Photoperiodic response curves for plasma LH concentrations and age at first egg in female broiler breeders

The objective of this work was to define precisely the response curve for photoinduced luteinizing hormone (LH) release in feed-restricted meat-type (broiler) breeder females and to compare it with the photoperiodic response curve for advance in age at first egg (AFE). Birds with a mean body weight of 2.0kg at 20 weeks of age were transferred from an 8 to a 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 14 or 18-h photoperiod; change in plasma LH was measured 4d after photostimulation and subsequent individual AFE recorded. The first significant increase in LH secretion was seen in birds transferred to an 11.5-h photoperiod, but no further significant increases in LH were observed in birds transferred to longer photoperiods. A photoperiodic response curve based on a meta-analysis of changes in photoinduced LH secretion observed in this study and data from an earlier experiment using dwarf broiler breeders indicated a critical daylength of about 9.5h and a saturation daylength of approximately 13h. Similarly, the first significant advance in AFE occurred in birds transferred to an 11-h photoperiod, but with no further significant increases seen in birds transferred to photoperiods >11h. A response curve for photoinduced advances in AFE was produced by meta-analysis using data from the present study and from an earlier investigation involving fewer, more widely spaced photoperiods. It is concluded, in female broiler breeders, that the photoperiodic response curves for photoinduced LH release and AFE are similar, with the point at which the responses begin to rise steeply (classical critical daylength) occurring at 9.5h and the asymptote (classical saturation daylength) at 13h. Functionally, however, the minimum photoperiod to achieve a significant change in either LH secretion or advance in AFE is between 11 and 11.5h.     

Neuroendocrine control of reduced persistence of egg-laying in domestic hens: evidence for the development of photorefractoriness.

Egg-laying in hens exposed for more than 11 months to photostimulatory daylengths was intermittent and associated with a reduction in numbers of yellow-yolky ovarian follicles. Old laying hens (105 weeks) had lower concentrations of luteinizing hormone (LH) in the pituitary gland and plasma and reduced pituitary gland responsiveness to chicken LH-releasing-hormones (LHRH-I and II) in vivo when compared with young laying hens (28 weeks). Four weeks after transfer from 14 to 8 h light/day, egg production almost stopped in old, but not in young hens, although plasma LH concentrations decreased in all birds. After transfer from 14 to 20 h light/day, plasma LH increased in young, but not in old, hens, without a change in the rate of egg production. Reproductive function was enhanced in old hens returned to long days after induction of a moult and ovarian regression by reducing daylength and dietary restriction. Moulted hens had a greater rate of egg production, higher concentrations of plasma LH and a greater pituitary-gland responsiveness to LHRH-II in vivo than unmoulted control hens. After transfer from 14 to 8 h light/day, egg-laying decreased more rapidly in unmoulted than in moulted hens; transfer to 17 h light/day increased egg production in moulted, but not in unmoulted, birds. Induction of ovarian regression in old hens by dietary restriction alone also enhanced reproductive function after the dietary restriction was relaxed. Egg-laying was more persistent in hens brought into lay for a second year by transferring them from 3 to 11 h light/day than in hens transferred from 3 to 20 h light/day. Egg production was stimulated in hens maintained on 3 or 11 h light/day for 42 weeks, after transfer to 20 h light/day. Egg production ceased in hens maintained on 20 h light/day for 46 weeks, after transfer to 3 h light/day. These observations are consistent with the view that poor persistence of laying in hens less than 2 years old and exposed continuously to long days is caused, in part, by a reduction in hypothalamic-gonadotroph function. This reduction in neuroendocrine function may be due, in part, to the development of relative photorefractoriness.     

A decreased capacity of hepatic growth hormone (GH) receptors and failure of thyrotrophin-releasing hormone to stimulate the peripheral conversion of thyroxine into triiodothyronine in sex-linked dwarf broiler hens

The effect of two different doses of thyrotrophic releasing hormone (TRH) upon the plasma levels of growth (GH) and thyroid hormones in both sex-linked dwarf (dw) and normal (Dw) broiler hens was determined. In normal hens, 1.5 and 24 microg TRH/kg increased the GH plasma concentrations after 15 min. Plasma concentrations of T3 increased significantly 1 h after TRH injection, whereas T4 concentration decreased after 2 following injection of 24 microg/kg TRH. In dwarf hens both doses of TRH increased the plasma concentrations of GH and the GH response lasted longer. However, TRH was ineffective in raising T3 and T4 levels. Saline-injected dwarf birds showed no differences in plasma T4 and T3 levels in comparison with normal hens. A smaller number of hepatic cGH receptors was found in dwarf hens, whereas the affinity of the hepatic GH receptor was not influenced by the genotype. It is concluded that the sex-linked dwarf broiler hen is unable to respond to a TRH-induced GH stimulus probably because of a deficiency in hepatic GH receptors resulting in a failure to stimulate the T4 to T3 converting activity.     

Modulation of gonadotrophin and prolacrin secretion by daylength and breeding behaviour in free-living starlings, Sturnus vulgaris

Plasma levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin were measured by radioimmunoassay in plasma samples collected from free-living starlings, Sturnus vulgaris , trapped in nest-boxes. By leaving some nest-boxes undisturbed, and repeatedly destroying nests in others, birds from a single-brooded population were trapped whilst nest-building, incubating or feeding nestlings, at different times throughout the normal breeding season. In both males and females trapped whilst nest-building, plasma LH and prolactin levels increased progressively from mid March until late May. In females sampled during incubation, LH and FSH levels were high throughout May but decreased in early June. Prolactin levels were highest in late May. In both males and females trapped during mid May, LH levels were highest in these birds which were nest-building at this time and lowest in those feeding nestlings, FSH did not change significantly, and prolactin was low in those birds which were nest-building and high in those incubating or feeding nestlings. In female starlings from a double-brooded population, levels of LH and FSH were much lower whilst feeding the second brood than when feeding the first brood, whereas prolactin levels were similar. The results suggest that incubation and feeding behaviour inhibit the increase in LH secretion caused by increasing daylength, but stimulate prolactin secretion in excess of that caused by increasing daylength.     

Photoperiodic history and a changing melatonin pattern can determine the neuroendocrine response of the ewe to daylength

The reproductive neuroendocrine response of Suffolk ewes to the direction of daylength change was determined in animals which were ovariectomized and treated with constant release capsules of oestradiol. Two groups of animals were initially exposed to 16 or 10 h light/day for 74 days. On day zero of the study, when one group of ewes was reproductively stimulated (elevated LH concentrations) and the other reproductively inhibited (undetectable LH concentrations), half the animals from each group were transferred to an intermediate daylength of 13 h light/day. The remaining ewes were maintained on their respective solstice photoperiods to control for photorefractoriness. LH concentrations rose in animals experiencing a 3 h decrease in daylength from 16L:8D to 13L:11D while LH concentrations fell to undetectable values in those that experienced a 3 h increase in daylength from 10L:14D to 13L:11D. The photoperiodic response of the Suffolk ewe, therefore, depends on her daylength history. Such a result could be explained if the 24-h secretory pattern of melatonin secretion, known to transduce photoperiodic information to the reproductive axis, was influenced by the direction of change of daylength. Hourly samples for melatonin were collected for 24 h 17 days before and three times after transfer to 13L:11D. The melatonin secretory profile always conformed to daylength. Therefore, the mechanism by which the same photoperiod can produce opposite neuroendocrine responses must lie downstream from the pineal gland in the processing of the melatonin signal.     

Abolition of seasonal testis changes in the Ile-de-France ram by short light cycles: relationship to luteinizing hormone and testosterone release

The purpose of this study was to examine the relationships between testis weight and the luteinizing hormone (LH) and testosterone releases in rams subjected to 14 successive 2-mo artificial light cycles in which daylength increased from 8 to 16 h in one month and decreased from 16 to 8 h the following month. Testis weights were measured fortnightly. Serial bleedings were performed during 6 to 8 h the last three 2-mo light cycles, when daylengths were 8, 12 (increasing), 16 and 12 h (decreasing) and plasma LH and testosterone were measured by radioimmunoassay. The overall mean testis weight, continuously high and equal to 314 g per testis, was not correlated with daylength. Mean LH levels and LH pulse height varied significantly according to daylength (P < 0.05 and P < 0.01, respectively) and were maximal on short days (8 h). Both mean and maximal testosterone levels presented variations according to daylength (P = 0.05). LH and testosterone pulsatility were quite high (equivalent to 7 to 10.3 pulses/day); however, mean plasma testosterone levels remained low. It is proposed that in rams under 2-mo light cycles, frequent but short stimulation of LH release by decreasing daylength phases allows maximal testis weight, while the interruption of LH stimulation by increasing daylengths prevents overstimulation of testosterone that can inhibit the LH release by negative feedback. Rams a under 2-mo light regimen presented a persistent high testis weight, indicating that seasonality had been broken down in the Ile-de-France male which is normally a marked seasonal breeder.     

Short photoperiod inhibits winter growth of subtropical grasses

Grass development is influenced by length of photoperiod, but no direct measurements under natural conditions exist on mass accumulation in response to photoperiod by subtropical grass species. Grasslands of the subtropics are a major resource, but their growth is inhibited substantially during the short-photoperiod months. This research was designed to examine the consequences on grass production under field conditions when the limitation of short photoperiod is artificially removed. Lights, which extended the daylength to 15 h, were placed over plots of four subtropical forage grasses representing three species (Paspalum notatum Flugge; Cynodon dactylon L.; Cynodon nlemfuensis Vanderyst) to measure their mass accumulation in response to extended photoperiod in a 2-year experiment. Forage yields in all grasses at 5-week harvests during the time of shortest daylength were increased up to 6.2-fold more than the yield under the natural daylength. For the 4.5-month period of shortest daylength in each year, forage yields were increased for all grasses with one grass having a yield increase of 3.6-fold under the extended photoperiod as compared to natural daylength. These results demonstrated that selection of grasses that are insensitive to photoperiod could substantially increase forage yield of subtropical grasslands to benefit animal production and enhance carbon sequestration.     

Environmental regulation of the reproductive system in a flexibly breeding Sonoran Desert bird, the Rufous-winged Sparrow, Aimophila carpalis

We investigated reproductive regulation in male Rufous-winged Sparrows, Aimophila carpalis, a Sonoran Desert passerine that breeds after irregular summer rains. Field and captive data demonstrate that increased photoperiod stimulates testicular development in March and maintains it until early September. Free-living birds caught in July and placed on captive long days (16L: 8D) maintained developed testes for up to 7 months, and free-living birds caught in September, during testicular regression, redeveloped testes when placed on captive long days, indicating that these birds were still photosensitive. Captive birds on long days maintained testicular development when exposed to temperatures mimicking those occurring during regression in free-living birds. In free-living birds, testicular development was observed during spring and summer, but unless this was associated with rainfall, breeding (indicated by juveniles) did not occur. Large increases in plasma luteinizing hormone (LH) in free-living males were correlated with heavy rainfall in July/August, when the birds bred, and in November, when they did not breed. In captive birds, plasma LH concentrations were unresponsive to photoperiodic changes, but may have responded to social cues. Plasma prolactin concentrations were directly correlated with photoperiod in free-living birds, but an effect of photoperiod on prolactin secretion was not seen in captive birds. It is concluded that male Rufous-winged Sparrows use long photoperiods to stimulate and maintain testicular development, but exposure to long photoperiods does not terminate breeding by inducing absolute photorefractoriness. The specific timing of reproductive behaviors is apparently determined by elevated plasma LH coinciding with long day stimulated gonad development.     

Growth, protein synthesis and plasma concentrations of growth hormone, thyroxine and triiodothyronine in dwarf, control and growth-selected strains of broiler-type domestic fowl

Body weight, tissue weight and plasma hormone concentrations were determined at 1, 3, 6, 12 and 21 weeks of age in two dwarf strains and one control strain of broiler chickens. Protein synthesis, accretion and degradation rates were determined in the control strain with age. Within each strain, plasma growth hormone (GH) concentrations were greater at 1 and 3 weeks of age and consequently decreased with age. Plasma GH concentrations were greater in the sex-linked dwarf chicken during pubescence and maturity (12 and 21 weeks) compared to the autosomal dwarf and control chickens. Circulating concentrations of 3,5,3' triiodothyronine (T3) were depressed by 70% in sex-linked dwarf birds compared to controls, while thyroxine concentrations did not differ at most time points. These findings support the suggestion that sex-linked dwarf chickens have reduced peripheral conversion of T4 to T3.     

Effects of ovariectomy, gonadal steroid replacement and photoperiod on plasma beta-endorphin in the ewe

The effects of gonadectomy, photoperiod and ovarian steroid replacement on plasma beta-endorphin concentrations were studied in 2 experiments using mature ewes. In Experiment 1 ovariectomy resulted in an increased plasma beta-endorphin concentration. Changing the photoperiod from long to short daylength had little effect on beta-endorphin, but transfer from short to long photoperiod caused a fall in the beta-endorphin level. In Experiment 2 ovariectomy again caused an increase in plasma beta-endorphin. Treatment with progesterone, estradiol or a combination of both failed to reduce the level to preovariectomy values, although LH concentrations were reduced. No diurnal rhythmicity in beta-endorphin secretion was observed.     

Relative photorefractoriness, prolactin, and reproductive regression in a flexibly breeding sonoran desert passerine, the rufous-winged sparrow, Aimophila carpalis

We tested the hypothesis that adult male rufous-winged sparrows, Aimophila carpalis, exhibit relative photorefractoriness. This condition results in partial loss of sensitivity to photoperiod as a reproductive stimulus after prolonged exposure to long photoperiods and is similar to the mammalian condition called photoperiodic memory. Captive birds were exposed either to 8 h of light/16 h of dark per day (8L) or to 16L for 11 weeks and were then exposed either to 8L, 13L, 14L, or 16L. Testicular diameter, plasma luteinizing hormone (LH), and plasma prolactin (PRL) were measured to assess reproductive system activity in response to photostimulation. In free-living birds, testicular diameter, plasma LH, and PRL were compared in birds caught in September in a year when birds were breeding and in a year when birds were not breeding to further evaluate the role of PRL in the termination of seasonal breeding. Testes completely developed after transfer from 8L to 14L or to 16L and partially developed after transfer from 8L to 13L. However, after 11 weeks of 16L exposure, transfer to 14L caused partial regression and transfer to 13L caused complete regression of the testes. Plasma LH increased in all birds that were transferred from 8L to a longer photoperiod. PRL showed a weak response to longer photoperiod treatment and was elevated in birds after chronic 16L exposure in comparison to birds exposed to chronic 8L. These data indicate that male rufous-winged sparrows lose sensitivity to photoperiod after long photoperiod exposure consistent with the relative photorefractoriness and photoperiodic memory models. Lower PRL in birds that developed testes on 13L and 14L compared to birds that regressed testes on 13L and 14L are consistent with the hypothesis that PRL regulates relative photorefractoriness. However, PRL does not appear to regulate interannual differences in the timing of testicular regression.     

Photoperiod requirements for puberty differ from those for the onset of the adult breeding season in female sheep

Reproductive responses to photoperiod were directly compared in mature ewes and in their spring-born twin female lambs. All females were ovariectomized and treated with oestradiol implants before transfer into artificial photoperiod; serum LH concentrations and pulsatile LH patterns provided an index of neuroendocrine reproductive activity. Mothers were transferred from natural photoperiod to artificial long days (16 h light:8 h dark) at the summer solstice so that no decrease in photoperiod would be experienced. These ewes began reproductive activity synchronously at the expected time in the autumn. One of each pair of twin lambs was treated exactly as the mothers; to determine the normal timing of puberty the remaining twin was maintained in a photoperiod simulating the natural decrease in daylength. In all 6 control lambs experiencing the simulated natural photoperiod, reproductive activity occurred synchronously at 28 +/- 1 weeks of age (2 October +/- 7 days). However, in their twin sisters which did not experience a decrease in photoperiod, only 2 of 6 lambs had begun reproductive activity by the end of the experiment at 52 weeks of age (March), and these were both delayed relative to their twin control lambs exposed to decreasing daylength. Therefore, a decrease in photoperiod is necessary for the normal timing of puberty in the spring-born, female sheep, whereas seasonally anoestrous, mature sheep can enter the breeding season at a normal time in the absence of decreasing photoperiod. We suggest that the requirement for a decreasing photoperiod by the spring-born lamb reflects its limited photoperiodic history as compared to the adult.     

Contribution of increasing and decreasing daylength to the photoperiodic control of LH secretion in the Ile-de-France ram

Sexually mature Ile-de-France rams were exposed to an 8-month light regimen in which the daily light increment and decrement were constant and equal to 7 min/day. Daylength therefore varied from 6 to 20 h. The animals were allotted to two groups of 12 rams each and submitted to the same light regimen but 4 months out of phase. Blood was collected every 40 min for 10 h, on 15 occasions at various intervals during the second light cycle. Plasma LH and on some occasions plasma testosterone concentrations were measured by radioimmunoassay. In both groups the number of LH pulses increased significantly as daylength increased from 11:40 to 20:00 h (P less than 0.01) but because their amplitude was low the mean plasma LH increase, although significant, was moderate. As daylength started to decrease (from 20:00 to 18:30 h), the frequency of LH pulses further increased (P less than 0.05) and the pulse amplitude doubled so that mean plasma LH values increased abruptly and remained high until the decreasing light photoperiod reached 11:40 h. Thereafter, the frequency and amplitude of LH pulses decreased and the mean plasma LH dropped to one-fourth of its maximal value. Mean plasma testosterone concentrations were low during most of the 8-month cycle but increased steeply when daylength decreased from 11:40 to 6:00 h.(ABSTRACT TRUNCATED AT 250 WORDS)     

Do endogenous opioid peptides mediate the effects of photoperiod on release of luteinizing hormone and prolactin in ovariectomized ewes?

Three experiments were done to determine if endogenous opioid peptides (EOPs) mediate the effects of photoperiod on release of luteinizing hormone (LH) and prolactin (Prl) in ovariectomized (OVX) ewes. Intravenous infusions of 0.5 naloxone X h-1 X kg body weight-1 for 3.5 h increased (P less than 0.01) mean plasma concentrations of LH and decreased (P less than 0.025) mean interpulse interval (period) of LH pulses in OVX ewes exposed to long day lengths (16L:8D). Infusions of either 1.0 or 2.5 mg morphine-SO4 X h-1 X kg-1 for 3 h increased (P less than 0.005) the period of LH pulses and increased (P less than 0.005) concentrations of Prl in OVX ewes during the breeding season. In OVX ewes exposed to long (16L:8D) or short (8L:16D) day lengths infusions of naloxone increased (P less than 0.05) mean concentrations of LH, whereas morphine decreased (P less than 0.01) mean concentrations of LH. These effects were attributed to changes in period of LH pulses (P less than 0.001). The drug X photoperiod interactions were not significant for LH parameters. Naloxone did not affect Prl release in either long- or short-day groups, but morphine increased (P less than 0.001) Prl release during long and short day lengths. The effect of morphine on Prl release was more pronounced in ewes exposed to long day lengths than in those exposed to short day lengths. In conclusion, EOPs inhibit the LH pulse generator in OVX ewes. However, it is doubtful that the EOPs mediate the steroid-independent effects of photoperiod on LH release. The results also suggest that photoperiod may influence Prl release via opiate neurons.     

Gonadal cycles and levels of luteinizing hormone in wild Spur-winged geese, Plectropterus gambensis

The gonadal cycles of wild Spur-winged geese at Barberspan, South Africa, were examined histologically. Changes in 13 testicular parameters and penis weight and 12 ovarian and oviducal parameters were quantified. Concentrations of luteinizing hormone (LH) in the plasma were measured in both sexes. It was found that, in terms of their histology, the gonadal cycles of Spur-winged geese fit conventional avian patterns. The occurrence of follicular atresia in the ovary is discussed.
In males, the testes of juvenile birds (< 1 yr. old) begin developing at least a month later than those of adults; in juvenile females the ovary often does not develop at all. However, in both sexes, LH levels in juveniles appear to rise simultaneously with those in adults and to reach similar values.
It seems likely that LH release and gonadal recrudescence in Spur-winged geese are under the dual control of photoperiod and some rain-associated factor(s). However, additional cues are required to stimulate the final stages of ovarian development in females. These are probably suitable water depth, suitable nesting cover, rainfall per se and having a mate.     

Seasonal changes in some plasma hormones in pigeons: diurnal variation under natural photoperiods with constant or seasonally changing ambient temperature

The diurnal variations of several plasma hormones and free fatty acids (FFA) were studied during periods in summer and winter for pigeons reared either outdoors or indoors. The latter were subjected to constant temperature and naturally varying photoperiods. A significant seasonal variation in the mean daily levels of triiodothyronine (T3), thyroxine (T4), corticosterone (B), lutropin (LH) and FFA was seen in the outdoor birds and in the T4 and B levels of indoor birds. The diurnal variation of hormone levels was generally more pronounced in winter in both groups. Cold ambient temperature significantly decreased the plasma LH level and potentiated the increasing effect of short photoperiod on plasma B level. Diurnal variation of plasma FFA level seems to be under the control of photoperiod, without any effects due to the ambient temperature. No significant correlation was found between FFA and GH concentrations.     

Influence of ingested petroleum on the reproductive performance and pituitary-gonadal axis of domestic ducks (Anas platyrhynchols)

1. The chronic ingestion of a sublethal dose (5%) of dietary North Sea crude oil delayed the onset of lay in adult Khaki Campbell ducks transferred from a short (8L:16D) to long day (16L:8D) photoperiod and greatly reduced the rate of oviposition and quality (weight and shell thickness) of the eggs subsequently laid. 2. Refeeding the oil-fed birds with the uncontaminated control diet stimulated the rate of egg production and improved egg quality, but in both cases not to the level in the controls. 3. Food intake and the plasma calcium level in the petroleum-fed birds were reduced, but these effects are unlikely to be causally responsible for the adverse effects of petroleum on avian reproduction. 4. Gonadotrophic (luteinizing hormone, LH) hormone secretion in the oil fed birds was not suppressed and the impairment of reproductive performance was not due to low plasma LH levels. 5. The reduced rate of lay in the oil-fed birds was accompanied by low gonadal steroid (progesterone) levels. The detrimental effects of oil and reproduction may be due to direct or indirect effects on the ovary or shell gland.     

Episodic gonadotropin secretion in the mature fowl: serial blood sampling from unrestrained male broiler breeders (Gallus domesticus)

Forty-week-old male broiler breeders were used in two experiments. Males were reared as recommended by the breeder, housed in individual cages, and cannulated to facilitate blood sampling. In experiment 1, blood samples were collected at 10- min intervals for 4 h commencing the day of cannulation (Day 0) and for 12 h on each of Days 1 and 2. In experiment 2, blood samples were collected at 10-min intervals for 8 h on Day 1. After centrifugation, plasma was stored at -20 degrees C until LH, FSH (experiment 1 and 2), testosterone, and corticosterone (experiment 1) concentrations were determined by RIA. Different statistical methods used to identify hormone secretion profiles revealed a characteristic pulsatile pattern of LH and FSH in plasma. However, LH pulses were more frequent and had greater amplitude than FSH pulses. Less than 32% of the FSH pulses were associated with LH episodes. Conversely, the association between LH and testosterone pulses averaged 83% in birds with testis weight greater than 10 g. Concentrations of corticosterone tended to increase after cannulation and remained elevated for only 3-4 h. Our data indicate that LH, FSH, and testosterone secretion is pulsatile in male broiler breeders. Additionally, LH pulses are associated with testosterone episodes but not with FSH pulses. The pulsatile pattern of FSH secretion, which is unique from those of LH, in adult males suggests that FSH secretion is independently regulated in the adult male fowl.     

Evidence for a circadian oscillation in the gonadal response of the tropical weaver bird (Ploceus philippinus) to programmed photoperiod.

The effect of an asymmetrical skeleton photoperiod scheduleee was studied on the gonadal development in a tropical finch, the weaver bird (Ploceus philippinus). The schedul comprised a short nonstimulatory primary photoperiod of 6 hrs and a secondary much shorter lightperiod given as a 15-min light pulse at different times in the dark period. The light pulse 11 hrs after the basic period resulted in gonadal stimulation, while light pulse in contimuation with the basic period or 8, 14 or 21 hrs after the basicperiod was not stimulatory. The "photoinducible phase" was much more precisely outlined by shifting the birds from an 8-h to a 10-h pulse, and from an 11-h to 12-h pulse and was found to be very short, lasting about an hour falling between 11 and 12 hrs after the primary light period. The short photoinducible phase may be of an adaptive value since in the tropics the difference between the shortest and the longest daylength is also rather small (3 hrs and 15 min at Varanasi, 25degrees N). Clearly the weaver bird possesses a fine time-measuring device involving an endogenous circadian rhythm in photosensitivity. In nature, spermatogenesis in this bird also begins in March when the daylength exceeds 11 hrs (thus perhaps coinciding with the photoinducible phase). In rather small, photoperiod may not serve as a cue to trigger seasonal reproductive periodicity, it seems that photoperiod can act as a Zeitgeber for the initiation of spermatogenesis in the weaver bird at least.     

Effects of male courtship and gonadal steroids on pair formation, egg-laying, and serum LH in Canvasback ducks (Aythya valisineria)

The effects of social stimuli on the reproductive physiology of female birds have been widely studied. This study examined the endocrine and behavioural responses of female Canvasbacks ( Aythya valisineria ) to changes in the behavioural and endocrine states of the male, as signalled by his courtship displays. Male Canvasbacks exposed to female conspecifics exhibited a variety of courtship displays which increased significantly with testosterone administration. However, the testosterone-increased courtship displays of males did not increase female receptivity or serum LH, or stimulate reproductive development or egg-laying in unpaired females placed with them. Females confined in cubicles within a pen that held breeding pairs and a free-swimming flock had higher serum LH levels and laid more eggs than did females in cubicles in another pen without breeding pairs or a free-swimming flock. This suggests that social factors, such as vocalizations from a flock and breeding pairs, may increase serum LH and egg-laying in unpaired females. Oestradiol implants in female Canvasbacks did not elevate receptivity to courting drakes, nor did they stimulate pair formation, nesting activity, or egg-laying behaviour. Testosterone implants in drakes and oestradiol implants in females decreased serum LH in each group. These results show that increasing the amount of courtship is not adequate to stimulate pairing behaviour or increases in reproductive hormones. Thus, our results suggest that female receptivity to male courtship may be a key determinant of pair formation and consequent synchronization of the behaviour patterns between members of a pair of wild birds. Our results may further suggest a dissociation between effects of courtship and pair formation in wild Canvasbacks.