird order engage in some sort of facultative brood reduction.

• Obligate brood reduction is common mostly in large birds.

• Active brood reduction more prevalent in shore and water birds.

• Relationship between altricial and precocial birds and forms of brood parasitism seems unclear.

Questions:

• What forms of infanticide exist in birds?

• What motives drive infanticide?

• How does infanticide (Brood Reduction) compare across taxa?

• What ultimate reasons explain evolution of infanticide (Brood Reduction)?

Why Brood Reduction? (Ultimate Approach)

• Environmental Instability– Insufficient food availability

• Physiological/Phylogenetic constraints– Ovulation speed/ timing.– Egg Size– Nutrient allocation limitations.

Brood Reduction = Fitness Fail-safe

Conclusions

• Numerous reasons motivate various types of infanticide in birds.

• Birds seem to love infanticide, and when they don’t, they may encourage young to kill each other instead.

• Parental Infanticide may be a useful ‘tool’ for maximizing yearly fitness.

Acknowledgements• Becky Stephenson• Adam Radcliffe• Christine, Glenn and JodyLee• RedBull

Brief List of Works Cited

Forbes, L.S. (1991) Insurance offspring and brood reduction in a variable environment: The costs and benefits of

pessimism. Oikos. 62(3): 325-332

Gill, Frank B. Ornithology: Second Edition. 1990. W.H. Freeman and Company: New York.

Mock D.W. (1995) The evolution of parental optimism.Trends in Ecology & Evolution. 10:130