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Non-learned defenses in naïve populations of songbirds

The hooded warbler ( Wilsonia citrine ), a bird that has relatively recently become a target of parasitism by brown-headed cowbirds, is significantly more responsive to stuffed cowbirds than to the stuffed veeries ( Catharus fuscescens ). Even young, naïve breeders were able to differentiate between the cowbird threat and the harmless veery, and this ability did not vary with the age of the birds. This suggests that the basis for determining the cowbird threat is not learned since age and experience seem to play no part in recognition of cowbirds, but is instead based primarily on a non-learned, genetic component. Given the short time in which the bird has served as host to cowbirds, it is likely that non-learned behavior has evolved quickly as a defense. If such behavior is sufficient to ward off parasites, then learning to differentiate between eggs may not even be necessary.

Discussion questions:

  1. Explain the meaning of each word in the term “obligate interspecific brood parasite.”
  2. What is the difference between a generalist and a specialist brood parasite?
  3. Why is the concept of coevolution important for understanding avian brood parasitism?
  4. How have imprinting experiments demonstrated learning among host birds?
  5. What is an explanation for rejection behavior that is not based on learning through experience?
  6. Explain a situation in which parasitic birds have been known to exploit defensive imprinting.
  7. Describe how experimenters have shown unlearned defensive strategies against parasitism.
  8. Explain how costs and benefits affect whether birds adopt an acceptor or rejecter strategy.
  9. What is the response of birds to different levels of intraclutch variation, and why is this adaptive?
  10. How do levels of parasitism affect adopted acceptor/rejecter strategies?
  11. Explain how the Mafia Hypothesis explains the importance of cost in rejection decisions by hosts.
  12. How do levels of parasitism affect the selection for learning-based defenses?


  • acceptor - In terms of avian brood parasitism, a strategy of response to foreign eggs in which a species always accepts parasitic eggs. This usually occurs when the cost of mistaken recognition and rejection are high, so it is more advantageous to accept all eggs.
  • rejecter - In terms of avian brood parasitism, a strategy of response to foreign eggs in which a species attempts to distinguish parasitic eggs from its own eggs and then reject them. In cases where the risk of mistake are low, such as in species with low intraclutch variation, learning to reject foreign eggs is favored even at low levels of parasitism.
  • obligate interspecific brood parasite - A species of bird that must parasitize nests of other species in order to reproduce. Brood parasites fool host birds into accepting their eggs and then raising the young as their own.
  • passerine - A member of the order Passeriformes, which comprises a number of small perching birds. This group makes up a majority of bird species.
  • coevolution - a system of interactions in which multiple parties exert selective forces upon each other and drive each other’s evolution.
  • mimicry - an imitation of something for purposes of protection or deception. In this context, parasitic birds have varying levels of egg mimicry; while cuckoos can have highly mimetic eggs, cowbird eggs can be extremely different from their hosts.
  • acceptance threshold - the point at which eggs satisfy a host bird’s criteria for “self” eggs and are accepted
  • intraclutch - characteristic of eggs that were laid in the same clutch. For example, intraclutch variation refers to the extent to which eggs laid in one clutch vary from each other.
  • mafia hypothesis - Sometimes, when hosts reject eggs, the parasitic birds return to depredate nests. The cost prompts some hosts to adopt an acceptor approach as it is better to accept a few parasitic chicks than to lose everything.
  • generalist - In this context, a species of bird that parasitizes many different species of hosts, such as the brown cowbird.
  • specialist - In this context, a species of bird that parasitizes specific hosts. The common cuckoo’s females each specialize on a certain species of host, though each female’s preference may be different.
  • evolutionary lag hypothesis - Hypothesis on why some birds in a species adopt acceptor strategies while others adopt rejecter strategies. It states that rejection is always adaptive but the behavior may not yet have had enough time to become fixed within the species, which is why some will still accept parasitic eggs.


  • Sorenson, M.D. and Payne, R. B. Molecular Genetic Perspectives on Avian Brood Parasitism (2002). Integrative and Comparative Biology 42(2):388-400.

    Sorensen and Payne analyzed genetic data of many avian brood parasites in regards to promiscuity and host-specificity. Cuckoos were found to be loyal parasites of several host species over evolutionarily significant time periods.

  • Marchetti, K., Nakamura, H., Gibbs, H. L. (1998). Host-Race Formation in the Common Cuckoo. Science 282( 5388):471-472.
  • Friedmann, H. and Kiff, L. F. (1985). The Parasitic Cowbirds and Their Hosts. Proceedings of the Western Foundation of Vertebrate Zoology 2( 4):225-302.
  • Anderson M. G., Moskat C., Ban M., Grim T., Cassey P., et al. (2009) Egg Eviction Imposes a Recoverable Cost of Virulence in Chicks of a Brood Parasite. PLoS ONE 4(11): e7725.
  • Hoover, J. P. and Robinson, S.K. (2007). Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs. Proceedings of the National Academy of Sciences of the United States of America. 104, 4479–4483.

    Hoover and Robinson manipulated the nests of breeding prothonotary warblers, experimentally changing egg number and accessibility. They found that nests where parasitic eggs had been removed were much more likely to experience depredation and conclude that this increased cost of rejection would prompt certain birds to be more accepting of all eggs.

  • Rothstein, SI (1975). Evolutionary Rates and Host Defenses Against Avian Brood Parasitism. The American Naturalist 109(966) 161-176.

    Rothstein provides an evolutionary explanation for why some species accept parasitic eggs while others reject them. The method by which hosts are parasitized and whether they may have surviving young will affect acceptance/rejection behavior, as some birds are more likely to accept the parasitic eggs and cut losses.

  • Vogl W, Taborsky M, Taborsky B, Teuschl Y, Honza M (2002). Cuckoo females preferentially use specific habitats when searching for host nests. Animal Behaviour 64 843-850 Part 6.

    Vogl et al. describe how cuckoo females imprint on habitats that they grow up in and parasitize birds in similar habitats later, developing specificity to certain hosts to which their egg appearances may be better adapted.

  • Sealy, S. G., Bazin, R. C. (1995). Low-Frequency of Observed Cowbird Parasitism On Eastern Kingbirds - Host Rejection, Effective Nest Defense, Or Parasite Avoidance. Behavioral Ecology. 6(2) 140-145.
  • Lotem, A., Nakamura, H. and Zahavi, A. (1992). Rejection of cuckoo eggs in relation to host age: a possible evolutionary equilibrium. Behav. Ecol. 3: 128-132

    One of the first studies relating host age with rejection rates of parasitic cuckoo eggs, Lotem et al. manipulated the clutches of reed warblers and found that inexperienced breeders were more likely to accept false eggs and less likely to abandon the clutch if one of their own eggs was returned. The authors argue for a imprinting-based learning mechanism.

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  • Strausberger, B.M., Rothstein, S.I. (2009). Parasitic cowbirds may defeat host defense by causing rejecters to misimprint on cowbird eggs. BEHAVIORAL ECOLOGY 20(4) 691-699.

    Strausberger and Rothstein investigated the effect of parasitizing early in the laying stage and found that some species were more likely to accept parasitic eggs as their own, especially in places where parasite frequency was high. This led them to conclude that the hosts were induced to mistakenly imprint on cowbird eggs.

  • Moskat, C., Hauber, M.E. (2007). Conflict between egg recognition and egg rejection decisions in common cuckoo (Cuculus canorus) hosts. ANIMAL COGNITION 10(4) 377-386.
  • Marchetti, K (2000). Egg rejection in a passerine bird: size does matter. ANIMAL BEHAVIOUR. 59 877-883 Part 4.
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  • Hoover JP, Yasukawa K, Hauber ME(2006). Spatially and temporally structured avian brood parasitism affects the fitness benefits of hosts' rejection strategies. ANIMAL BEHAVIOUR 72:881-890 Part 4
  • Hauber ME, Yeh PJ, Roberts JOL (2004). Patterns and coevolutionary consequences of repeated brood parasitism. PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON SERIES B-BIOLOGICAL SCIENCES 271:S317-S320
  • Lawes MJ, Marthews TR (2003). When will rejection of parasite nestlings by hosts of nonevicting avian brood parasites be favored? A misimprinting-equilibrium model. BEHAVIORAL ECOLOGY 14(6) 757-770.
  • Honza M, Prochazka P, Stokke BG, Moksnes A, Roskaft E, Capek M, Mrlik V (2004). Are blackcaps current winners in the evolutionary struggle against the common cuckoo? JOURNAL OF ETHOLOGY 22(2):175-180.
  • Langmore NE, Hunt S, Kilner RM (2003). Escalation of a coevolutionary arms race through host rejection of brood parasitic young. NATURE 422(6928):157-160
  • Grim T (2002). Why is mimicry in cuckoo eggs sometimes so poor?. JOURNAL OF AVIAN BIOLOGY. 33(3):302-305
  • Rodriguez-Girones, MA; Lotem, A (1999). How to detect a cuckoo egg: A signal-detection theory model for recognition and learning. AMERICAN NATURALIST. 153(6):633-648.
  • Soler JJ, Martinez JG, Soler M, Moller AP (1999). Genetic and geographic variation in rejection behavior of cuckoo eggs by European magpie populations: An experimental test of rejecter-gene flow. EVOLUTION. 53(3):947-956.
  • Lotem A (1993). Learning To Recognize Nestlings Is Maladaptive For Cuckoo Cuculus-Canorus Hosts. Nature. 362 (6422):743-745.
  • Sharon A. Gill, Diane L. Neudorf And Spencer G. Sealy (1997). Host Responses To Cowbirds Near The Nest: Cues For Recognition. Animal Behaviour 53(6):1287-1293
  • Davies NB, Brooke MDL, Kacelnik A (1996). Recognition errors and probability of parasitism determine whether reed warblers should accept or reject mimetic cuckoo eggs. PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON SERIES B-BIOLOGICAL SCIENCES 263(1372):925-931
  • May Rm, Robinson Sk (1985). Population-Dynamics Of Avian Brood Parasitism. American Naturalist 126(4):475-494.

About the author

a photo of the author.

Mary Mao grew up as the daughter of immigrants in the small town of Oxford, Mississippi. It was in this rural setting that she was first exposed to wildlife in the surrounding woods and forests and became interested in the ecosystems in which local fauna and flora. She now resides in Houston, Texas as an undergraduate at Rice University majoring in Biochemistry and Cell Biology and hopes to attend medical school after graduation in spring 2010.

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Source:  OpenStax, Mockingbird tales: readings in animal behavior. OpenStax CNX. Jan 12, 2011 Download for free at http://cnx.org/content/col11211/1.5
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