| Titre : | Methodical aspects of investigating migratory activity in Emlen-funnels |
| Auteurs : | F. Nievergelt, Auteur ; F. Liechti, Auteur |
| Type de document : | Article |
| Dans : | Journal of Ornithology (141(2) APR 2000) |
| Article en page(s) : | 180-190 |
| Langues: | Anglais |
| Résumé : | A widely applied method of experimentally determining migratory activities and directions is the so called Emlen-funnel cage, using typewriter correcting paper (Tipp-Ex(C)) for the registration. We investigated methodical biases, with respect to its construction, environmental condition and the determination of activity and mean direction. The experimental birds were slightly attracted by the joint of the Tipp-Ex(C) paper. Small songbirds left two different kinds of claw marks: clearly visible ones (cover of the typewriter correcting paper scratched off), and vague ones, which only can be detected in appropriate light conditions (cover not scratched off but pressed). Small songbirds (e. g. Reed Warblers Acrocephalus scircapeus. Willow Warblers Phylloscopus trochilus) produced almost exclusively weak scratches, and therefore would often be classified mistakenly as inactive, if only the clearly visible scratches were counted. The relative number of well visible scratches was positively correlated with increasing atmospheric humidity, since the cover of the typewriter correcting paper is more likely to be scratched off when humidity is high. Consequently, the bird's activity is underestimated in dry air conditions. In a sample of 55 experiments no difference was found in mean direction and concentration, when preferred individual directions were(1)estimated visually, (2) assumed to be the angles with most scratches, and (3) calculated by vector addition of the single scratches. Repetitive counts of the scratches in single experiments resulted in an error for the individual me:ln vector of +/- 12 degrees (90 %-confidence intervall). |
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