Dissertation

Squirrels in suburbia: the avian impacts of urban grey squirrels sciurus carolinensis

Bonnington, Colin

2013

Overview

The grey squirrel Sciurus carolinensis is one of the most invasive species globally, mainly due to its impact on the native red squirrel Sciurus vulgaris. However, anecdotal evidence suggests that the grey squirrel may also have a negative impact on birds through its role as nest predator and competitor for food resources. Furthermore, recent evidence suggests that the grey squirrel abundance may be related to the nest failure rate of some woodland song birds. The grey squirrel is considered as being abundant in urban environments within its non-native range, but studies investigating grey squirrel habitat use and impact on birds within these environments is surprisingly lacking. This thesis is the first to determine the urban habitat use of grey squirrels in its non-native range, and to investigate the species’ role as a nest predator and food competitor to song birds. I found that habitat characteristics, including canopy cover and the proportion of seed-bearing trees are important in influencing squirrel habitat use and density; with bird feeders important only in the winter. I provide evidence that although squirrels are likely to depredate song bird nests, and are identified as a potential threat by nesting parent birds, they do not influence the nesting territories, clutch size or the overall nest failure rates of song birds. Furthermore, comparisons with other potential nest predators determined that corvids and domestic cats Felis catus have a greater influence on nesting birds, including rates of nest failure. The final element of this thesis (food competition) found that grey squirrels do displace birds from supplementary feeders, although the magnitude of this displacement (in terms of interference competition) depends on factors such as the number of alternative supplementary feeders and the number of grey squirrels. Overall my findings suggest that presently grey squirrels are having a limited influence on urban song birds, in terms of both nest predation and food competition; with negligible evidence that squirrels are suppressing avian populations. This thesis is a preliminary investigation into urban grey squirrels within their exotic range, and the findings should be used as a basis for further studies into the issue. Future research to determine any developing associations between grey squirrels and urban avian assemblages are important, particularly when one considers that the grey squirrel exotic range is rapidly extending across Europe, with populations of urban grey squirrels expected to rise.

• Publisher: ProQuest Dissertations & Theses
• Subject: Ecology