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The extent to which circadian rhythms are involved in photoperiodic time measurement in quail is enigmatic, and earlier investigations have produced results consistent with an hourglass clock or one involving damped circadian oscillators. To address the problem further, the present authors carried out a direct comparison between the clocks in quail and those in starlings. Starlings possess strongly self-sustaining circadian oscillators. In Experiment 1, comparisons of testicular growth were made between the two species when birds were exposed to light:dark (LD) 6:30, LD 6:18, and LD 18:6. Starlings grew their testes rapidly under both LD 6:30 and LD 18:6, and they became photorefractory (under LD 6:18, the testes remained undeveloped). Quail grew their testes rapidly under LD 18:6 but did not do so under LD 6:30 or LD 6:18. In Experiment 2, entrainment of the activity rhythm under cycles of LD 6:30 was investigated by measuring the phase of the rhythm after release into constant darkness (DD). Birds were exposed to either 10 cycles or 11 cycles of LD 6:30 prior to DD. Starlings maintained their 24-h rhythmicity under LD 6:30 and always free ran from the phase of the subjective day By contrast, quail showed circadian activity approximately 24 h after every light pulse and free ran from the phase of the last light pulse received. In Experiment 3, phase response curves (PRCs) were generated to 6-h light pulses. The species were strikingly different; starlings produced a Type 1 PRC, whereas quail produced a Type 0 PRC. More important, in quail the 6-h light pulse had the same effect regardless of circadian time and in almost every case activity free ran from the position of the 6-h light pulse. The results in quail are consistent with the photoperiodic time measurement system being based on a weakly self-sustaining (rapidly damping) circadian system that is invariably reset by 6 h of light, whereas in starlings the pacemakers are strongly self-sustaining. The results support the notion that hourglass pacemakers can be highly damped circadian pacemakers.

In photoperiodic species, short daylength resonance cycles of modulo t + ½t (t = 24 h) behave like long days because they entrain the circadian system so that alternate light pulses coincide with the photoinducible phase (ø1) in castrated quail. However, while a long-day response after exposure to a single long daylength is readily detected by a rise in plasma LH (photoinduction), long-term exposure to LD 6:30 is ineffective in this respect. To discover whether this occurs because of unusual entrainment, circadian rhythms in quail and starlings were investigated. Whereas starlings entrained in the expected way with alternate pulses falling at different circadian phases, activity bouts in quail appeared to follow 24 h after successive light pulses. Because of this, activity was examined in free-running conditions to confirm that the pacemaker in quail was indeed being reset to a constant phase (reset to circadian time [CT] 0) by successive pulses. Examination of the circadian rhythms of plasma melatonin secretion under LD 6:30 also showed a resetting to CT 0. The positioning of all light pulses at the same circadian phase in the early subjective day explains the lack of photoinduction in quail since ø1in the early subjective night phase remains unilluminated. A second feature in quail is that when the length of the photophase is gradually increased within T36h cycles, there is a progressive increase in the degree of photoinduction although the photophase length remains well below the critical daylength for induction in normal T24h cycles. We therefore tested whether ø1is reset to a constant phase by successive pulses in LD 6:30, and that this phase is also advanced relative to light onset so that photophases shorter than the critical daylength can interact with ø1to cause induction. Such a reduction in critical daylength relative to successive LD 6:30 pulses was confirmed by transferring quail to various types of long day and measuring the change in LH secretion. When the long-day test was replaced with continuous light, stimulation of LH secretion occurred 5-7 h earlier in quail pretreated with LD 6:30 and LD 6:54 compared to quail pretreated with LD 6:18 or LD 6:42, implying that ø1had been markedly phase advanced under resonance cycles.

The earliest detectable event in the photoperiodic response of quail is a rise in luteinizing hormone (LH) secretion beginning at about hour 20 on the first long day. The timing of this rise was measured in castrated quail after entrainment to short daylengths which cause significant phase angle differences in the circadian system: (1) LD 2:22 and LD 10:14, and (2) LD 3:21 (T = 24 hr) and LD 3:24 (T = 27 hr). The quail were then exposed to 24 hr of light (by delaying lights-off ), and the time of the first LH rise was measured; it was similar in all schedules. Quail were also entrained to LD 3:21 or LD 3:24 and then given a single 6-hr nightbreak 6-12, 7-13, or 13-19 hr after dawn. The earlier pulse was marginally more inductive in the 27-hr cycle. Thus the entrainment characteristics of the photoinducible rhythm (Φ1) in quail appear very different from those of the locomotor circadian rhythm, and raise doubts as to whether Φ1 is a primary circadian oscillator.

Experimental photoperiods, presented either once only or repeatedly, were used to assess the oscillatory and hourglass properties of the photoperiodic clock in Japanese quail. Gonadectomized quail on 8-hr daylengths respond to a single skeleton photoperiod consisting of two 8-hr light pulses separated by 2 hr of darkness (i.e., LDLD 8:2:8:6) with a marked increase in secretion rate of luteinizing hormone (LH). This response suggests that the second light pulse interacts with a "photoinducible phase" (Φi) lying some 10-16 hr from "dawn" (start of the first light pulse). If, however, groups of quail maintained on 8-hr daylengths are transferred to continuous darkness (DD), and the position of the Φiis sought by a single 8-hr light pulse applied at various times on the first or third day of DD, then an increase in circulating LH is, at best, barely detectable. It would appear that a strongly responsive Φidoes not recur rhythmically in DD. Instead, the light pulse apparently acts primarily as a "dawn" signal that triggers a single cycle of photoinducibility, since a second 8-hr light pulse, placed to begin 2 hr after the end of the first, induces a large increase in plasma LH. Similar results are obtained if any single 8-hr light pulse presented to animals held in darkness is preceded, 10 hr earlier, by a short "dawn" light signal. Such dawn signals can be effective when very short; a pulse of only 30 sec can cause a subsequent Φi. The dawn pulse is effective at any circadian phase and leads to a single cycle in photoinducibility. In contrast, a much longer light pulse (perhaps not less than 4 hr) is needed to interact with Φiif significant gonadotropin secretion is to be stimulated.In confirmation of the findings described above, we found that Nanda-Hamner lighting schedules have remarkably little effect in stimulating gonadotropin secretion in gonadectomized quail. There is, for example, a very marked difference between the effectiveness of "resonating" schedules such as LD 6:6, which stimulates a high LH secretion rate since each "inductive" light pulse is preceded by an appropriate "dawn" signal, and a theoretically effective schedule such as LD 6:30, which induces a very small response by comparison. Such schedules (even theoretically noninductive ones) can, however, be made very highly inductive if alternate light pulses are preceded by an appropriately positioned 15-min light pulse to act as "dawn."