On Photoperiod and Captive Breeding of Northern Peregrines

year.Migratory species (at least) when placed on long daylengths put on weight -develop their fat deposits (Wolfson 1959)-presumably in preparation for migration.The length of time it takes for the birds to come into breeding condition depends on a number of factors (light intensity, species, etc.).While the testes of males will develop fully in response to long days, the ovaries of females will not develop fully solely due to photoperiodic stimulation.For full ovarian development females, apparently, also require "psychic" factors-nesting material and/or a nesting site and, perhaps, a photoactivated male also (Farner 1959).Many species respond to the rather long days of a natural spring by breeding.However, before the longest day of summer is reached, their gonads have regressed and they have lost their ability to breed until the following year (see Schwab 1970: Fig. 1 ).The rather long spring days "turn on" the birds; the even longer days near June 21 appear to "turn them off."Various researchers have shown that by holding the birds at a certain daylength, they can be kept in breeding condition for many months.Male Starlings (Sturnus vulgaris).fromCalifornia held on daylengths of 10V2 or 11 hours remained in breeding condition "for at least 15 months" (Schwab 1970).With 12 hours of daylight they came into breeding condition and then passed through it in a couple of months, as in the wild.That is, the longer day apparently caused their breeding cycle to eventually end, whereas slightly shorter daylength caused the birds to remain in breeding condition for over a year (until the experiment was ended).The daylengths required for this response seem to vary with latitude (where the birds originated) and with species.Some species, when held for many months on winter (short) daylengths, have eventually started to come into breeding condition.This response has led to the suggestion that birds summate or "store up" something from daylengths.Apparently, when they have "stored up" enough, they will come into breeding condition.This is the "summation hypothesis."The period of darkness each day may be as important as the period of light.A number of experiments have shown that a long night (short day) which is broken by a period of light (even a short period of light) can affect the bird in the same way as a short night (long day). Summer 1972 Nelson-Photoperiod and Breeding of Peregrines 59still long enough to cause appreciable stimulation of the birds.Sexual activities in the autumn apparently vary from year to year depending on weather conditions.Farner (1954, cited by Engels 1959) suggests that for North American species which winter south of the equator, the refractory period may be rather long, ending sometime after the South American summer solstice (December 21).The birds could then be triggered to migrate northward by the still long (though shortening) days.Engels (1959) found that Bobolinks (Dolichonyx oryzivorus) (breeding 40-50 degrees N, "wintering" 15-25 degrees S) possess a photorefractory period.Birds held on short ("winter") days for eight weeks, then placed on 14-hour ("summer") days, came into breeding condition more quickly if the "winter" (refractory period) days were 10 hours instead of 12 hours long.Shorter "winter" days appear to hasten the end of the refractory period.Most experiments with photoperiod appear to have been conducted with sudden changes from one daylength to another (e.g. from 10 hours suddenly to 14 hours).This has probably been done more as a matter of convenience than a result of experimental design.However, as will be pointed out below, it appears that from gradual changes in daylength more can be learned and some particular problems may be avoided.At the moment, in attempting to breed raptors, we must be primarily concerned with achieving successful reproduction.Experimentation into the causes and internal mechanisms can come later.Wild birds respond to gradual changes in photoperiod and it should be expected that they will also respond best to gradually changing photoperiod in captivity. Annual Cycle of Northern PeregrinesFor a northern Peregrine the year is broken into four distinct periods: migration south, "wintering," migration north, and breeding.It is with the breeding season that we are primarily concerned in captive breeding, but. the other periods of the year are probably equally important in setting the stage for breeding.In Figure 1 some of the available data on migration and "wintering" of northem Peregrines are summarized.Only the more important points will be considered here.

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