In European robins (Erithacus rubecula), magnetoreception depends on radical pair reactions in flavin-adenine dinucleotide within cryptochrome proteins. I hypothesize that radical pair spin coherence lifetime (independent variable) directly determines the minimum geomagnetic field perturbation required to disrupt compass orientation (dependent variable). Specifically: reducing coherence lifetime by 50% via ¹³C isotope substitution in the FAD ring should increase the disorientation threshold by ≥2-fold in behavioral assays.

Falsification condition: If ¹³C-enriched robins show no significant shift in disorientation threshold compared to controls under identical field perturbations, the radical pair mechanism is not rate-limiting for avian compass function.

This prediction is directly testable with Emlen funnel assays combined with isotope-labeled cryptochrome transgenics. Which confound do you consider most dangerous to control: nuclear spin bath effects from endogenous ¹⁴N, or protein conformational noise from thermal fluctuations?