Introduction
Ultraviolet (UV) is the part of the electromagnetic radiation spectrum that lies between 100 and 400 nm and is divided into three types according to wavelength: UVA (315-400 nm), which lies immediately below visible light and also called blacklight; UVB (280-315 nm) or erythemal UV, and UVC (100-280 nm), sometimes called germicidal or bactericidal UV (CIE, Reference CIE1987). Ultraviolet radiation can only be measured with a radiometer because light meters do not record radiation below 400 nm (the lower limit of human vision), and so the usual unit of measurement is mW/m². All forms of UV are emitted by the sun, but the majority (0.99) of the UV that reaches earth is UVA; all of the UVC and most of the UVB are screened out by the ozone layer in the atmosphere. Ordinary window glass is partially transparent to UVA but virtually opaque to UVB, passing about 0.90 of UV above 350 nm, but blocking > 0.90 of UV below 300 nm. Within the domestic fowl's range of retinal photosensitivity (350-780 nm), UV typically forms 0.06-0.07 of sunlight, 0.03-0.04 of white fluorescent light, < 0.01 of light from a sodium lamp, but 0.97 of the emission from a blacklight-blue lamp; it is almost totally absent from candle and incandescent light (Lewis and Morris, Reference LEWIS and MORRIS2006b). Recently, however, a specialist fluorescent lamp has been introduced that emits UVA and visible light in proportions similar to those found in sunlight (Arcadia Bird-lamps (Arcadia lamps, Croydon, CR0 1XP, UK)). Prior to its introduction, the only way to provide UVA to poultry was to use a combination of blacklight-blue and normal lamps, but this resulted in both an uneven distribution of UV and white light and the creation of areas of illumination in which the concentration of UV far exceeded that required. New lamps provide a more practical and economical method of supplying UV to poultry, and so it is apposite that a review is made of the relevance of, and responses to, UV by the various classes of poultry.
UV and animal physiology
Ultraviolet radiation can have both beneficial and detrimental effects, but these vary with wavelength: for example, whereas UVA and UVB induce the synthesis of cholecalciferol (vitamin D3) from 7-dehydroxycholesterol in the skin, UVA, and more so UVB, can damage collagen fibres, destroy vitamin A in skin, cause sunburn, and initiate molecular changes that may lead to skin cancer; artificially generated UVC can be used for microbial sterilization purposes, but an overexposure to UVC causes ocular damage and severe facial sunburn. Although UVB and UVC pose greater risks to human health than UVA, all forms of ultraviolet can be dangerous, and so the International Commission on Non-Ionizing Radiation Protection (ICNIRP) has set limits of 30 J m-² (0.001 W m-² effective irradiance) ultraviolet radiation for unprotected skin and 10 kJ m-² for retinal exposure in an 8-h period (ICNIRP, 2004). Exposure levels should be checked before anyone is asked to work in an environment in which UV radiation is being emitted.
Avian vision and extra-retinal photoreception
All forms of ultraviolet radiation are invisible to humans, but poultry, in common with other avian species, possess an extra retinal cone that allows the transmission of radiation at wavelengths shorter than 400 nm (Bowmaker et al., Reference BOWMAKER, HEATH, WILKIE and HUNT1997), and this, together with the optical clarity of their lens and humours to radiation between 320 and 400 nm (Govardovskii and Zueva, Reference GOVARDOVSKII and ZUEVA1977; Hart et al., Reference HART, PARTRIDGE and CUTHILL1999), enables them to see in UVA. This ability was first demonstrated in domestic fowl by Wortel et al. (Reference WORTEL, RUGENBRINK and NUBOER1987) using an electrophysiological test and later confirmed by Prescott and Wathes (Reference PRESCOTT and WATHES1999a) using a psychophysical test (Figure 1).
Relative spectral sensitivity of domestic fowl (bold data: Prescott and Wathes, Reference PRESCOTT and WATHES1999a) and humans (faint data: CIE, Reference CIE1987) from radiation between 350 and 800 nm (light ranges from 400 to 780 nm).
Retinal reception of wavelengths of radiation between 350 and 780 nm allows the bird to see and to modulate various behavioural activities. Additionally, retinally perceived UVA suppresses the activity of serotonin N-acetyltransferase and the release of melatonin in the pineal gland of dark-adapted birds, a process that, in contrast to white light reception, does not involve dopamine but is activated by stimulation of retinal N-methyl-D-aspartate (NMDA) glutamate receptors (Rosiak and Zawilska, Reference ROSIAK and ZAWILSKA2005). The second site of avian photoreception is the suprachiasmatic nucleus (SCN) of the hypothalamus, sometimes referred to as the extra-retinal or deep encephalic photoreceptor. Light can penetrate plumage, bone, and cranial tissues to reach the base of the brain and, together with, though to a lesser extent than, light received from the retina via neural signals, is used to regulate the bird's sexual activity by controlling the secretion of GnRH (Scanes, Reference SCANES1984) and GnIH hormones (Ciccone et al., Reference CICCONE, DUNN, BOSWELL, TSUTSUNI, UBUKA, UKENA and SHARP2004).
The efficiency with which light penetrates to the hypothalamus is dependent upon its wavelength, with shorter wavelengths experiencing more absorption by the cranial tissues than longer wavelengths (Benoit, Reference BENOIT1964; Hartwig and van Veen, Reference HARTWIG and van VEEN1979; Foster and Follett, Reference FOSTER and FOLLETT1985). Some failures of shorter wavelengths of radiation to elicit a photosexual response may be due to their suboptimal ambient illuminance rather than a lack of a response to the wavelength per se (Benoit et al., Reference BENOIT, WALTER and ASSENMACHER1950). For example, Foster and Follett (Reference FOSTER and FOLLETT1985), using optic fibres to shine light directly on the hypothalamus of quail, found that blue light at an intensity of 1.7 x 1010 photons/m².s not only succeeded in inducing a rise in plasma Luteinizing Hormone (LH) but did so using only one twentieth of the intensity required for red light (most stimulatory ambient colour of light) to achieve the same response.
Behavioural responses
Light, including UVA, which falls on the retina of the avian eye allows the sensation of sight and influences behaviours such as peer recognition, mate selection, the establishment and maintenance of social hierarchies, social encounters, and locomotor and feeding activities.
In preference tests, poultry have not found UVA aversive; indeed, Moinard and Sherwin (Reference MOINARD and SHERWIN1999) reported that, irrespective of whether they had been reared with or without supplementary UVA, young turkeys chose white fluorescent light supplemented with UVA in preference to white fluorescent light on its own. It may also have been a preference for UVA that encouraged White Leghorn laying hens (Widowski et al., Reference WIDOWSKI, KEELING and DUNCAN1992) and turkey males (Sherwin, Reference SHERWIN1999) to choose a standard fluorescently illuminated environment over an incandescent one; white fluorescent light typically has a UVA content of between 0.03 and 0.04 whilst incandescent has none (Lewis and Morris, Reference LEWIS and MORRIS2006b). However, the supplementation of continuous incandescent illumination with 12 h of added UVA from a blacklight-blue lamp inexplicably reduced feed intake in laying hens during the period of UVA exposure, even though the total daily feed intake was not significantly different from that of non-UVA-supplemented controls (Lewis et al., Reference LEWIS, PERRY and MORRIS2000a). The 0.75 proportion of UVA in the lighting environment provided in this study exceeded tenfold that found naturally in sunlight, and so there may have been some aversion to the spectral balance of the light rather than to UVA per se.
Domestic fowl (Prescott and Wathes, Reference PRESCOTT and WATHES1999b) and turkeys (Sherwin and Devereux, Reference SHERWIN and DEVEREUX1999) have markings on their plumage which reflect UVA, and, in domestic fowl at least, reflectance occurs with the same efficiency as that of red light (Prescott and Wathes, Reference PRESCOTT and WATHES1999b). Variations in the colour and saturation of these markings in both domestic fowl and turkeys may be used in the recognition of peers and provide visual cues for use during mate selection and before agonistic interactions. Widowski et al. (Reference WIDOWSKI, KEELING and DUNCAN1992) observed that White Leghorn laying hens performed proportionately more preening under fluorescent than under incandescent (no UVA) illumination and hypothesised that the birds’ plumage may have appeared different under the two types of light (our knowledge of the involvement of UV in avian vision and its effect on their behaviour was minimal when this study was reported).
Sherwin and Devereux (Reference SHERWIN and DEVEREUX1999) noted a coincidence of the development of UV-reflective markings with the onset of injurious pecking at specific locations on turkey poults: for example, in the prepatagium at about 8 d and at the base of the tail between 15 and 20 d. These findings led the authors to suggest that, in the absence or insufficiency of UV radiation, potentially UV reflective areas of plumage might appear dark and that the unnaturally dull appearance, especially if contrasted with brighter surrounding plumage, encourages pecking activity. Subsequently, Sherwin et al. (Reference SHERWIN, LEWIS and PERRY1999) and Lewis et al. (Reference LEWIS, PERRY, SHERWIN and MOINARD2000b) demonstrated that UVA supplementation (60-160 mW/m² at bird-head height) of either fluorescent or incandescent white light minimised injurious pecking in intact (not beak-trimmed or de-snooded) turkey males, especially when given in conjunction with environmental enrichment and at an illuminance of 5 lux. Although these findings demonstrated the ameliorative effect of UVA on the incidence of injurious pecking, Lewis et al. (Reference LEWIS, PERRY, SHERWIN and MOINARD2000b) concluded that the interactions among UV supplementation, illuminance, and material environmental enrichment were complex and that these precluded the provision of a simple lighting solution to injurious pecking.
The involvement of UVA in mate selection, which was originally demonstrated in non-domesticated birds (Bennett et al., Reference BENNETT, CUTHILL, PARTRIDGE and MAIER1996, Reference BENNETT, CUTHILL, PARTRIDGE and LUNAU1997), has been observed to play an important role in the sexual activity of broiler breeders (Jones et al., Reference JONES, PRESCOTT, COOK, WHITE and WATHES2001). Supplementation of white fluorescent light with light from a specialised UVA-emitting lamp increased both the number of matings and the amount of locomotor activity performed by broiler breeder males, and, in a second study, female broiler breeders spent more time inspecting cockerels when illuminated with 0.15 UVA light than with a 0.02, 0.44 or 0.58 UVA environment. So UVA is clearly implicated in the transmission of signals and communication associated with sexual activity, and its absence or minimal presence in a lighting environment, for example in incandescently illuminated poultry houses, is likely to jeopardise the efficiency with which visual cues are transmitted to mediate mate choice and mating behaviour (Jones and Prescott, Reference JONES and PRESCOTT2000).
Growth responses
Light mainly influences growth, particular during the first three weeks, through an indirect effect on feeding activity, for example, as longer photoperiods are associated with higher feed intakes (Lewis and Morris, Reference LEWIS and MORRIS2006a). However, when birds are fed diets that are deficient in vitamin D3, growth is beneficially affected by UVA and UVB radiation through a facilitation of vitamin D3 synthesis in the skin. Sub-optimal growth and severe leg weakness are the first signs of a vitamin D3 deficiency in young poultry (Leeson and Summers, Reference LEESON and SUMMERS2001), but surprisingly the small amount of UVA present in white fluorescent light has been shown to be sufficient to ensure normal development (Edwards et al., Reference EDWARDS, ELLIOT, SOONCHARERNYING and BRITTON1994; Elliot and Edwards, Reference ELLIOT and EDWARDS1997; Edwards, Reference EDWARDS2003; Ledwaba and Roberson, Reference LEDWABA and ROBERSON2003). Ledwaba and Roberson (Reference LEDWABA and ROBERSON2003) observed an improvement in feed conversion efficiency, elevated serum calcium, and improved phytate phosphorus retention in broilers fed a vitamin D3 deficient diet when illuminated with white fluorescent light. The inclusion of rye in a diet is known to reduce the availability of vitamin D3, but a 10 minute daily exposure to supplemental UVB radiation was noted to be sufficient to diminish the antagonism and to allow normal growth in young broilers (Mac-Auliffe and McGinnis, Reference MAC-AULIFFE and McGINNIS1976).
When male turkeys were given nutritionally complete diets, feed intake, growth, and feed conversion efficiency were similar for all groups, irrespective of whether they had been illuminated with incandescent or warm-white fluorescent lamp, and with or without UVA supplementation (60-160 mW/m² intensity at bird-head height) (Lewis et al., Reference LEWIS, PERRY, SHERWIN and MOINARD2000b). The absence of any direct effect of UV on growth when nutritionally adequate diets are fed was also demonstrated in broilers which had been continuously exposed to UVA radiation from fly traps (Hogsette and Wilson, Reference HOGSETTE and WILSON1999).
Surprisingly, continuous exposure to UVC radiation was reported to have accelerated the growth rate of broilers in three of four trials and to have improved feed conversion efficiency in all four, but had no effect on feed intake (Perek and Heller, Reference PEREK and HELLER1970). However, in a repeat study conducted to investigate possible links between continuous UV exposure and the incidence of ocular abnormalities, Barnett and Laursen-Jones (Reference BARNETT and LAURSEN-JONES1976) observed no difference in performance between experimental and control groups. As poultry have no retinal reception of UVC and UVC does not induce the dermal synthesis of vitamin D3, it is difficult to understand how UVC could have improved the performance in the first study, however, maybe it resolved an unknown microbial contaminant in the feed.
Reproductive responses
Sexual maturation, rate of egg production, and the timing of the ovulatory cycle are modulated by the photoinduced activity of the hypothalamo-pituitary axis. We have seen in an earlier section (avian vision and extra-retinal photoreception) that the SCN of the avian hypothalamus principally uses directly perceived light to regulate its output of GnRH (Siopes and Wilson, Reference SIOPES T.D. and WILSON1980a) and that the efficiency with which radiation penetrates directly to the hypothalamus decreases proportionately with wavelength. Ultraviolet radiation has a shorter wavelength than visible light, and so photosexual responses to direct UV reception are likely to be weak or non-existent, especially at the relatively low light intensities prevalent in controlled environment poultry houses, although the situation may be different in natural lighting conditions where the intensity of UVA will be far higher; the intensity of UVA on an autumn morning at latitude 50°N exceeded 1000 mW/cm² (recorded by the senior author). Notwithstanding that direct cranial penetration is the main route for hypothalamic light reception, retinally received light can be important at low illuminances when photic penetration through the skull and cranial tissues is reduced (Siopes and Wilson, Reference SIOPES T.D. and WILSON1980b), and so UVA may be involved in the photosexual response in these circumstances.
A greater persistency of egg production was observed in broiler breeders kept in open-sided housing and exposed to a mixture of fluorescent and natural light when they had been reared in lightproof rooms under dedicated ‘bird-lamps’ (0.07-0.10 UVA) than under white fluorescent lighting. It was hypothesised from these findings that the UVA had, in some way, delayed the onset of adult photorefractoriness (Lewis et al., Reference LEWIS, GHEBREMARIAM and GOUS2007). Contrastingly, in two further studies of broiler breeders conducted at the University of KwaZulu-Natal, but this time in lightproof adult housing, exposure to UVA (2.5±0.22 mW/m²) from special bird-lamps in either or both the rearing and laying periods failed to have any effect on egg production or egg weight (Table 1). However, the fertility of eggs laid by birds illuminated by bird-lamps during the laying period, irrespective of the lighting environment in rearing, was 2% significantly higher than for eggs produced under white fluorescent lighting. Interestingly, this improvement in fertility was of a similar magnitude to that reported by Jones et al. (Reference JONES, PRESCOTT, COOK, WHITE and WATHES2001) for broiler breeders given 0.17 supplemental UVA. It is postulated that these enhancements in fertility were consequences of an increase in mating activity stimulated by better peer-recognition and a more effective transmission of sexual cues.
Mean age at sexual maturity, eggs numbers, egg weight, and mortality for broiler breeders reared on white light and transferred to white or white and UVA light (Trial 1) or reared on white or white and UVA light and transferred to white or white and UVA light (Trial 2) at 20 weeks.
* Illumination from Philips compact warm-white fluorescent lamps (8W in rearing and 23W in the laying period). Illuminance in laying period: 25±1.9 lux, 0.8±0.04 mW/m² UVA.
† Illumination from 20W compact Arcadia Bird-lamps. Illuminance in laying period: 21±1.9 lux, 2.5±0.22 mW/m² UVA.
Further evidence of a lack of effect of UV on hypothalamic activity are similar egg-laying performances by caged laying hens exposed to UVA from continuously operated UV fly traps fitted with blacklight or blacklight-blue lamps (Hogsette et al., Reference HOGSETTE, WILSON and SEMPLE-ROWLAND1997) and floor-housed broiler breeders illuminated by white fluorescent light supplemented with UVA from specialist lamps (Jones et al., Reference JONES, PRESCOTT, COOK, WHITE and WATHES2001). Additionally, a daily supplementation of white incandescent light with 8 h or 12 h of UVA (140 mW/cm²) failed to modify the timing or entrainment of the ovulatory cycle in caged laying hens (Lewis et al., Reference LEWIS, PERRY and MORRIS2000a).
Although UVA does not appear to be able to elicit a photosexual response, the inability may be more due to a sub-optimal intensity at the hypothalamus than to an insensitivity of hypothalamic photoreceptors to UVA per se. Although Foster and Follett (Reference FOSTER and FOLLETT1985) did not include wavelengths of UV when they tested the quail's hypothalamic sensitivity to light direct to the surface of the hypothalamus via optic fibres, their finding that the hypothalamus was twelve times more sensitive to violet light than to red light (the most sexually stimulatory ambient wavelength) suggests that the avian hypothalamic photoreceptors could be responsive to UVA if its intensity at the skull surface equalled or exceeded the 6.5 x 10-7 μE.cm-2.s-1 (≡ 3.9 x 1015 photons/m-².s-1) threshold necessary for white light to induce a photoperiodic response.
Laying hens fed a vitamin D3 deficient diet suffer a drop in egg production within 2-3 weeks (Leeson and Summers, Reference LEESON and SUMMERS2001), however, when exposed to UVB irradiation, egg production, fertility, and hatchability were maintained at levels typical for birds fed nutritionally complete diets (Hughes and Payne, Reference HUGHES and PAYNE1924; Hart et al., Reference HART, STEENBOCK, LEPKOVSKY, KLETZEIN, HALPIN and JOHNSON1925; Hendricks et al., Reference HENDRICKS, LEE and GODFREY1931; Titus and Nester, Reference TITUS and NESTER1935; Mussehl and Yung, Reference MUSSEHL and YUNG1942). Contrastingly, there were no effects on egg production, egg weight, shell quality, or hatchability when egg-type breeders were fed nutritionally complete diets and illuminated with incandescent white light supplemented with UVB (Carson and Beall, Reference CARSON and BEALL1955). It is likely that the benefits derived from the provision of UVB in the earlier studies were hygienic consequences of the synthesis of vitamin D3 from 7-dehydroxycholesterol in the skin and not direct effects of UVB per se.
In an experiment in which breeding turkeys were illuminated for 14 h daily with incandescent light, UVB radiation, or a combination of incandescent and UVB, UVB-only birds were the last to mature and had the poorest egg production, whilst the combined groups laid most eggs and had the best persistency (Carson and Junnila, Reference CARSON and JUNNILA1953). The later maturity and inferior egg production of the UVB-only birds were undoubtedly a consequence of minimal hypothalamic photoreception, whilst the antirachitic property of UVB may have facilitated some synthesis of vitamin D3 if the mash and grain feeding programme used had been marginal for D3. This also explains why the combined treatment performed better than the incandescent-only treatment (incandescent lamps do not emit UV). These responses to UVB radiation are not surprising, because there is no retinal reception, and penetration to the hypothalamus would be trifling, thus confirming the non-involvement of UVB in reproductive activity.
Bizarrely, in each of five studies conducted between 1945 and 1949, laying hens exposed to a mixture of fluorescent and incandescent light supplemented with UVC radiation had significantly better egg production than white-light-only controls or hens provided with supplemental UVA or UVB (Barott et al., Reference BAROTT, SCHOENLEBER and CAMPBELL1951). UVC is certainly unable to induce a photosexual response at the hypothalamus and, unlike UVA and UVB, does not stimulate the synthesis of vitamin D3 in the skin to rectify any dietary deficiency. Support for these findings being anomalous comes from Carson and Beall (Reference CARSON and BEALL1955) who reported that egg production, egg weight, shell quality, and hatchability for laying hens given supplemental UVB, UVC, or a combination of UVB and UVC were no different from controls illuminated with incandescent light only.
Sanitary responses
Most of the benefits of UV radiation to health in poultry arise from either the ability of UVA and UVB to induce the synthesis of vitamin D3 from 7-dehydroxycholesterol in the skin or from the germicidal properties of UVC.
Rather surprisingly, the relatively small amount of UVA emitted by white fluorescent lamps has been shown to induce sufficient synthesis of vitamin D3 to minimise the incidences of rickets and tibial dyschondroplasia (TDP) and to maintain normal levels of bone ash in broilers and turkeys fed diets which were deficient in vitamin D3 (but adequate in calcium and phosphorus). Although the recommended level of vitamin D3 for broiler diets is 5 μg/kg (NRC, Reference NRC1994), the concentration needs to be increased to ≥ 20 μg/kg when there is no exposure to UVA (Edwards et al., Reference EDWARDS, ELLIOT and SOONCHARERNYING1992, Reference EDWARDS, ELLIOT, SOONCHARERNYING and BRITTON1994; Edwards, Reference EDWARDS1993; Elliot and Edwards, Reference ELLIOT and EDWARDS1997; Ledwaba and Roberson, Reference LEDWABA and ROBERSON2003); fluorescent illumination can therefore have a sparing effect on the levels of vitamin D3 required in diets. However, it seems that the UV radiation alone cannot prevent TDP in all genotypes. When two lines of broilers that had been divergently selected for a high or low incidence of TDP were fed diets adequately fortified with vitamin D3 and illuminated with white fluorescent light, the UVA component failed to reduce the incidence of TDP in the highly-susceptible line (Mitchell et al., Reference MITCHELL, EDWARDS and McDANIEL1997). It also appears that when broilers are fed a vitamin D3 deficient diet, they need to be exposed to the UVA component of fluorescent illumination from soon after hatching, even if exposure is subsequently limited to one day in three (UVA filtered out on the other two days). When the initial exposure to the UVA was delayed until 4 d of age, bone ash at 16 d was reduced and the incidences of rickets and TDP were increased relative to birds given UVA exposure from day one (Edwards, Reference EDWARDS2003).
Koch and Koch (Reference KOCH and KOCH1941) found that skin from the feet and legs of chicken contained eight times as much 7-dehydroxycholesterol as body skin, and Tian et al. (Reference TIAN, CHEN, LU, SHAO and HOLICK1994) found that, following whole-body irradiation of chickens with UVB, pre-vitamin D3 was present in the skin of the feet and legs, but not on the back. It is not surprising, therefore, that UVB radiation of caged broilers from below was twice as effective as that provided from above (Edwards, Reference EDWARDS2003). It was also concluded from this study that vitamin D3 synthesised in the skin in response to UVA from white fluorescent lighting was used more effectively than the equivalent amount of D3 provided by the diet.
The UVA content of white fluorescent light has been reported to improve phytate phosphorus retention and to increase sera calcium concentration (Edwards, Reference EDWARDS1993; Ledwaba and Roberson, Reference LEDWABA and ROBERSON2003), and to minimise antagonism between vitamins A, D and E in the diet (Aburto and Britton, Reference ABURTO and BRITTON1998). A 10-min daily exposure to UVB radiation has also been demonstrated to be sufficient to negate the antagonism between rye and vitamin D3 in broiler diets, and, as a result, to normalise bone ash content (Mac-Auliffe and McGinnis, Reference MAC-AULIFFE and McGINNIS1976).
The provision of supplemental UVC radiation completely protected White Leghorn chicks against an artificial aerogenic infection with Newcastle Disease (ND), with no symptoms of, or mortality from, the disease. Furthermore, the protection against ND by UVC did not interfere with the chicks’ immune response to subsequent inoculation with ND vaccine (Perek and Heller, Reference PEREK and HELLER1970).
Pathological responses
Constant exposure to UVA radiation from the blacklight or blacklight-blue lamps of fly traps had no effect on the eye pathology of laying hens (Hogsette et al., Reference HOGSETTE, WILSON and SEMPLE-ROWLAND1997). However, continuous exposure to UVC radiation produced a mild conjunctivitis, but without any apparent visual impairment, in broiler chickens (Perek and Heller, Reference PEREK and HELLER1970) and laying hens (Barott et al., Reference BAROTT, SCHOENLEBER and CAMPBELL1951), and a roughening of the surface of the cornea and a thickening of the corneal epithelium but with no signs of vision impairment or buphthalmia in broilers exposed to continuous UVC radiation and 23 h of incandescent light (Barnett and Laursen-Jones, Reference BARNETT and LAURSEN-JONES1976). These findings are not surprising because over-exposure to UVC radiation can cause corneal damage in humans (Sheng and Birkle, Reference SHENG and BIRKLE1995).
Conclusions
Various studies of the responses of wild birds to UVA exposure, for example, circadian activity in canaries (Pohl, Reference POHL1992) and mate selection in zebra finches (Bennett et al., Reference BENNETT, CUTHILL, PARTRIDGE and MAIER1996) and starlings (Bennett et al., Reference BENNETT, CUTHILL, PARTRIDGE and LUNAU1997), prompted Bennett and Cuthill (Reference BENNETT and CUTHILL1994) to conclude that the main functions for retinal UVA sensitivity were orientation, foraging, and sexual selection. The identification of UV vision in more than 30 avian species also led Bennett and Cuthill to suggest that the ability to see in UV was probably a general property of all birds. The increased mating activity and facilitation of mate selection from the recognition of UVA-reflective markings on the plumage and fleshy appendages and enhancement of fertility in broiler breeders, the depressive effect of supplemental UVA on the feed intake of laying hens, the suppressive effect of retinally perceived UVA on serotonin activity and melatonin release in the pineal gland of dark-adapted chicken, the potential involvement of UVA in the onset of photorefractoriness in broiler breeders, and the reduction of injurious pecking in male turkeys confirm the involvement of retinally perceived UVA in the behavioural activities of poultry. However, the inability of UVA to induce testicular development in Mallard drakes, the lack of influence on sexual maturation or egg production in laying hens and broiler breeders, and the failure to phase-shift the timing of the ovulatory cycle in laying hens show that UV is not involved in the hypothalamic control of reproduction, essentially because of the poor penetration of intra-cranial tissues by short wave radiation.
The ability of UVA and UVB to synthesise vitamin D3 from 7-dehyroxycholesterol in the skin, especial of the legs and feet, and the beneficial effects of such a function on growth, the incidences of TDP and rickets, and the maintenance of normal bone-ash in birds fed cholecalciferol-deficient diets, makes this an important property of UV, especially in areas of the world where poultry are illuminated with natural light and the nutritional quality of poultry diets cannot be guaranteed. It is unlikely to be of relevance to developed poultry industries, even when illumination is provided by incandescent lamps, because diets usually have adequate levels of vitamin D3.
The improved reproductive performances of domestic fowl and turkeys exposed to UVC radiation observed in some of the early research are not likely to have been direct effects of UV on the activity of the reproductive system because avian retinal photoreceptors are not sensitive to UVC, and intracranial penetration of UVC to the hypothalamus is inconceivable; more likely these were consequences of some germicidal action by UVC. The ability of UVC to protect poultry against airborne viral infections, whilst interesting, is somewhat impractical and dangerous, and protection against most viral poultry diseases can be provided by vaccination.
Acknowledgements
We wish to acknowledge the generous donation of day-old chicks by Ross Poultry Breeders (South Africa), the supply of bird-lamps by Arcadia Lamps, UK, and the willing assistance of Raife Danisman in supervising the studies conducted at the University of KwaZulu-Natal.
