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.