John Dittami

Testosterone implants affect molt in male European starlings, Sturnus vulgaris

Male European starlings, Sturnus vulgaris, were implanted with varying dosages of testosterone in June, shortly before the natural onset of the molt. Dose-dependent effects on the primary, secondary, and body molts were found. In general, testosterone delayed the onset of molt and slowed its progress. Since the duration of the molt was relatively unaffected, the molt in testosterone-treated birds stopped before its completion. It was not resumed when the implants were removed in November. The plasma testosterone levels in all implanted birds tended to be lower during the molt than afterward. The opposite was true of thyroxine plasma titers. A negative correlation was found between testosterone levels during the molt and the final molt score, and a positive correlation was found between thyroxine and the final molt score.

Effects of nestboxes on LH, testosterone, testicular size, and the reproductive behavior on male European starlings in spring

The effect of nestboxes on the vernal increase and decrease in reproductive activity was investigated in male European starlings (Sturnus vulgaris), held in outdoor aviaries. Male-only groups with (group 1) and without (group 2) availability of nestboxes were compared for the amount of song, for testicular size and plasma titers of luteinizing hormone (LH) and testosterone. Testicular size and LH differed significantly between the 2 groups, being higher in males with nestboxes. Similar differences, as well as differences in nestbox-related behaviors (sitting in front of a nestbox, inspecting it, and nestbuilding) were also found when individuals of group 1, in possession of a nestbox, were compared to those without. In another group (2), the seasonal dependence of nestboxes as a modifying factor was investigated. These males were given access to nestboxes later in the season, than the males of group 1, after April 7. The effects on physiological parameters at this time of year were negligible in that only a transient increase in LH was seen compared to group 3. Nestbox-oriented behaviors did not reach the same intensities as in group 1. These results demonstrate the importance of nest-site availability in modifying behavioral and physiological functions during the vernal reproductive cycle. At the same time they indicate a seasonal dependence in these modifying effects.

The Effect of Pinealectomy on Circadian Plasma Melatonin Levels in House Sparrows and European Starlings

We determined 24-hr plasma melatonin profiles in intact, sham-pinealectomized, and pi nealectomized European starlings (Sturnus vulgaris) and house sparrows (Passer domesticus) in a light-dark (LD) cycle and in constant darkness (DD). In the intact and sham-pinealectomized birds of both species, a melatonin rhythm was found, with low levels during the day and high levels during the night. Pinealectomy abolished the nighttime peak of melatonin in both species; hence, levels were low at all times sampled. This uniform response of plasma melatonin to pinealectomy contrasts with the differential response of circadian activity rhythms to pinealectomy for these two species. In DD, locomotor activity in pinealectomized house sparrows is usually arrhythmic, whereas in starlings a rhythm usually persists. This suggests that in the latter species free-running circadian rhythms are not necessarily dependent on a rhythm in plasma melatonin. The same is true for the synchronized activity rhythm observed in pinealectomized birds of both species in LD, as well as for the damped rhythm that persists in pinealectomized house sparrows following an LD-to-DD transfer. The results are consistent with the hypothesis that the pineal and its periodic output of melatonin constitute only one component in a system of at least two coupled pacemakers. They also suggest that there are species differences in the relative role played by the pineal and other pacemakers in controlling circadian rhythms in behavior.

Postjuvenile molt in East African and Central European stonechats (Saxicola torquata axillaris, S. t. rubicola) and its modification by photoperiod

SummaryTwenty-eight stonechats of the European race (Saxicola torquata rubicula) from Austria and thirty-one stonechats of the Central African race (S.t. axillaris) from equatorial Kenya were handraised and subsequently investigated with regard to the temporal pattern of their postjuvenile molt. About one half of the birds of each race were held under their own native photoperiod and the other half under the photoperiodic conditions of the other race. The results demonstrated clear differences in the postjuvenile molt between the two races when birds were kept in the photoperiod under which they normally live. The African birds began to molt earlier and molted longer than their European conspecifics. The time course of postjuvenile molt was affected by photoperiod in both races as molt began and ended earlier under the equatorial photoperiod than under the European photoperiodic simulation. The question why the African birds showed strong photoperiodic reactions under these experimental conditions although in their natural environment they experience only minute photoperiodic variations, is critically evaluated: Three possible explanations are discussed: (1) the photoperiodic reaction may represent a relict from a time when ancestors of the tropical populations still lived in more temperate zones; (2) it may be due to the occasional immigration of conspecifics from populations living further north or south; (3) it may result from effects on a (possibly circadian) submechanism of the system controlling annual cycles which is normally affected by other environmental cues but can also be influenced by photoperiodic variations.

Seasonal reproduction, moult and their endocrine correlates in two tropical Ploceidae species

SummaryPopulations of Reichenows weavers and rufous sparrows living at the equator were investigated and compared for seasonality in breeding, moulting and fluctuations in the plasma titers of LH, testosterone (in males) and estradiol (in females). These species did not breed during the dry season and they commenced breeding at the onset of the heavy rains (April). In both species, the change from the dry season to the rainy season was accompanied by dramatic increases in gonadal size and steroidogenesis. Surprisingly, plasma titers of the hypophysial hormone LH were not affected.Differences were found in the length of the breeding season: in the weavers it was terminated in September whereas the sparrows bred through January, the end of the study. The differences between the two species in the absence of a pronounced dry season are assumed to be related to the sparrows ability to interrupt moult and renest in fall when environmental conditions are favourable. in the weavers, no differences in the seasonal changes in gonadal size and plasma titers of LH, T (males) and E2 (females) were found between ‘wet’ and ‘dry’ post-heavy rain periods (October–January) whereas differences were found in the sparrows.

Pinealectomy affects the circannual testicular rhythm in european starlings (Sturnus vulgaris)

SummaryPinealectomized and sham-operated European starlings were maintained for 16 months under a constant 12-h photoperiod and constant temperature conditions. In all birds, testicular width was measured at about monthly intervals and the onset and end of molt was determined. Shortly after the beginning of the experiment, the sham-operated birds went through a cycle of testicular growth and regression which was followed by a complete molt; subsequently most individuals initiated a second testicular cycle. Most of the pinealectomized birds, in contrast, failed to go through a second testicular cycle. Moreover, during the first cycle their testes regressed earlier than in the sham-operated birds and the subsequent molt was relatively advanced. In these respects the pinealectomized birds behaved like intact starlings under a 13-h photoperiod. Since pinealectomy probably changes the phase-relationship between circadian rhythms and the entraining light-dark cycle it is proposed that pinealectomy in the present experiment might have altered the phase-relationship between a circadian rhythm of photosensitivity and the light-dark cycle in such a way that the birds interpreted the 12-h photoperiod as a 13-h photoperiod.

Seasonal organization of breeding and molting in the Fiscal Shrike (Lanius collaris)

Free-living Fiscal Shrikes were investigated in Nakuru Kenya for seasonal patterns in breeding, gonadal size, molt and the plasma titers of LH, T in males and E 2 in females. Both breeding and molt were found to be seasonally restricted activities. A complete molt occurred from August to December after breeding had ceased. Although the gonads in some birds were well developed during the dry season, at the beginning of the year, no breeding was found until the onset of the heavy rains. It is proposed that severe drought inhibits reproductive activity and correspondingly the secretion of gonadal steroids (T and E 2) in Fiscal Shrikes without affecting increases in gonadal size. Der Fiskalwürger ist die häufigste afrikanische Würgerart. Er ernährt sich von Insekten, Reptilien, kleinen Säugetieren und Vögeln. VonBrown &Britton (1980) undMacDonald (1980) wurde wegen dieser Beutevielfalt vermutet, daß er auch unter ungünstigen Umweltbedingungen (Trockenheit) brüten kann. In Nakuru, Kenya, wurde untersucht, ob jahreszeitlich bedingte Unterschiede in Brutaktivität, Gonadengröße, Mauser und in Plasmatitern von LH, T und E 2 vorhanden waren. Brutaktivität und Mauser waren mit der Hauptregenzeit korreliert. Die Brutzeit endete mit Beginn der Mauser, die von August bis Dezember dauerte. Obwohl die Gonaden bei einigen Vögeln schon während der Trockenzeit (Januar bis März) gut entwickelt waren, gab es bis zur Regenzeit keinen Hinweis auf Fortpflanzungsaktivität. Es wird angenommen, daß eine strenge Trockenzeit die Ausschüttung von Testosteron (♂) und Estradiol (♀) verhindert, ohne die Gonadengröße zu beeinflussen. In Gegensatz zu den vorher genannten Autoren wurde keine Fortpflanzungsaktivität in Trockenzeiten festgestellt. Das beruht vielleicht auf dem in einzelnen Jahren unterschiedlichen Ausmaß der Trockenzeit.

Seasonal changes in follicle-stimulating hormone in a breeding population of barheaded geese, Anser indicus

Changes in the plasma titers of follicle-stimulating hormone were investigated in a free-living population of barheaded geese over the whole year. Adult males had pronounced seasonal changes from prebreeding levels of approximately equal to 20 ng/ml to a peak in the breeding season of about 300 ng/ml. Changes in females were only slight. An increase from nondetectable, approximately equal to 20 ng/ml to a mean of 32.2 ng/ml before egg-laying was found. The data are discussed with regard to the literature on free-living species and the previously published data on LH and testosterone levels in the same birds.

Starling circannual systems: are they arrested in long photoperiods ?*

SummaryIn the European starling,Sturnus vulgaris, circannual rhythms in gonadal size, molt and other related functions persist only in photoperiods close to 12 h, but are absent in longer or shorter daylengths. To find out whether the arrhythmia seen in long photoperiods results from an arrest of the underlying clock system, three groups of male starlings were held for 10, 14, or 20 months in a 13 h photoperiod and then transferred to a 12 h photoperiod. A control group was held in the 13 h photoperiod throughout the experiment for 28 months. During the initial exposure to the 13 h photoperiod, all birds went through a gonadal cycle, followed by a complete molt. Subsequently, the control birds retained small testes to the end of the experiment and there was no further molt. In contrast, most of the experimental birds re-initiated a testicular cycle, following transfer to the 12 h photoperiod and molted after its completion. The latency between the transfer to the 12 h photoperiod and the onset of testicular growth was not significantly different among the three groups, indicating that the underlying circannual clock had been arrested in the 13 h photoperiod and restarted in the 12 h photoperiod. The pattern of the second testicular cycle did, however, differ among groups. Particularly its amplitude decreased from group 1 to group 3, suggesting that the capacity of the birds to respond to a 12 h photoperiod decreased with increasing duration of exposure to the 13 h photoperiod.

Circannual Variations in Plasma Luteinizing Hormone Levels in Castrated Male European Starlings (Sturnus vulgaris)

Intact and castrated male European starlings were held for about 2 years in a constant 12-hr photoperiod and constant temperature conditions. At 1- to 2-month intervals, testicular width was measured by laparotomy, and blood samples were taken for analysis of plasma luteinizing hormone (LH). Most of the control birds went through at least one circannual cycle of testicular width and plasma LH concentration. In the castrates, a similar proportion of birds went through circannual LH cycles with periods indistinguishable from those of the controls. It is concluded that the testes and their hormones are not essential components of the mechanism that generates circannual gonadal cycles in male European starlings.