Much of the experimental research used, as model species, the Japanese quail and later wild-caught starlings. Achievements included the development of the first radioimmunoassay to measure bird luteinizing hormone (LH) in collaboration with Frank Cunningham (Reading University) and Colin Scanes. This key innovation made it possible to measure LH in 10 microlitres of plasma and so follow circulating hormone levels in individual birds exposed to photoperiods of many types. Using gonadectomized quail it was possible to show unequivocally that the underlying photoperiodic response in birds (but not mammals) is driven by brain circuits that are switched on an off by daylength. It also allowed the demonstration that measuring daylength involved a daily (circadian) rhythm in photosensitivity with the birds being responsive to light particularly 12 and 18 hours after dawn. If light fell at these hours then the day was read as “long”, if not then it was read as “short”.