Avian response to microclimate in canopy gaps in a bottomland hardwood forest.

Abstract - Microclimate may infl uence use of early successional habitat by birds. We assessed the relationships between avian habitat use and microclimate (temperature, light intensity, and relative humidity) in experimentally created canopy gaps in a bottomland hardwood forest on the Savannah River Site, SC. Gaps were 2- to 3-year-old group-selection timber harvest openings of three sizes (0.13, 0.26, 0.50 ha). Our study was conducted from spring through fall, encompassing four bird-use periods (spring migration, breeding, post-breeding, and fall migration), in 2002 and 2003. We used mist netting and simultaneously recorded microclimate variables to determine the influence of microclimate on bird habitat use. Microclimate was strongly affected by net location within canopy gaps in both years. Temperature generally was higher on the west side of gaps, light intensity was greater in gap centers, and relative humidity was higher on the east side of gaps. However, we found few relationships between bird captures and the microclimate variables. Bird captures were inversely correlated with temperature during the breeding and postbreeding periods in 2002 and positively correlated with temperature during spring 2003. Captures were high where humidity was high during post-breeding 2002, and captures were low where humidity was high during spring 2003. We conclude that variations in the local microclimate had minor infl uence on avian habitat use within gaps. Instead, habitat selection in relatively mild regions like the southeastern US is based primarily on vegetation structure, while other factors, including microclimate, are less important

Food abundance does not determine bird use of early-successional habitat.

Abstract. Few attempts have been made to experimentally address the extent to which temporal or spatial variation in food availability influences avian habitat use. We used an experimental approach to investigate whether bird use differed between treated (arthropods reduced through insecticide application) and control (untreated) forest canopy gaps within a bottomland hardwood forest in the Upper Coastal Plain of South Carolina, USA. Gaps were two- to three-year-old group selection timber harvest openings of three sizes (0.13, 0.26, and 0.50 ha). Our study was conducted during four bird use periods (spring migration, breeding, post-breeding, and fall migration) in 2002 and 2003. Arthropods were reduced in treated gaps by 68% in 2002 and 73% in 2003. We used mist-netting captures and foraging attack rates to assess the influence of arthropod abundance on avian habitat use. Evidence that birds responded to arthropod abundance was limited and inconsistent. In 2002, we generally captured more birds in treated gaps of the smallest size (0.13 ha) and fewer birds in treated gaps of the larger sizes. In 2003, we recorded few differences in the number of captures in treated and control gaps. Foraging attack rates generally were lower in treated than in control gaps, indicating that birds were able to adapt to the reduced food availability and remain in treated gaps. We conclude that arthropod abundance was not a proximate factor controlling whether forest birds used our gaps. The abundance of food resources may not be as important in determining avian habitat selection as previous research has indicated, at least for passerines in temperate subtropical regions

Appendix B. Number of arthropods collected per gap in treatment and control canopy gaps of three sizes in a bottomland hardwood forest in South Carolina (2002–2003).

Number of arthropods collected per gap in treatment and control canopy gaps of three sizes in a bottomland hardwood forest in South Carolina (2002–2003)

Appendix A. Mean arthropod abundance in six treated and six control canopy gaps by year at the Savannah River Site, South Carolina (2002–2003).

Mean arthropod abundance in six treated and six control canopy gaps by year at the Savannah River Site, South Carolina (2002–2003)

Appendix E. Percentage of vegetation cover and stem density in six treatment and six control canopy gaps at the Savannah River Site, South Carolina (2002–2003).

Percentage of vegetation cover and stem density in six treatment and six control canopy gaps at the Savannah River Site, South Carolina (2002–2003)

Appendix D. Number of individuals of each species (excluding recaptures) captured in experimental canopy gaps in South Carolina (2002–2003).

Number of individuals of each species (excluding recaptures) captured in experimental canopy gaps in South Carolina (2002–2003)

Appendix C. Mean arthropod abundance for all arthropods by treatment and period in six treated and six control canopy gaps at the Savannah River Site, South Carolina (2002–2003).

Mean arthropod abundance for all arthropods by treatment and period in six treated and six control canopy gaps at the Savannah River Site, South Carolina (2002–2003)