Birds provide one of the most common ways to encounter nature in cities, but few people wonder how their actions affect birds looking for love. A new study published in the journal Ecology and Evolution this month suggests that changes in suburban environments can affect the number of breeding pairs, who mates with whom, and even shape how a species evolves. The study tracked gray catbirds nesting in Washington, D.C.—area parks and in local backyards participating in a citizen science project called Neighborhood Nestwatch, which is run by the Smithsonian Conservation Biology Institute’s Migratory Bird Center.
Gray Catbird by Dan Garber
"When humans modify habitats, like in their own backyards or in cities, we are changing the canvas upon which evolution works," said Pete Marra, one of the study’s authors and an SCBI research scientist. "This paper provides evidence that evolutionary processes are likely changing right under our noses as we change habitats."
Native birds are often faced with a variety of human-modified habitats and many species have adjusted to living in these environments, said Thomas Ryder, the study’s lead author and a research ecologist at SCBI. Ryder expected to find that human modifications of habitat might influence mating patterns and the traits (such as feather color and body size) that are important in mate choice. For example, parks, which are larger and feature contiguous habitat, might be more conducive to mating than backyards, which are often patchier and provide limited nesting habitat. The researchers found that it was not habitat, per se, but the number of breeding pairs that influenced both mating patterns and the evolution of male traits.
Catbirds, like many other bird species that scientists consider "socially monogamous," actually rely on a time-tested method for improving their reproductive success: they seek mating opportunities outside their social pair-bond. The researchers found that male catbirds that settled in areas with more breeding pairs had higher mating success because of these "extra" mating opportunities. That means that when humans cut down trees, build houses, and take other actions that lower the density of bird populations, they can diminish reproductive opportunities and alter the course of evolution for specific traits.
"Through our actions, we’re influencing their reproductive strategies," Ryder said. "It’s really important to think about how evolution plays out in human-dominated contexts because we know very little about it; scientists usually study evolution in ideal environments. In environments altered by humans, there is this inevitable cascade of ecological changes that can influence evolution."
The Neighborhood Nestwatch program is designed to involve citizens in bird research from the comfort of their own backyards. Scientists visit the participants to band birds for identification purposes and train residents to observe and report bird behavior. D.C. area residents interested in joining the program can learn more on the Migratory Bird Center website.
In addition to Ryder and Marra, the paper’s other SCBI author is Rob Fleischer, head of SCBI’s Center for Conservation & Evolutionary Genetics. The other author on the paper is Greg Shriver from the Department of Entomology and Wildlife Ecology at the University of Delaware, Newark.
The Smithsonian Conservation Biology Institute plays a key role in the Smithsonian’s global efforts to understand and conserve species and train future generations of conservationists. Headquartered at a Smithsonian facility in Front Royal, Va., SCBI facilitates and promotes research programs based at Front Royal, the National Zoo in Washington, D.C., and at field research stations and training sites worldwide.
This article summarizes the information in this publication:
Thomas B. Ryder, Robert C. Fleischer, W. Greg Shriver and Peter P. Marra. 2012. The ecological–evolutionary interplay: density-dependent sexual selection in a migratory songbird. Ecology and Evolution.
Little is understood about how environmental heterogeneity influences the spatial dynamics of sexual selection. Within human-dominated systems, habitat modification creates environmental heterogeneity that could influence the adaptive value of individual phenotypes. Here, we used the gray catbird to examine if the ecological conditions experienced in the suburban matrix (SM) and embedded suburban parks (SP) influence reproductive strategies and the strength of sexual selection. Our results show that these habitats varied in a key ecological factor, breeding density. Moreover, this ecological factor was closely tied to reproductive strategies such that local breeding density predicted the probability that a nest would contain extra-pair offspring. Partitioning reproductive variance showed that while within-pair success was more important in both habitats, extra-pair success increased the opportunity for sexual selection by 39% at higher breeding densities. Body size was a strong predictor of relative reproductive success and was under directional selection in both habitats. Importantly, our results show that the strength of sexual selection did not differ among habitats at the landscape scale but rather that fine-scale variation in an ecological factor, breeding density, influenced sexual selection on male phenotypes. Here, we document density-dependent sexual selection in a migratory bird and hypothesize that coarse-scale environmental heterogeneity, in this case generated by anthropogenic habitat modification, changed the fine-scale ecological conditions that drove the spatial dynamics of sexual selection.
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