Evidence for corrective effects of afferent signals from the extraocular muscles on single units in the pigeon vestibulo-oculomotor system

The role of extraocular muscle (EOM) afferent feedback signals in the control of eye movement is still controversial. We recorded from 106 single units in the vestibular nuclei, oculomotor nuclei and reticular formation of 80 decerebrate, paralysed pigeons. EOM afferents were stimulated by passive eye movement (PEM) during vestibular stimulation by sinusoidal oscillation in the horizontal plane. We found that EOM afferent signals profoundly modified the vestibular responses of 91 (86%) of the single units recorded. As well as using PEM to simulate eye movements similar to saccades, we moved the eye in a manner which mimicked the slow phase of the vestibulo-ocular reflex (artificial VOR, AVOR). We have found evidence that, as well as providing signals closely related to the parameters of eye movement, PEM alters the vestibular responses of cells during AVOR in a manner which suggests that EOM afferent signals may play a corrective role in the moment-to-moment control of eye movement in the vestibulo-ocular reflex.

The visual acuity for the lateral visual field of the pigeon (Columba livia)

Previous studies on pigeons indicated that the visual acuity for the frontal visual field was much higher than that for the lateral one. Suspecting that the poor values for the lateral field were due to suboptimal testing conditions, we determined the lateral field acuity in eight head-fixed pigeons with high-contrast square-wave gratings. An instrumental conditioning task with water as reinforcer and mandibulation as an operant was used. Subjects achieved a mean acuity value of 12.6 c/deg. The results show that the acuity of the lateral visual field is only slightly lower than that of the frontal field. These data provide a psychophysical basis for ecological observations that pigeons and most other birds gaze laterally when scrutinizing small and distant objects.

Visually triggered neuronal oscillations in the pigeon: an autocorrelation study of tectal activity

In this study we describe visually triggered gamma oscillations in the optic tectum of awake pigeons. This study was motivated by the potential relevance of synchronous oscillatory responses in perceptual binding in a laminated structure other than the mammalian neocortex. Tectal responses were recorded as local field potential and multiunit activity by differential filtering. The local field potential was analysed by computing its autocorrelation function and spectral power with a moving window applied to single response sweeps. The temporal structure of the spike trains was evaluated by computing averaged autocorrelograms. A damped sine wave function was fitted to the autocorrelograms in order to quantify the degree of oscillation of both signals. Epochs of significant oscillatory activity were observed in the local field potential in 60% of the trials (n = 39). In all trials, significant oscillations occupied approximately 10% of the time the stimulus was present. The oscillatory events in both the local field potential and the multiunit activity had frequencies in the range of 20-50 Hz. It is important to emphasize the great variability in the frequency and in the probability of occurrence of the oscillatory responses from trial to trial, which makes the oscillatory behaviour of the tectal activity highly non-stationary. The oscillatory activity we describe in the avian tectum has characteristics similar to those reported in the mammalian neocortex. These findings from a fully awake animal strengthen the universality of oscillations as a possible carrier for synchronization of activity in the constitution of neuronal assemblies.

[Spontaneous respiration versus IPPV in pigeons]

Veterinary textbook on anaesthesia in birds that volatile anaesthetics can accumulate in the air sac system during spontaneous respiration of birds anaesthetized with inhalational anaesthetics. In order to quantify the extent of accumulation, air samples were collected from the endotracheal tube and from the left abdominal air sac of ten pigeons anaesthetized with O2, NO2, and isoflurane. In this way, O2, CO2, and isoflurane concentrations were measured continuously at two places, during both spontaneous respiration and positive pressure ventilation. During spontaneous respiration, CO2-concentrations were higher in the air sac than in the endotracheal tube in 60% of the animals. The average difference for the whole group was 0.8 volume per cent; individual differences were as high as 2.9 volume per cent. The concentration of isoflurane was on average 1.1 volume per cent lower in the abdominal air sac than in the endotracheal tube. During mechanical ventilation, the percentage of CO2 was lower in the abdominal air sac than in the endotracheal tube in all pigeons. At the same time, the differences between isoflurane concentrations in the endotracheal tube and air sac decreased. Thus the results of our study with isoflurane-anaesthetized pigeons do not support the suggestion that volatile anaesthetics accumulate in the air sacs of spontaneously breathing birds anaesthetized with inhalation anaesthetics. The results also show that, under these conditions, CO2 concentrations in the endotracheal tube can only be measured reliably during positive pressure ventilation.

Evidence for a separate food-entrainable circadian oscillator in the pigeon

In Experiment 1, four pigeons lived in a metabolic chamber on a 12h:12h LD cycle where they maintained a reduced body weight by consuming a daily ration of food presented at the eighth hour of the photophase. Body temperature (Tb) and oxygen consumption (Vo2) increased prior to the daily feeding. The possibility that a food-entrainable oscillator timed these anticipatory responses was tested by four manipulations, conducted in successive phases, each of which involved eliminating the regularly scheduled food presentation, which is the putative entraining stimulus for such an oscillator, while the 12h:12h LD cycle remained in effect. The manipulations, and their outcomes, were: when fasting was imposed for 3 days, the anticipatory responses continued to occur; when ad lib feeding was allowed for 11 days, the anticipatory responses were mostly eliminated; when fasting was reimposed for 5 days, there was evidence that the anticipatory responses reoccurred; and, when the time of the daily feeding was phase-shifted earlier in the photophase for 8 days, anticipatory responses persisted at the original feeding time and simultaneously developed at the new feeding time. In the first phase of Experiment 2, key pecking by two pigeons produced food only during hours 9-11 of the daily photophase (12h:12h LD). In this condition, Tb increased and key pecking occurred in anticipation of the daily period of food availability. Evidence for a food-entrained oscillator was sought in a second phase when constant dim light (LL) was imposed without changing the hours of food availability.(ABSTRACT TRUNCATED AT 250 WORDS)

Whole body exposures to a phosphoric acids aerosol: I. Spontaneous activity effects in wild rodent and avian species

Two inhalation-chamber studies were conducted to assess acute (2-h out-of-chamber) and subacute (< or = 6 d postexposure) spontaneous activity effects of whole-body phosphoric acids aerosol exposure(s) in black-tailed prairie dogs (Cynomys ludovicianus) and rock doves (Columba livia). The aerosol was generated using a red phosphorus/butyl rubber (RP/BR) mixture under development as a military obscurant. Each study involved (1) 3 RP/BR target concentration groups [0.0 (controls), 1.0, and 4.0 mg/L], (2) 24 prairie dogs or rock doves (8/group), with gender included as a factor, (3) a successive 3-phase paradigm (2 d preexposure; 4 and 2 d of about 80 min/d exposures to RP/BR for prairie dogs and rock doves, respectively; and 6 d postexposure), and (4) infrared detection of the rodents'/birds' home-cage movements. In-chamber atmospheres were uniform and acceptable for all exposures; median aerosol mass concentrations ranged from 0.76 to 0.89 mg/L and 3.46 to 3.74 mg/L for the 1.0 and 4.0 mg/L groups, respectively, with median phosphoric acid (H3PO4) readings of between 67.2 and 74.3%; median particles were < or = 0.85 microns. Mortality was negligible; no prairie dogs died, but 1 male rock dove died on d 3 postexposure to two 4.0 mg/L target concentrations of RP/BR aerosol. Group x session interactions were significant for the acute activity counts of both species. The acute mean ambulatory (e.g., walking) counts of prairie dogs and the acute mean ambulatory and horizontal (e.g., preening) counts of rock doves exposed to 4.0 mg/L RP/BR aerosol were relatively less than those of the other groups after the first 2 or 1 exposures, respectively. Nevertheless, acute session means for all groups approximated or exceeded the 23 h/d activity measured during the pre- and postexposure phases--data indicating that chamber confinement caused a temporary, sharp increase in activity for both species irrespective of RP/BR aerosol concentrations. No RP/BR concentration-related, subacute shifts in the activity of the rodents/birds were observed.

Behavioural effects of ablation of the pigeon-equivalent of the mammalian prefrontal cortex

Six pigeons were trained to perform delayed alternation and brightness discrimination. Three of them underwent ablation of the posterodorsolateral neostriatum (PDLNS) which is believed to correspond to the mammalian prefrontal cortex. In the other three pigeons hyperstriatal lesions were induced by local injections of ibotenic acid. Ablation of PDLNS impaired performance of delayed alternation much more than did the hyperstriatal lesion. In brightness discrimination, a mild impairment occurred only on the first postoperative session and only in the PDLNS group. We conclude that the ablation of PDLNS in pigeons and of the prefrontal cortex in mammals induce similar impairments. Thus, the prefrontal cortex appears not to be a privilege of mammals, but may appear in different architectonic variants in all "higher" vertebrates.

Heritability estimates of blood pressure in genetically selected White Carneau pigeons

Seven White Carneau (WC) pigeon families of the high blood pressure (HBP) line with 51 offspring and six families of the low blood pressure (LBP) line with 57 offspring were used to estimate heritability of the blood pressure trait. Pigeons from the HBP line had significantly higher systolic blood pressures than the LBP line at ages 1 through 6 months. No significant effect of age on blood pressure was observed in either the HBP line or the LBP line. Systolic, diastolic and mean arterial pressures were 155 +/- 6.4 and 122 +/- 5.7 and 139 +/- 6.1, respectively, for male parents and 163 +/- 9.3 and 125 +/- 7.1 and 143 +/- 8.3, respectively for female parents. The mid-parent mean averaged across pairs was 159 +/- 7.3 and 123 +/- 5.9, and 141 +/- 6.6, for systolic, diastolic, and mean arterial pressures, respectively. The average systolic, diastolic, and mean arterial pressures for offspring were 159 +/- 5.9, 132 +/- 5.5, and 147 +/- 5.6, respectively. Blood pressure measurements did not differ greatly between the offspring and the mid-parent mean, indicating that offspring inherit blood pressure levels similar to those of their parents. Mean heart rates were 189 +/- 12.2, 202 +/- 14.2 and 197 +/- 9.9 for the male parents, female parents and offspring, respectively. Heritability estimated by the regression of offspring on the mid-parent mean was 0.55 +/- 0.18, 0.69 +/- 0.19 and 0.61 +/- 0.18, 0.82 +/- 0.19 for systolic, diastolic and mean arterial pressure and heart rate, respectively. The results suggest that genetic factors play a significant role in influencing blood pressure in WC pigeons.

Retrograde transneuronal transport of the fluorescent dye rhodamine beta-isothiocyanate from the primary and centrifugal visual systems in the pigeon

The transneuronal labeling properties of the fluorescent dye Rhodamine beta-isothiocyanate (RITC) were investigated in the pigeon following the intraocular injection of the tracer either alone or in combination with kainic acid (RITC/KA). The RITC was transported bidirectionally from the eye producing both orthograde axonal and terminal labeling of the primary visual system (PVS) and retrograde labeling of the centrifugal visual system (CVS) comprising the n. isthmo-opticus (NIO) and associated ectopic neurons (EN) as well as the tractus isthmo-opticus (TIO). In addition, RITC-positive cell bodies were identified in layers 9/10 of the optic tectum, n. isthmi parvocellularis (Ipc) and the hyperstriatum accessorium (HA) subdivision of the telencephalic visual Wulst. In the RITC/KA experiments, the orthograde transport of the dye within the PVS was selectively suppressed and this coincided with the absence of somatic labeling in n. Ipc and HA. The results are explained in terms of transneuronal transport of RITC, involving the terminal uptake and subsequent retrograde axonal transport of the tracer within second-order neurons in n. Ipc and HA which project to the PVS and in tectal layers 9/10 projecting to the CVS. Moreover, the transneuronal transport of RITC, as demonstrated using the present experimental conditions, appears to be very specific and only labeled distant cell populations known to project to either the PVS or CVS.

The dopaminergic innervation of the pigeon caudolateral forebrain: immunocytochemical evidence for a 'prefrontal cortex' in birds?

The dopaminergic (DA) innervation of the caudal telencephalon of the pigeon was investigated with an antiserum against glutaraldehyde-conjugated dopamine. It was found that the DA-like fibers were distributed within the Paleostriatum augmentatum and the dorsal Archistriatum in a dense meshwork of fibers, while most of the remaining part of the caudal forebrain was innervated by dopaminergic axons which were coiled up like baskets around unlabelled neurons. Within the basket-type innervated structures, the Neostriatum caudolaterale (Ncl) could be distinguished by the high density of its dopaminergic fibers. Retrograde tracer injections into Ncl revealed afferents from the Area ventralis tegmentalis (AVT) and the n. tegmenti pedunculo-pontinus pars compacta (TPc). Since large numbers of DA-like perikarya could be detected in AVT and TPc, it is supposed that these two structures constitute the main source of the dopaminergic innervation of the Ncl. Previous studies had suggested that the Ncl represents an avian equivalent to the mammalian prefrontal cortex. The present results reveal an organization similar to that of the mesocortico-prefrontal system and would thus strengthen this hypothesis.

[Horizontal optokinetic nystagmus in th pigeon during static tilts in the sagittal plane]

The mode of symmetrical position-related otolith influences modified the parameters of optokinetic nystagmus in pigeons. The symmetry of temporonasal-nasotemporal responses was due to increase in the NT-stimulation efficiency. The parameters of optokinetic nystagmus seems to depend on the direction of the otolith membrane shifts.

Direct and indirect "cortico"-rubral and rubro-cerebellar cortical projections in the pigeon

In birds the red nucleus is the most rostral cell group in the brain having projections to all levels of the spinal cord (Cabot et al., Prog. Brain Res., 57:79-108, 1982), but its sources of afferents are incompletely known. In order to determine these, a series retrograde and anterograde tracing experiments was carried out, largely with cholera toxin B-chain conjugated to horseradish peroxidase. The results show that a sparse and diffuse projection to the red nucleus arises from deep regions of the hyperstriatum accessorium (HA) of the anterior Wulst, and that a much more dense projection arises from the caudal part of the nucleus principalis precommissuralis and the medial part of the medial spiriform nucleus (SpMm). These last two sources were themselves shown to receive a substantial projection from HA of the anterior Wulst. The red nucleus was also shown to project upon the cerebellar cortex of lobule VI, and SpM upon the cerebellar cortex of lobules VI through IX (Karten and Finger, Brain Res., 102:335-338, 1976; Clarke, J. Comp. Neurol., 174:535-552, 1977). Double retrograde labelling experiments with fluorescein and rhodamine labelled latex microspheres injected into the cerebellar cortex and spinal cord showed that the rubrocerebellar cortical neurons are a different population from, although intermixed with, the rubrospinal neurons.

Circadian rhythms of pineal melatonin release in the pigeon measured by in vivo microdialysis

Circadian rhythms of pineal melatonin release were measured in freely moving pigeons (Columba livia) by in vivo microdialysis. The birds were placed in light-dark cycles with 12 h of light and 12 h of darkness (LD 12:12) or continuous dim light (LLdim) after LD 12:12. Although the level of melatonin was various, daily changes of melatonin with higher levels during the dark and lower levels during the light were observed in all of the birds examined. The daily changes of melatonin persisted in LLdim, indicating circadian nature of pineal melatonin release. Moreover pineal melatonin release was inhibited by acute exposure of light during the dark. These results indicate that microdialysis is useful for studying circadian pineal melatonin rhythms of birds.

Circadian feeding and locomotor rhythms in pigeons and house sparrows

Feeding and locomotor activities were measured simultaneously in homing pigeons (Columba livia) and house sparrows (Passer domesticus). Feeding, as well as locomotor activity, was found to be regulated by a circadian clock in both of these species. Implantation of melatonin-filled capsules or exposure to constant light abolished feeding and locomotor rhythms in both species. Removal of the pineal gland from pigeons did not abolish either rhythm, whereas pinealectomy abolished both feeding and locomotor rhythms in house sparrows. Although feeding rhythms were generally more robust than locomotor rhythms in both of these species, different feeding and locomotor free-running periods were not observed within any individual pigeon or house sparrow. These results are consistent with the hypothesis that each of these species has a single pacemaker that regulates the timing of feeding and locomotor activity, but they do not rule out the possibility that separate clocks regulate these behaviors.

Head orientation in pigeons during landing flight

Landing flights of pigeons were video recorded or filmed, and frame-by-frame measurements were made of the angle of the head relative to the horizontal, and of the position of the perch in the visual field. The angle of the head increases above that seen in free flight, to a value which is correlated with the trajectory of approach to the perch. As a result, the perch is fixated 20-25 degrees above the beak early in landing flight. The possible significance of the behaviour is discussed in relation to specialised retinal areas and to lower-field myopia.

Visual discrimination performance after lesions of the ventral lateral geniculate nucleus in pigeons (Columba livia)

Pigeons were trained to perform simultaneous pattern and color discrimination tasks. After their training was completed, bilateral electrolytic lesions were made in the ventral lateral geniculate nucleus (GLv). Following the surgery, they were retrained to their preoperative performance levels. Lesions of GLv caused no deficits in pattern discrimination performance. The birds which had been trained for discrimination of red vs. magenta showed a slight decline in their performance. This impaired performance on color discrimination was not, however, as severe as that of a bird with lesions in the nucleus rotundus. These results suggest that GLv plays some role in the detection of short wavelengths of light.

In vivo studies on the effects of ovine corticotrophin-releasing hormone, arginine vasotocin, arginine vasopressin, and haloperidol on adrenocortical function in the racing pigeon (Columba livia domestica)

This study investigates the effects of in vivo administration of pituitary stimulants in racing pigeons (Columba livia domestica). Plasma corticosterone concentrations were measured following intravenous administration of ovine corticotrophin-releasing hormone (oCRH) (0.1, 1, 10, and 100 micrograms/kg), arginine vasopressin (AVP) (0.01, 0.1, 1, and 10 micrograms/kg), arginine vasotocin (AVT) (0.01, 0.1, 1, and 5 micrograms/kg), and haloperidol (0.05, 0.5, and 5 micrograms/kg). Compared with mammals the pituitary-adrenocortical system of the pigeon appeared to be less sensitive to stimulation with oCRH, although high doses were well tolerated and gave a clear response. Both AVP and AVT stimulated corticosterone secretion, AVT in a more pronounced dose-dependent manner than AVP. The natural neurohypophysial peptide in birds, AVT, was less well tolerated in high doses than AVP. The clear response to haloperidol indicates that the hypothalamo-pituitary-adrenocortical system is under dopaminergic inhibition. Of the pituitary stimulants tested AVP (10 micrograms/kg) and oCRH (100 micrograms/kg) are the most appropriate for testing the integrity of the pituitary-adrenocortical system in the pigeon.

Color mixing in the pigeon (Columba livia) II: A psychophysical determination in the middle, short and near-UV wavelength range

Pigeons were trained to discriminate between spectral lights and additive mixtures in the 350-560 nm spectral range using a successive "autoshaping" discrimination procedure [introduced in Palacios, Martinoya, Bloch & Varela, Vision Research, 30, 587-596 (1990)]. Dichromatic mixtures were found in the short and near UV region, but not in the middle-wave region. Our results suggest that color vision in the pigeon involves the active participation of five different primary mechanisms, which are differentially active in the yellow- and red-sensitive retinal fields.

The shell region of the nucleus ovoidalis: a subdivision of the avian auditory thalamus

The connectivity of a region surrounding the established thalamic auditory nuclei, n. ovoidalis (Ov) and n. semilunaris parovoidalis (SPO), was explored in the ring dove by using the anterograde tracers, Phaseolus vulgaris leucoagglutinin (PHAL) and biocytin, and the retrograde tracer, fluorogold. The Ov-SPO surround received a projection from a cell group along the interface of the auditory midbrain and the n. intercollicularis, as revealed with PHAL and biocytin, and was composed of neurons exhibiting a common morphology. These features and the presence of overlapping projections from different portions of the Ov-SPO surround suggest that this region comprises a functionally discrete area, which we term the Ov shell. Single unit recording within the shell established the existence of acoustically responsive units. Both PHAL and fluorogold labeling revealed a robust projection from the Ov shell to the caudomedial hypothalamus. Major telencephalic projections of the shell terminated within the ventral paleostriatal complex, "end-zones" of the field L, the caudomedial hyperstriatum ventrale, and regions immediately dorsal and lateral to the auditory neostriatum. Except for a portion of the shell bordering medial ovoidalis, PHAL injections into the shell also labeled fibers within the caudolateral neostriatum and along the lateral neostriatal rim. The connectivity of the Ov shell suggests that this region may integrate auditory pathways with brain regions associated with endocrine mediated behavior. In addition, the shell may constitute a source of converging input to several levels of central auditory pathways.

Effect of lesions in the ectostriatum and Wulst on species and individual discrimination in pigeons

Pigeons were trained on species (pigeon vs. quail) or individual (pigeon vs. pigeon) discrimination in an operant chamber with a video screen. Still video images were used as discriminative stimuli. After the pigeons had accomplished the discrimination tasks, they received lesions of the Wulst or the ectostriatum. While damage to the Wulst did not disrupt either task, the ectostriatal damage caused deficits selectively in individual discrimination. These dissociations between the tasks and between the lesion sites are comparable to the previous experiments on natural and pseudoconcept. Species discrimination seems to share common brain mechanisms with other natural concept discrimination.

The importance of comparative studies and ecological validity for understanding hippocampal structure and cognitive function

Building from the premise that hippocampal cognitive function has been molded by natural selection under natural environmental conditions, it is argued that traditional laboratory studies likely do not reveal the richness and complexity of hippocampal function. Research on the role of the hippocampal formation in the navigational behavior of homing pigeons is offered as an example to illustrate the advantages of using an ecological approach to understand hippocampal function. It is further proposed that dissimilarities in hippocampal anatomy, physiology, and neurochemistry found between species reflect species differences in the range of functions served by the hippocampal formation, as well as possibly the molecular and cellular mechanisms that support such functions. These differences notwithstanding, it is suggested that, from an evolutionary perspective, the primary function of the hippocampal formation is a role in some aspect of spatial cognition. Dissimilarities in hippocampal structure and function among extant species are viewed as resulting from differences in evolutionary selective pressure and evolutionary history.

Effects of arginine vasotocin on cardiorespiratory and thermoregulatory responses in the pigeon

1. The effects of intramuscular injections (10 ng, 100 ng, 1 mu or 10 mu/pigeon) of two preparations of synthetic arginine vasotocin (AVT), deamino-dicarba-arginine vasotocin (ddAVT) and arginine vasotocin diacetate tetrahydrate (AVTdt) on heart rate (HR), breathing frequency (BF), oxygen consumption (VO2), cloacal and foot temperatures (Tc and Tf) and shivering (S) were studied in the pigeon housed in a respiratory flow chamber maintained at a temperature of 22.5 degrees, during a 4 hr post-injection period. 2. In general, with the exception of the lowest dose, AVT-treatment produced hypothermic responses, as was indicated by Tc and Tf. 3. Both ddAVT and AVTdt produced increases in HR and BF when administered in high concentrations (10 micrograms/pigeon), the effect being more potent with AVTdt than ddAVT; with lower doses, the effects were considerably less pronounced. This increase in HR and BF is attributed to a probable compensatory response to the hypothermic effect. 4. With a 10 micrograms dose, ddAVT produced a greater decline in VO2 than AVTdt. Although other doses too evoked a drop in VO2 at certain post-injection time points, the effect was of lesser magnitude. 5. No significant shivering response was elicited, in spite of the drop in Tc. 6. It is suggested that, besides the antidiuretic function of AVT, its hypothermic effect would be of considerable importance in thermal homeostasis during flight in birds.

Overlap of somatic and visual response areas in the Wulst of pigeon

The somatic and visual response areas of the Wulst were investigated electrophysiologically in pigeons. Somatosensory neurons are distributed in the hyperstriatum accessorium (HA), the hyperstriatum intercalatus superior (HIS) and the hyperstraitum dorsal (HD), mainly in HA. The radial nerve response area is relatively larger and overlaps the sciatic nerve area. Visual neurons are located in HA, especially the more superficial part of HA. In the Wulst, the somatic response area overlaps the visual area and there is somatosensory-visual convergence.

Effects of physiological cycles of infused melatonin on circadian rhythmicity in pigeons

The role of the hormone melatonin in the circadian system of pigeons (Columba livia) was investigated. Using an automatic infusion system, melatonin at physiological levels was delivered for 10 h each day to cannulated, pinealectomized (P-X) pigeons in constant darkness. These cyclic infusions of melatonin entrained feeding rhythms in P-X pigeons while vehicle infusions were ineffective entraining agents. When the retinae of P-X pigeons were removed (E-X), feeding rhythms were abolished in constant darkness. When cyclic melatonin infusions were delivered to these birds (E-X and P-X), feeding rhythmicity was restored whereas vehicle infusions alone did not restore rhythmicity. When melatonin infusions were terminated in E-X/P-X pigeons, feeding rhythms persisted for several days but eventually decayed. Blood melatonin levels were measured in both P-X and E-X/P-X birds infused cyclically with exogenous melatonin and were found to be within the physiological range both in level and pattern. These results strongly suggest that endogenous melatonin, released by the pineal gland and the retinae, regulates the timing of feeding rhythms by entraining other oscillators in the circadian system of the pigeon.