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Are Flashy Male Birds Threats to Their Own Species?

John Roach
for National Geographic News
April 8, 2003
 
For bird species whose males and females differ in color, guys with the brightest feathers tend to have the greatest lady luck. This natural mating game however puts entire local populations at risk of dying out, according to a new study.

The finding confirms the idea that the extraordinary lengths an animal will go to woo a mate, such as the peacock who spends time and energy to maintain his extravagant tail feathers, comes at a price in terms of survival.

"We found that to be true," said Paul Doherty, a wildlife ecologist at Colorado State University in Fort Collins who co-authored the study appearing in the current issue of Proceedings of the National Academy of Sciences.

This idea is a key component of the sexual selection theory put forth by Charles Darwin in 1871 to explain the evolutionary process that favors seemingly outlandish adaptations.


This behavior at first confused Darwin, who found such extravagant displays to fly in the face of natural selection in which the strongest survive. Traits such as bright feathers take time and energy to maintain and make the birds more vulnerable to predators.

Darwin's theory on sexual selection suggests the purpose of these traits is to allow a species to find a mate and thus pass on its genes to the next generation. In theory, this potential to carry one's genes forward outweighs the cost to individual longevity.

"Much of the theory of sexual selection depends on there being a cost, such that only the best individuals can display or have the best display," said Doherty. "Lesser-quality individuals either die or cannot bear the time or energy costs and get little reproductive success."

And, according to the study, although extravagant displays are costly in terms of extinction, bird species are able to make up for the loss of local population by quickly re-colonizing the area.

"You have species perhaps winking out more frequently, but at least in North American populations they seem to be coming back again by colonization events," said Jim Nichols, a study co-author with the U.S. Geological Survey Patuxent Wildlife Research Center in Laurel, Maryland.

Nils Chr. Stenseth and Glenn-Peter Saetre of the University of Oslo in Norway write in an accompanying perspective that the insight on bird populations gained from this study is "both novel and highly interesting from an ecological point of view."

Breeding Bird Survey

The study by Doherty and colleagues is based on statistical analysis of information recorded over the past 21 years by thousands of volunteers who participate in the North American Breeding Bird Survey, an annual bird count.

The volunteers wake at dawn during the height of the North American bird breeding season and drive a pre-determined 24.5 mile (39.4 kilometer) back-road route with stops at 0.5 mile (0.8 kilometer) intervals. At each stop the volunteers count all the birds they can see or hear.

The survey was designed to give researchers an annual index of abundances for each species, said Keith Pardieck, national coordinator for the breeding bird survey at the Patuxent Wildlife Research Center.

"You have indices that can be compared through time, for example to see where robins are increasing or decreasing," he said. This data, he added, allows researchers to devise conservation strategies for specific species.

Doherty and colleagues took the breeding bird survey data and looked at in a way that allowed them to ask questions about bird populations at the community level.

The researchers analyzed the bird species within six orders that are active during the day and thus readily detected by the survey. Each individual species was placed either into the community of having males and females of different color (dichromatic) or males and females of the same colored feathers (monochromatic).

According to the researchers' analysis of this data, bird species whose males and females differ in color have a 23 percent higher local extinction rate than do species whose males and females do not differ in color.

Despite the local extinction pressure, the total number of dichromatic species did not decrease over the 21-year study period. Rather, the researchers found that the local extinction rates were matched by local species turnover rates.

"There is a pretty good correlation. Where extinction rates are highest, turnover rates are highest," said Doherty.

Bird Conservation

This finding suggests that as a local population of a dichromatic species begins to decline another population moves in and takes over the habitat, keeping the species population stable.

Based on these results, Doherty and colleagues suggest that human activities that restrict the movement of bird populations from one habitat to another, such as logging or suburban sprawl, may put global populations of dichromatic bird species at risk of extinction.

"Colonization is dependent on movement and dispersal, so anything that would disrupt the free flow of individual birds from one local community to another could have deleterious effects," said Doherty.

Nichols said that one of the next steps in the research is to devise a methodology that permits researchers to "ask questions at a species-specific level" rather than the community level and thus get an idea of what species might be at risk.

Another interesting point raised by the study that Stenseth and Saetre say is worthy of further investigation is the regional differences in extinction and turnover rates across North America.

For example Southern Texas and the prairie region of the north-central part of the continent have much more stable community structures than other regions. There is little difference in extinction and turnover rates between dichromatic and monochromatic species in these regions.

"A major challenge for future studies would be to unravel the underlying ecological processes that determine the relative stability of communities," write Stenseth and Saetre.

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National Geographic Bird Resources:
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Recent "Birder's Journal" Stories from Robert Winkler:
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Additional Information from Related Web Sites:
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Environmental Protection Agency: Bird Conservation
 

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