What are we missing with only ground-level mist nets? Using elevated nets at a migration stopover site

Abstract

Mist nets deployed in a standard ground-level fashion capture birds approximately 0.5-2.6 m above the ground. In habitats where the vegetation extends above this height, standard mist deployment may inadequately sample the targeted avian community and age- and sex-classes within species. Such sampling biases may raise questions regarding studies based on data from mist-net captures. To determine if birds were equally likely to be captured by mist nets at different heights, we constructed a series of paired ground-level and elevated mist nets (hereafter net rigs) at a research station in western New York State. Net rigs were operated during 14 migration seasons from 2000 to 2006 (spring and fall each year), and 19,735 birds of 118 species were captured. Capture rates were significantly higher in ground-level nets, but 12 species were only captured in elevated nets. Of 44 species with at least 50 captures, 25 species were more likely to be captured in the ground-level nets and two species in the elevated nets. For four of 18 species, more birds were captured in the elevated nets during fall migration than during spring migration. We conclude that standard ground-level placement was more efficient in capturing birds in the secondary growth habitats that we sampled. However, ground-level nets may not adequately sample the entire targeted community or all age- or sex-classes within species. SINOPSIS. Que informaci´ on estamos perdiendo con el uso de redes de niebla al nivel del suelo. Uso de redes elevadas en un punto de parada de migratorios Redes de niebla colocadas de forma estandar capturan aves que vuelan entre 0.5 y 2.6 m, sobre el nivel del suelo. En hdonde la vegetacise extiende sobre la altura previamente indicada, las redes de niebla colocadas de forma estandar, pudieran muestrear inadecuadamente la comunidad de aves y la estructura de edad y sexo en las poblaciones. Estos sesgos de muestreo pueden levantar interrogantes sobre estudios que utilizen de forma usual redes de niebla. Para determinar si las aves eran capturadas en igualdad de condiciones, con redes colocadas a diferentes alturas, colocamos redes contiguas, a la altura cotidiana y otras elevadas en una estacide investigaci´ on en el estado de Nueva York. De 2000-2006 operamos redes elevadas (durante la primavera y el oto˜