Studies have been conducted to study the impact of
urbanization of habitat fragmentation of specific bird species, the Blue Jay
and the Golden-cheeked Warbler. This urbanization has caused some bird communities
to be more susceptible to nest predation and nest parasitism (other birds
stealing existing bird nests). It’s been identified that the Blue Jay, among
other species, is a potentially important nest predator on open-nesting
songbirds in fragmented habitats. The increased
urbanization introduces the Blue Jay into Warbler’s habitat.
Urban Development
-> Blue Jay presence -> Golden-cheeked Warbler Absence
To assess this hypothesis of the negative correlation
between Blue Jays and Golden-cheeked Warblers, Engels and Sexton chose 14
tracts of land for their consensus, and waited to watch and listen for the
species of bird that were present in specific time intervals.
Here is a bar graph showing the amount of consensus points that recorded Golden-cheeked Warblers and Blue Jays in 1991 and 1992.
The results showed that there was a significant
negative correlation between the Blue Jays and Warblers. The Warblers only
breed in mature juniper-oak woodlands along the Balcones Escarpment of the
Edwards Plateau in central Texas. Recent development has resulted in the
wide-spread loss and fragmentation of these woodlands. As a result, the overall
population size of the Golden-cheeked Warblers has declined over the past few
decades. The density of the warblers near other certain urban areas has also
declined, suggesting that the warbler is being affected by factors other than
the urbanization. Although there is a significant negative correlation between
the two birds, this clearly does not establish causation. Many birds may be
responsible for stealing eggs and nests of the warblers. It is speculated that
the foraging activities of Jays over and through the canopy of the warblers’
juniper-oak woodland habitat may be enough to deter successful territorial
establishment by male Golden-cheeked Warblers (which defend territories from
high song perches) or may inhibit successful attraction of mates.
Engels, T. M., & Sexton, C. W. (1994). Negative correlation of blue jays and golden-cheeked warblers near an urbanizing area. Conservation Biology, 8(1), 286-290.
What I really find interesting about this article is the extreme difference between Warblers only, Jays only, neither species, and both species. I think it's also worth noting that while there were relatively minor tracts of land that had both Warblers and Jays, this number did increase between 1991 and 1992, which perhaps shows the affect of urbanization clearing out the land of Warblers and forcing them to reside in the same areas as the Jays.
ReplyDeleteYou stated that increased urbanization has introduced Blue Jays into the habitat of Warblers, which I find particularly interesting. I am curious about the further effect of human interaction in these environments. Did urbanization or possibly sub-urbanization decrease the existence of some predators, while simultaneously leading to an increase in the existence of other predators? In what way does human action directly impact these relationships by car usage or home building?
ReplyDelete