Richard S. Donham

Physiological Preparation for Autumnal Migration in White-Crowned Sparrows

Termination of breeding, the development of photorefractoriness, subsequent postnuptial molt, and fat deposition precede autumnal migration in White-crowned Sparrows (Zonotrichia leucophrys). The timing of these postreproductive functions differs from that of analogous vernal functions because they are neither synchronous within the population nor uniformly coincident with environmental stimuli. We hypothesize that the expressions of autumnal functions are internally coupled and that they result from a separate process initiated by the increasing vernal daylength. Our data concerning the effect of various photoregimes on testicular regression, postnuptial molt and fat deposition of Z. 1. gambelii are consistent with the following generalizations: (1) on daylengths that exceed 16 h these processes begin after 40-60 days of photostimulation, inversely related to daylength; (2) transfer of photostimulated (20L 4D) males to an intermediate daylength of 12 h near the end of the testicular growth phase blocks the expression of autumnal functions, although their expression is not blocked if the birds are photorefractory when transferred; (3) males transferred from 12L 12D to 20L 4D in various stages of testicular development begin postnuptial molt after a fixed number of days regardless of the stage of testicular development at the time of transfer; (4) rates of postnuptial molt and fat deposition are inversely related to the daylength to which the birds are exposed when these functions occur, which may explain the acceleration of preparation for autumnal migration in late-starting individuals. Therefore, the systems that control autumnal processes appear not only to be independent of those that regulate analogous vernal processes, but also appear to rely more on internal coupling to integrate these functions than do the latter. The physiological preparation that precedes migration usually involves several functions that require coordination--activation (spring) or inactivation (autumn) of the reproductive system, hyperphagia, and fat deposition, and usually at least a partial molt (Stresemann and Stresemann 1966, Farner and Lewis 1971, Murton and Westwood 1977, Wingfield and Farner 1980). These processes occur in a species-specific order and are temporally separated in small avian forms, although the extent of separation varies among species (Stresemann and Stresemann 1966, Payne 1972, Farner et al. 1980). Although the mechanisms that ensure that these functions occur at the proper time and in the proper sequence have been extensively studied in the migratory races of the Whitecrowned Sparrow (Zonotrichia leucophrys), they remain only partially understood. Differences in preparation for vernal and autumnal migration under natural conditions suggest that different control mechanisms are employed in these two seasons. Preparation for spring migration is synchronized within a population of Z. leucophrys wintering at a given latitude because it appears to be initiated in direct response to increasing daylength (Farner and Wilson 1957, King 1961, King and Farner 1965, Mewaldt and King 1978b). In contrast, preparation for autumnal migration is less synchronous within populations because it does not begin until the birds finish nesting. If a clutch or brood is lost, a pair may renest and delay onset of postnuptial molt, although there is a compensatory acceleration of the molt (Wingfield and Farner 1979). In addition, the decreasing daylength of late summer and autumn differs from lengthening days of spring as predictive information for fall migration (King 1963). Because of (1) lack of synchrony among individuals with the same environmental information and (2) individual differences in rate of preparation due to renesting, i seems likely that this sparrow relies on external information to coordinate its post-repro-

Plasma luteinizing hormone and the development of ovarian follicles after loss of clutch in female mallards (Anas platyrhynchos).

Abstract The plasma level of LH and the extent of development of ovarian follicles were analyzed in incubating female Mallards. In both wild and game-farm stock, incubation was associated with a significant decline in plasma levels of LH from those of laying females. Within 1 day after removal of eggs, LH levels had increased to levels indistinguishable from those of laying females. The mean diameter of the largest follicle in wild females on the tenth day of incubation was 5.3 mm; it was 5.2 mm in game-farm stock at the same stage. Three days following removal of eggs, the mean of the largest follicles of wild-stock hens had increased to 14.0 mm and those of game-farm stock to 12.7 mm.

Daylength and Control of Seasonal Reproduction in Male Birds

Reproduction in most species of birds is discontinuous, or seasonal. Because young are produced during the season that favors their survival (i.e., when environmental conditions are optimal), seasonal reproduction doubtless represents an adaptation to a periodic environment. Although any source of environmental information that reliably predicts forthcoming favorable conditions may be used to trigger reproduction and/or to synchronize it with the optimum season, daylength, because of its consistent and precise relationship to season, especially at middle and high latitudes, seems particularly well suited for such purposes. The importance of daylength was demonstrated experimentally more than 60 years ago when Rowan (1925) reported that extending the photoperiod induced testicular growth and courtship song in slate-colored juncos exposed to the rigors of winter in central Alberta! Since that initial demonstration, photoperiodic control of reproduction has been documented for about sixty species of birds in fifteen families (Lofts et al. 1970, Farner 1975). All of the known photoperiodic species are “long-day” breeders. Although other sources of information (e.g., behavioral interactions, weather) may be important in the fine-tuning and eventual success of seasonal reproduction (Moore 1982, Wingfield et al. 1983, Runfeldt and Wingfield 1985, Wingfield 1985), daylength is clearly the primary proximate factor that controls the onset of the breeding season in most temperate-zone species of birds. We review here the physiological basis of the photoinduced testicular cycle, including the termination of the reproductive effort.

Changes in testicular and plasma androgens with photoperiodically induced increase in plasma LH in the house sparrow

Abstract Testicular growth, plasma levels of reproductive hormones, and testicular content and concentration of androgens were measured in male house sparrows before and after transfer from short (8L:16D) to long (16L:8D) days. Plasma levels of LH and the testicular content of testosterone increased rapidly following transfer to long days, and remained elevated for at least 21 days. In contrast, the testicular concentration of testosterone rose intially and then decreased as the testes increased in size. Elevated levels of testosterone did not appear in the plasma until Day 21 of photostimulation. Levels of DHT in the plasma and in the testis were lower than those of testosterone throughout the period of photostimulation. These results suggest that testosterone is sequestered in the testis of the house sparrow after photostimulation prior to its appearance in the blood.

Physiological Basis of Repeated Testicular Cycles on Twelve-Hour Days (12L 12D) in White-Crowned Sparrows Zonotrichia leucophrys gambelii

The testes of male white-crowned sparrows, Zonotrichia leucophrys gambelii, subjected to a photoregimen of 12-h days (12L 12D) undergo quasiannual cycles of repeated growth and regression. These cycles decrease successively in period and amplitude and are not accompanied by postnuptial molts. This suggests that they are not physiologically equivalent to those of birds subjected to natural photocycles. Our results demonstrate that transfer of white-crowned sparrows undergoing testicular regression on 12L 12D to long days (20L 4D) induces both testicular growth and increases in plasma levels of luteinizing hormone at normal rates for 20L 4D. These data indicate that testicular regression on 12L 12D must be induced by mechanisms other than the development of photorefractoriness, which is the normal cause of regression on natural photocycles. This suggests very strongly that daylength functions as a driver of the independent components of the annual cycle rather than, as in some other species, a Zeitgeber for an endogenous circannual cycle.