| Titre : |
Breeding seasons and reproductive physiology of tricolored blackbirds and redwinged blackbirds |
| Type de document : |
texte imprimé |
| Auteurs : |
Payne, Robert B., Auteur |
| Editeur : |
RUC |
| Année de publication : |
1969 |
| Importance : |
137 p. |
| Format : |
26 cm. |
| Langues : |
Anglais (eng) |
| Index. décimale : |
598 Oiseaux |
| Résumé : |
Tricolored blackbirds (Agelaius tricolor) nest in central California both in spring and, in smaller numbers, in autumn. The closely related sympatric red-winged blackbirds (A. phoeniceus californicus) breed only in spring. The tricolored blackbirds breed both in years of late summer rains and in dry years, nest-both in the hills and in irrigated agricultural lands. The autumnal breeding season of tricolored blackbirds is of particular interest to concepts of the physiological control of sexual activity; gonadal development of songbirds in temperate regions with predictable seasonal rainfall is generally thought to be controlled by the increasing daylengths of springtime.
The annual testis cycles of the two species are nearly identical with the exception of a recrudescence in some male tricolored blackbirds in early autumn at the completion of molt. While spermatogenesis is similar in other birds, the histological details of testieular regression at the end of the breeding season in redwinged blackbirds and in males feeding their young in tricolored blackbirds indicate both a massive lipoidal degeneration in the tubules and also a pronounced invasion of the interstitium and the tubules with lymphocytes, polymorphonuclear granular leucocytes, macrophages, and fibroblasts. The infiltration by these cells characteristic of injured tissues argues against a proposed secretory (progestin) function of the steroidal, post-nuptial seminiferous tubule. The cytological changes in the epithelioid interstitial cells of the testes, including lipid and cholesterol activity, suggest a histophysiological basis for androgen secretion both during seasonal testicular enlargement and in the breeding season.
Species differences in ovarian development through the nesting cycle are correlated with differences in the time required for nest-building, but in both species the clutch size is determined by means of the differential rate of growth of ovarian follicles by the first day of laying. Renesting of individual blackbirds within a week of the loss of an unsuccessful clutch is possible apparently through the retention of relatively small, lightly yolked ovarian follicles in the ovary through in-cubation and the early period of parental care.
Breeding of females of both blackbirds involves deposition of fat as an energy reserve before the final stages of ovarian development. Restriction of mass yolk deposition in the ovary to the precise number of follicles that are ovulated and to the four days immediately before ovulation is important in the energy balance of breeding females. Females lose their body fat deposits and decrease significantly in body weight during laying, incubation, and the period of parental care. The post-nuptial molt in summer is completed in about two months in both species; molt is delayed in late-nesting blackbirds. Changes in fat and weight in early winter indicate a strain on the energy balance, and adaptive flexibility of the breeding-molting schedule occurs also in autumn with the absence of a second postnuptial molt after the autumnal breeding season in tricolored blackbirds.
Experimental studies show that males and females of both species develop large gonads in response to long daylengths. The rate of growth of the testes of wild blackbirds in spring corresponds closely to the rate predicted from the experimentally determined photoperiodic response. Development of ovarian follicles up to 2.0 mm diameter in females on long photoperiods is independent of male song and display. Tricolored blackbirds held under natural seasonal daylength for nine-teen months were sexually active in spring but remained inactive during autumn. The majority of males in early autumn are refractory to long photoperiods. Sexually active males in the field in autumn are thought to be mainly the individuals that nest successfully early in spring as the testes of males feeding young in spring regress earlier than do the testes of other males. Some younger males develop large testes in their second autumn. Male tricolored blackbirds showed gonadal enlargement in autumn in experimental conditions only when the phases of their refractory periods were altered, but all experimental females developed enlarged ovaries in early autumn when fed live grasshoppers or soaked by simulated rains.
The difference in breeding seasons in these two blackbirds is related to species differences both in physiological control of sexual activity and in ecology. Gonadal development and nesting in tricolored blackbirds are apparently controlled in part directly by rainfall, an abundance of food, and social interaction, as well as by daylength. Rainfall and food are temporary features of the environment and so have little value of prediction of future environmental conditions in the long run to species such as the redwinged blackbirds which have extended periods of territorial establishment, mate selection, and nest-building. The abbreviated nesting |
Breeding seasons and reproductive physiology of tricolored blackbirds and redwinged blackbirds [texte imprimé] / Payne, Robert B., Auteur . - RUC, 1969 . - 137 p. ; 26 cm. Langues : Anglais ( eng)
| Index. décimale : |
598 Oiseaux |
| Résumé : |
Tricolored blackbirds (Agelaius tricolor) nest in central California both in spring and, in smaller numbers, in autumn. The closely related sympatric red-winged blackbirds (A. phoeniceus californicus) breed only in spring. The tricolored blackbirds breed both in years of late summer rains and in dry years, nest-both in the hills and in irrigated agricultural lands. The autumnal breeding season of tricolored blackbirds is of particular interest to concepts of the physiological control of sexual activity; gonadal development of songbirds in temperate regions with predictable seasonal rainfall is generally thought to be controlled by the increasing daylengths of springtime.
The annual testis cycles of the two species are nearly identical with the exception of a recrudescence in some male tricolored blackbirds in early autumn at the completion of molt. While spermatogenesis is similar in other birds, the histological details of testieular regression at the end of the breeding season in redwinged blackbirds and in males feeding their young in tricolored blackbirds indicate both a massive lipoidal degeneration in the tubules and also a pronounced invasion of the interstitium and the tubules with lymphocytes, polymorphonuclear granular leucocytes, macrophages, and fibroblasts. The infiltration by these cells characteristic of injured tissues argues against a proposed secretory (progestin) function of the steroidal, post-nuptial seminiferous tubule. The cytological changes in the epithelioid interstitial cells of the testes, including lipid and cholesterol activity, suggest a histophysiological basis for androgen secretion both during seasonal testicular enlargement and in the breeding season.
Species differences in ovarian development through the nesting cycle are correlated with differences in the time required for nest-building, but in both species the clutch size is determined by means of the differential rate of growth of ovarian follicles by the first day of laying. Renesting of individual blackbirds within a week of the loss of an unsuccessful clutch is possible apparently through the retention of relatively small, lightly yolked ovarian follicles in the ovary through in-cubation and the early period of parental care.
Breeding of females of both blackbirds involves deposition of fat as an energy reserve before the final stages of ovarian development. Restriction of mass yolk deposition in the ovary to the precise number of follicles that are ovulated and to the four days immediately before ovulation is important in the energy balance of breeding females. Females lose their body fat deposits and decrease significantly in body weight during laying, incubation, and the period of parental care. The post-nuptial molt in summer is completed in about two months in both species; molt is delayed in late-nesting blackbirds. Changes in fat and weight in early winter indicate a strain on the energy balance, and adaptive flexibility of the breeding-molting schedule occurs also in autumn with the absence of a second postnuptial molt after the autumnal breeding season in tricolored blackbirds.
Experimental studies show that males and females of both species develop large gonads in response to long daylengths. The rate of growth of the testes of wild blackbirds in spring corresponds closely to the rate predicted from the experimentally determined photoperiodic response. Development of ovarian follicles up to 2.0 mm diameter in females on long photoperiods is independent of male song and display. Tricolored blackbirds held under natural seasonal daylength for nine-teen months were sexually active in spring but remained inactive during autumn. The majority of males in early autumn are refractory to long photoperiods. Sexually active males in the field in autumn are thought to be mainly the individuals that nest successfully early in spring as the testes of males feeding young in spring regress earlier than do the testes of other males. Some younger males develop large testes in their second autumn. Male tricolored blackbirds showed gonadal enlargement in autumn in experimental conditions only when the phases of their refractory periods were altered, but all experimental females developed enlarged ovaries in early autumn when fed live grasshoppers or soaked by simulated rains.
The difference in breeding seasons in these two blackbirds is related to species differences both in physiological control of sexual activity and in ecology. Gonadal development and nesting in tricolored blackbirds are apparently controlled in part directly by rainfall, an abundance of food, and social interaction, as well as by daylength. Rainfall and food are temporary features of the environment and so have little value of prediction of future environmental conditions in the long run to species such as the redwinged blackbirds which have extended periods of territorial establishment, mate selection, and nest-building. The abbreviated nesting |
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