By Arthur C. Risser, Jr., Ph.D.
There is no more regularly occurring event in nature than the changing of daylight to darkness. Since the beginning of time, this event has had a profound influence on the seasonality and cyclicity of all organisms. Birds in general and north temperate birds in particular have evolved to gear their annual activities to seasonal changes and specifically to those changes related to day length. Seasonal day length changes provide the most reliable information indicating the approach of a suitable breeding season. However, photoperiodic responses are modified by many environmental inhibitors and accelerators such as temperature, weather conditions, food supply and behavioral interactions. The photo refractory phase of the annual breeding cycle which is present in many, but not all species, acts as a safety mechanism to prevent unseasonal breeding.
Ever since Rowan’s 1920 – 1930’s early work with Juncos in Alberta, Canada, scientists have been intrigued with the physiological mechanisms and the behavioral correlates to changes in day length. Rowan’s early work demonstrated beyond a doubt that increasing the number of daylight hours after the short photoperiod of fall and winter was stimulatory to the migratory and reproductive responses. With added illumination, he was able to trick his captive Oregon Juncos into breeding in the dead of winter even when there was snow on the ground. It is now standard procedure for poultry people to hasten sexual maturity and maintain production by manipulating the lighting scheme in the hen house. Those of us dealing with captive stock should, where possible, consider utilizing this powerful stimulus as a tool to promote reproduction in our captive birds.
This report will give a very brief and general summary of the vast literature which exists relative to photoperiod, discuss some situations in which this type of manipulation has worked or is work= ing and then mention how we as aviculturists may be able to utilize light manipulation to our advantage. This summary is based on two excellent reviews. (See Lofts and Murton, 1968, Zoological Society, London, 155, 327-394; Farner and Follett, 1966, Journal Animal Science, 25, supplement.)
We could be very teleological and state that a bird’s primary purpose in nature is to maximize the offspring it leaves. To do this, a species must accumulate enough energy to make eggs and then provide food to the young that hatch. To achieve breeding condition and anticipate a season of optimum food availability, birds have evolved response mechanisms to various appropriate environmental stimuli which signal the approach of a suitable season. It is, therefore, recognized that the increasing day length which is stimulatory to a great many species is tempered by other factors such as temperature and rainfall which can lead to the emergence of an important dietary item. While increasing day length is stimulatory to gonadal development, it is equally true that under the influence of long summer days, gonads of many species spontaneously regress as the hormone secretions from the anterior pituitary gland diminish and bring about a period of sexual quiescence called the “refractory period”. It has also been demonstrated, however, that some species of birds lack a true refractory condition.
Evidence for gonad stimulation by light and the following refractory period has come through the efforts of physiologists and field ecologists working first with natural populations and then with captive birds to characterize the gonadal cycles of a variety of species. These investigations have meant either harvesting a representative sample of birds from a population at a given latitude, measuring the gonads (or later, examining the bird’s gonads by laparotomy and putting them back into the population). Then light manipulation experiments (especially on White-crowned Sparrows, House Sparrows, starlings and House Finches) helped elucidate the photoperiod mechanism in operation. Some species received more attention than others because that particular species was superabundant or an economic pest, and investigations into breeding cycle might provide information on how to shut off its breeding. Probably sixty to seventy north temperate species in widely separate orders have been shown experimentally to depend on light stimulation to bring about gonadal recrudescence. But where the male may make a full response to increases in day length and undergo active spermatogenesis, females may not give full response and may depend on other factors (i.e., stimulus provided by presence of mate, nesting site, etc.) for full breeding expression.
Basically, the mechanism bringing about the gonadal response is as follows. Appropriate day length is perceived by external receptors and transmitted to the hypothalamus of the brain where the information is further processed. A circadian or 24-hour clock – which is still largely a mystery – influences the hypothalamus, and special chemical signals are then sent to the anterior pituitary which, in turn, releases gonadotropic hormones (FSH and LH) which signal the gonads to undergo egg and sperm production. In turn, the gonads produce sex hormones which bring about certain secondary sexual characteristics and the appropriate reproductive behavior.
One interesting aspect coming out of experimental work is that(at least in some species) the photoperiod response depends on stimulation being received during a light sensitive stage of the circadian rhythm. In other words, light stimulus provided at the wrong time in the species’ 24-hour rhythm may have no response.
There is also considerable field evidence gathered by ecologists to indicate that the breeding cycle may be modified by such environmental factors as temperature, weather and food, but none of these factors bring about any gonad response in the absence of proper light stimulation. Temperature (and even more importantly, a mild winter) has pushed several European species through a winter breeding season when normally they would be inhibited because of poor weather conditions. It is thus important to note that not only is increasing day length stimulatory but so also is the decreasing daylight of late fall and winter to which some species would respond minimally. Under extremely favorable conditions it is conceivable that this autumnal response could become productive. Usually, however, a variety of environmental factors are inhibitory and shut down this autumnal surge before it is completed.
Complicated behavioral factors may also influence the final stages of breeding. Apart from requiring the right habitat and the possession of a territory, most birds depend on a host of reciprocal displays between mates to synchronize and regulate the final stages of reproduction. Males may initiate the breeding season by assuming a reproductive condition before females. The final timing of the season (oviposition) depends on the female receiving the appropriate stimuli from the male and her environment. Stereotyped behaviors can cause specific hormone secretions, a fact well worked out by several studies conducted mainly on captive doves and canaries, although there are still countless details to be settled.
But what of the refractory period or period of sexual quiescence? Anatomically, this period is marked by rapid degeneration of gonadal activity and the accumulation of cholesterol-rich fat tissue in the testes. It involves a condition during which hormone response cannot be brought about by photo-stimulation and has probably evolved to prevent breeding at inappropriate times. (In captivity, however, our objective is to try to provide optimum conditions on a year-round basis). Once a bird emerges from its refractory state, it is once again photo-sensitive. The duration of this refractory period varies among species. It can be as short as six weeks (Quelea) or as long as three or four months. Many temperate zone species, i.e., the mallard, regain their photo-sensitivity in the autumn but remain sexually inactive due to such inhibitory factors as cold temperatures, lack of food and diminishing day lengths which are prevalent in their environment.
In the bobolinks (a trans-equatorial migrant) it has been shown that exposure of autumn birds to an eight-week period of ten hours daylight before stimulating with fourteen hours of light will advance the development of breeding condition by two months over birds maintained on fourteen hours daylight all the time. Trans-equatorial migrants encounter refractory-breaking day lengths in crossing the equator, but since it takes time to go through a refractory phase, southern hemisphere breeding by such birds is an exception rather than a rule. Moreover, some species may lack a photo refractory period entirely and thus be capable of being artificially stimulated throughout the year (as with some species of quail).
It is, therefore, important to emphasize that in any photo-periodic manipulations on captive birds we must be cognizant of this refractory period and use shortened periods of exposure to light as a means to dispel it.
Experiments by Wolfson (see Photoperiodism and Related Phenomenon in Plants and Animals, 1959, AAAS.) have shown that the duration of the daily photoperiod determines (1) the rate at which testicular growth and spermatogenesis proceeds and thus the time of year when it occurs, (2) the degree of response and (3) the duration with which a response persists before the refractory phase begins. Furthermore, the refractory period is dependent on the photoperiod in a quantitative way, short photoperiods hastening its early dispersal and long ones prolonging it.
The importance of photoperiod varies from place to place. Tropical and equatorial breeding cycles are considerably different from those of temperate species. In high latitudes (northern North America, Europe and the Arctic Circle) photoperiod is the main regulator of sexual cycles in many species but in low latitudes, where seasonal changes in day lengths are less, the position is not quite so clear, and factors in the immediate environment are often more important. According to investigations by Alden Miller (formerly Museum of Vertebrate Zoology, Berkeley) in equatorial latitudes (5° south – 5° north), photoperiodic fluctuations may have little or no influence on birds’ reproductive rhythms, but between 5° and 10° north and south latitude, a prevailing influence of photoperiod can be detected.
In equatorial and tropical environments, where annual fluctuations in daily photoperiod are slight or absent, seasonal rains and associated changes in vegetation act as the environmental synchronizers to reproduction. In Indonesia, for example, most species, including evergreen inhabitants, breed from January to March in West Borneo and March to July in Java, depending on the rainy season in the two areas. In contrast, Central American birds mostly breed between April and June, before or early in the seasonal rains, while in southwest Ecuador breeding begins in the onset of rains in the December – April period. Many African land birds have breeding patterns associated with rainfall and show a variety of preferences in different areas. Several Galapagos Finch species brought back to California and held captive by Dr. Robert Orr at the California Academy of Science were stimulated to sing when artificial rain fell on their cages. It is highly likely that the tropical situation reflects a relative insensitivity to and perhaps lack of dependence on photo stimulation. However, some species may still have the capacity to respond to photo stimulation as well as to other types of stimulus. The trick is to determine what the proper stimulation is. A similar breeding response to sporadic rainfall has been noted in many arid or semi-arid species in Australia so that approximately thirty out of sixty species examined in detail attained full breeding condition within a month of rainfall having occurred. Moreover, Australian semi-arid birds studied experimentally showed that they can be affected by variable photoperiods, though under natural conditions other stimuli seemed to be stronger and capable of overriding the effects of light. These opportunistic species have also evolved the ability to respond quickly to suitable breeding conditions by undergoing rapid gonadal maturation as juveniles.
Although not always the case, domestication and captive conditions can have a definite influence on how responsive species will be to photoperiod changes. The common pigeon has had a domestic history dating back perhaps to 3,000 B.C. Under the proper stimulatory conditions, common pigeons (and some of their well-known relatives such as the Collared Dove) may have no refractory period. This may also be true of the Red Jungle Fowl and certainly of some of its phylogenetically close relatives (e.g., some quail).
But what of captive conditions where we have been known to move birds from the southern to the northern hemisphere and vice versa. In general, those species with restricted photoperiodically controlled breeding seasons modify their cycles to suit the day length changes in their new environment.
It is often the case that for the first season an unseasonal response is shown, presumably because the hormone apparatus has been stimulated inappropriately during transfer to the new environment. Thus, we may find southern hemisphere birds coming out of quarantine and laying eggs immediately upon entry into a new aviary condition. (Then there is the case of Silver Gulls taken from Australia to a zoo in the United States. For two years the birds nested in November, then changed to breeding in spring and summer. Seemingly more remarkable is that after twenty years of captivity their descendants reverted to autumn breeding.) Other species such as the emu, budgerigar and European Stork when moved from one hemisphere to another, retain their original breeding season despite a change in the light regime. Other species when moved across the equator, however, shift their breeding season by six months, suggesting that their inherent annual reproductive rhythm can indeed be modified.
Given the condition under which many zoos and private aviculturists keep captive birds, day length is simply one variable which can be controlled. With indoor bird exhibits or housing situations where birds are kept indoors all the time or for a large portion of the year, manipulation of photoperiod can be an easy matter. Timing clocks to turn lights on and off at predetermined intervals are inexpensive and can even be purchased with mechanisms which will automatically change light period on an annual basis to correspond with any predetermined latitude. With today’s modern solid state electronics, it is further possible to provide lighting systems which mimic normal dawn and dusk twilight periods. Ability to provide dawn and dusk twilight is particularly important in arboreal species but not so important for ground dwelling species.
Several captive breeding programs are currently employing photo-period manipulation in their reproductive programs. (1) Patuxent Wildlife Research Center: Whooping Cranes and Masked Bobwhite, (2) Bronx Zoo: Puffins, (3) Cornell University: Peregrine Falcons, (4) Sea World, San Diego: Penguins.
While photoperiod manipulation offers an opportunity to enhance or modify reproduction in captive birds, the following suggestions are offered. (1) Become familiar with the natural history of the species to be subjected to light manipulation. Not all captive birds respond the same. Not only are there species differences but also individual differences within the species. (2) So nearly as possible, try to determine where the captive stock came from. Different populations may have evolved with different conditions. (3) Try to set up replicate experiments with controls. Repeatable experiments lend statistical validity to any results. This may be particularly hard since with some of our exotic birds the sample size is very small.
From an avicultural point of view, day length manipulation may have several advantages. (1) The captive environment can be made to mimic more exactly the conditions under which the species lived in the wild. It is quite possible, therefore, to influence the time at which breeding starts. (2) It may be possible to force a species into a seasonal reproduction and thereby stagger husbandry efforts. (3) By initiating breeding earlier, it may be possible to promote more than the usual number of clutches in a season.