04 June 2009

Familiarity breeds contempt: Mockingbirds recognize and react to repeat intruders

ResearchBlogging.orgHumans are a fact of life for many, many parts of the natural world. This doesn't always have to be a bad thing -- some critters adapt to human-dominated landscapes pretty well. A paper in this week's PNAS, for instance, shows that Northern Mockingbirds nesting on a busy university campus learn to differentiate between uninterested passers-by and people who repeatedly disturb the nest site [$-a].


Northern Mockingbird with fledglings
Photo by mjmyap.
When potential predators come too close to a nest, wild birds try to distract the threat with harassing alarm calls, diving attack flights, or "flushing" to draw attention away. This doesn't work so well if you're nesting near a busy sidewalk; you'd spend all you time trying to drive off passers-by who pose no real threat. And, as Levey et al. show in the new paper, mockingbirds seem to have solved this problem by reacting more strongly to people who approach the nest repeatedly.

The experimental evidence is elegant: Individual researchers approached occupied mockingbird nests on four consecutive days, and recorded the birds' reactions. The birds flushed farther, gave more alarm calls, and attacked more often with each repeat visit. When a new person approached the nest on a fifth day, though, the birds' reactions were equivalent to their behavior on the first day. Furthermore, nesting mockingbirds flushed farther if their nests were near less-busy sidewalks. This isn't evidence for an evolved response, but for learning; and it suggests that mockingbirds are able to recognize individual humans, and apply that familiarity in assessing the danger posed when someone approaches the nest.

This is just one example of the evolved and learned adaptations the living world has made in response to human activity. Last year, for instance, a study showed that a French wildflower has evolved wingless seeds in response to urban growing conditions -- although winged, wind-dispersed seeds do better in the wild because they're less likely to compete with their siblings, in a heavily paved environment the best spot to germinate is more likely to be close to the parent plant. And perhaps one of the best-known examples of natural selection in action is the increased frequency of dark-colored peppered moths in response to industrial pollution. Nature is nothing if not flexible.

References

Cheptou, P., Carrue, O., Rouifed, S., & Cantarel, A. (2008). Rapid evolution of seed dispersal in an urban environment in the weed Crepis sancta Proc. Nat. Acad. Sci. USA, 105 (10), 3796-9 DOI: 10.1073/pnas.0708446105

Grant, B.S., Owen, D.F., & Clarke, C.A. (1996). Parallel rise and fall of melanic peppered moths in America and Britain Journal of Heredity, 87, 351-7 DOI: http://jhered.oxfordjournals.org/cgi/content/abstract/87/5/351

Levey, D., Londono, G., Ungvari-Martin, J., Hiersoux, M., Jankowski, J., Poulsen, J., Stracey, C., & Robinson, S. (2009). Urban mockingbirds quickly learn to identify individual humans Proc. Nat. Acad. Sci. USA, 106 (22), 8959-62 DOI: 10.1073/pnas.0811422106

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