I just came across a recently-published paper (see citation below for Perlut et al. 2008) that documents some of the surprising ways in which human-caused changes in habitat can impact birds. In Bridget Stutchbury’s book, “Silence of the Songbirds” (on Amazon.com) that I mentioned last week, she explains that birds need neighborhoods not just habitat. By that she means that they need to breed in places where there are lots of others of their species breeding close by.
It turns out that in most bird species, both males and females try to increase their breeding success and offspring quality by mating with the highest quality birds around themâ€”often not just their mate. When forests are fragmented, it becomes harder and more dangerous to fly from fragment to fragment to assess and mate with other birds thereby disrupting mating systems.
The authors of the recent paper were able to show that in the case of their study populations of Savannah Sparrow, hay harvesting changed the potential evolutionary pressures by changing the social system (more polygynous pairings in undisturbed habitats as compared to more monogamous pairings in cut-over hayfields).
To quote from their paper:
“These changes doubled the strength of sexual selection, and as a consequence, potentially altered evolutionary processes of the population.”
The traditional view is that habitat loss and fragmentation can decrease the number of birds that survive and reproduce because of impacts like decreased resources and increased predators. Now we find that the social systems can be disrupted causing not just decreases in numbers but potentially some profound changes in evolutionary pressures and genetic variation in the species.
If you want to get all the details about why and how you may want to read:
Perlut, N.G., C.R. Freeman-Gallant, A.M. Strong, T.M. Donovan, C.W. Kilpatrick, and N.J. Zalik. 2008. Agricultural management affects evolutionary processes in
a migratory songbird. Molecular Ecology 17:1248-1255.
Stutchbury, B. 2007. Silence of the Songbirds. Walker Publishing Company, NY. (chapter 9-Living On the Edge).
On another note, I spoke about my new book “Birder’s Conservation Handbook: 100 North American Birds at Risk” on WICN Public Radio on Sunday, June 8 - click here to listen to my interview.