Our understanding of bird orientation guided by magnetic and visual cues is primarily based on Emlen funnel experiments. Migration-motivated birds jump in the direction they want to fly, and their feet leave marks on paper lining the funnel, which yields the preferred direction. Despite the low signal-to-noise ratio, this paradigm has proven instrumental for studying magnetoreception in birds. However, the high noise limits the questions that can be answered and there is no data-informed guideline for selecting sample sizes that have a high likelihood to be conclusive. Furthermore, differences in experimental design traditions limit comparison and reproducibility across studies, slowing down discovery. We performed a large meta-analysis across double-blind magnetic orientation studies with Emlen funnels performed at Oldenburg to statistically characterize Emlen funnel data and determine minimal sampling requirements for conclusive experimental design. The analysis confirms that pre-selecting migration-motivated animals before the real experiments start improves statistical power by reducing noise. We also highlight mathematical limitations of the widely used directionality measure 'r', due to lacking sample-size bias correction, and present realistic ranges for expected bird directedness in Emlen funnels. Combined, these results provide critical design and analysis guidelines for statistically informative magnetic orientation experiments.
Patmanidis et al. (Wed,) studied this question.