Migration monitoring operations traditionally use ground-level mist nets to capture birds. These nets are likely biased towards species that are active at the ground and shrub levels. As such, they may not adequately sample the avian community at a stopover site as many species tend to focus their activities above the height of the nets. In an effort to examine this potential sampling bias, we developed our elevated net project. Elevated nets are two standard mist nets -- one stacked on top of another -- on one set of poles. By stacking the nets, we now can sample from ground-level to approximately 6 meters above ground. We currently operate 6 elevated net assemblies during spring and fall migration, in addition to our standard array of ground-level nets.

Data from 1999 revealed that capture rates were significantly higher in the ground-level nets, although these nets failed to detect 5 uncommon species that were captured in the elevated nets. Ten species were significantly more likely to be captured in the ground-level nets than in elevated nets. No species were significantly more likely to be captured in the elevated nets. No significant age- or sex-related differences in capture height were detected. However, daily and seasonal shifts in capture height were detected for several species. We conclude from these preliminary analyses that standard ground-level net placement is more efficient in the secondary growth habitats that we sample, but may not adequately sample the entire avian community. Further data analyses are under way to examine the efficacy of the elevated net sampling method. 



The vertical distribution of captures in 1999 is represented in the chart above. Panel 1 is the bottom level of the lower net, located just above ground level. Panel 8 is the top of the elevated net, located at approximately 5.5 meters above the ground. Data are pooled from 5 elevated net assemblies, and show the highest capture rates in panel 2 (approximately knee-high to waist-high).