Sunday, 3 July 2011

Forest birds avoid territories with noise pollution

(Manassas National Battlefield, Manassas, Virginia, USA)

As any animal-watcher knows, different species have different preferences when it comes to habitat. Territory choice is usually driven by resource availability--animals need to find habitat where they can locate, for instance, preferred food items and nesting places. However, a recent study by researchers at the University of Delaware has shown that at least 2 species of birds choose their habitat based on amount of nearby traffic--or lack thereof.

The study was conducted in the Prince William Forest National Park and Manassas National Battlefield, both located in Prince William County, northern Virginia. In each park, researchers conducted surveys in order to determine occupancy rates of 8 breeding bird species: Acadian flycatcher, Empidonax virescens; Carolina wren, Thryothorus ludovicianus; great-crested flycatcher, Myiarchus crinitus; ovenbird, Seiurus aurocapilla; scarlet tanager, Piranga olivacea; white-breasted nuthatch, Sitta carolinensis; wood thrush, Hylocichla mustelina; and yellow-billed cuckoo, Coccyzus americanus). They focused their efforts on 15 roadside habitats and 15 mid-forest habitats in each of the 2 sites. In order to see whether the presence/absence of the birds was influenced by noise levels, researchers measured ambient noise levels and then examined recordings of each species' songs in order to see which vocalizations overlapped most with noise generated by passing traffic; they hypothesized that birds with the most-overlapped songs would be least common in sites near roads.

(Entrance to the Prince William Forest Park, Virginia, USA)

Of all 8 species surveyed, only 2 seemed to directly alter their habitat use based on the presence of noise: the white-breasted nuthatch and the yellow-billed cuckoo, both of which were approximately 10 times less likely to occur in noisy plots. These were also the two species whose songs were most overlapped by traffic noise, suggesting that the birds deliberately avoided roadside areas because they were aware that it would be more difficult for them to communicate there.

(White-breasted nuthatch, Sitta carolinensis)

(Yellow-billed cuckoo, Coccyzus americanus)

These results support findings of previous studies that have examined occupancy rates in other noisy sites, such as patches of forest around natural gas extraction machinery. What remains to be explored is the process by which animals pick and choose their habitats in areas near noise pollution. For instance, do they seek areas that have the quietest level of noise relative to the surrounding habitat, or are they happy with anything that is below a certain threshold--around 45 dB, perhaps, given that traffic noise here was found to range from 44-57 dB? Since we now know that some birds can adjust their songs in response to noise, we have to wonder whether is there any possibility that these species have enough vocal flexibility to find a way to utilize louder habitats--or are those areas completely off-limits as long as there is human traffic nearby?

Goodwin, S.E. and Shriver, W.G. Effects of traffic noise on occupancy patterns of forest birds. 2011. Conservation Biology 25(2):406-411.

Thanks to the following websites for providing the photos used in this post:





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