Temporal patterns of circulating LH and steroid hormones in a brood parasite, the Brown-headed cowbird, Molothrus afer.

The breeding behaviour of female Brown-headed cowbirds, Molothrus ater , is remarkably distinct from that of females of most other passerine species. Cowbirds are brood parasites that do not build nests, incubate eggs or feed young. Instead, they have an extended egg-laying stage, and deposit eggs in the nests of hosts within large breeding territories that are defended from other female cowbirds. In this communication, we report the changes in plasma levels of circulating hormones and in morphological characteristics that occur in free-living female Brown-headed cowbirds over a breeding season.
Plasma LH in female cowbirds increases at the onset of breeding and remains elevated for approximately eight weeks, at which time levels decline to basal values. This pattern is similar to that of follicle development, and is consistent with the prolific reproductive output by individuals of this species. Circulating E, levels are quite variable and do not demonstrate any discernible pattern, while plasma B is elevated during the egg-laying period and declines subsequently.
Testosterone titre (T) is highest early in the breeding season, when territorial interactions are generally most frequent. T levels decline slowly thereafter, reaching their lowest values at the season's end. Plasma DHT levels are slightly higher than those for T; DHT exhibits little change until late in the breeding season, when it declines. Fat scores are highest prior to egg laying, and fat reserves diminish as the breeding season progresses. In contrast, body weight increases during the reproductive season, probably as a result of developing eggs in the ovary. These results are consistent with the hypothesis that temporal patterns of hormone secretion are correlated with the expression of social systems.     

No Correlation Between Song and Circulating Testosterone Levels During Multiple Broods in the Domesticated Canary (Serinus canaria)

In anticipation of the breeding season male songbirds of the temperate zones undergo gonadal recrudescence in early spring that lead to elevated circulating testosterone (T) levels, positively correlated with an increase in aggressive and song behaviour. However, besides seasonal changes there are also marked fluctuations of T levels and song production within the breeding season. In many species, T levels and singing activity drop after pairing or after the first clutch is laid. Domesticated canaries (Serinus canaria) are multiple‐brooded with an extended breeding season, and males continue to sing after egg‐laying. So far, studies have mainly focused on the seasonality of T levels and song behaviour whereas the pattern of change throughout the breeding period is unknown. Here, we focused on the first and on the last brood of the breeding season. We measured plasma T levels in males at the different breeding stages and assessed song characteristics of males at both times. T levels fluctuated significantly throughout brood 1, being highest during the nest building stage compared with egg‐laying and feeding of young. No such changes occurred during the last brood. Temporal song characteristics changed between brood 1 and brood 3 with song length being the main contributor to explain these changes. Our data suggest that T mainly plays a role in mate attraction and initial nesting site selection but that elevated levels are not necessary for subsequent breeding attempts. Furthermore, temporal song characteristics are maintained independently of T levels, suggesting a threshold effect. Our results demonstrate behavioural and physiological plasticity of domesticated canaries during the breeding season and are consistent with previous findings in wild songbirds.     

Plasma levels of androgens and cortisol in relation to breeding behavior in parental male bluegill sunfish, Lepomis macrochirus

Like many teleosts, male bluegill sunfish (Lepomis macrochirus) provide sole parental care. To understand some of the proximate costs of parental care, we measured body condition and plasma levels of testosterone (T), 11-ketotestosterone (11KT) and cortisol in nesting bluegill males during pre-spawning, spawning and parental care stages. T and 11KT were at their highest mean levels during the pre-spawning period and decreased to lower levels early during the parental care period before rising again when the eggs hatched. Cortisol levels fluctuated across the breeding stages, but there was a noticeable increase from low levels on the day of spawning during the first 2 days of parental care when egg fanning is most intense. Levels of all hormones varied considerably among males, with androgen levels often correlating positively with a male's body condition. We also demonstrate, using a brood reduction experiment and repeated sampling of known individuals, that the presence of eggs affects hormone levels shortly after eggs hatch. Parental males in better body condition had higher levels of androgens during parental care. Males that were known to renest later in the season also had higher androgen levels and were in better body condition during the first nesting bout than males only known to have nested once. However, circulating levels of cortisol did not differ significantly between these groups. We discuss our findings in the context of proximate and ultimate costs of parental care and propose several reasons why elevated androgen levels may not be as incompatible with the expression of paternal care in male teleosts, as compared with avian and mammalian fathers.     

The influence of social cues on the reproductive endocrinology of male brown-headed cowbirds: field and laboratory studies

Captive male brown-headed cowbirds exposed to long days exhibit gonadal growth and have elevated plasma testosterone (T) levels. This photoperiodic response is enhanced if males are housed with female cowbirds: Photostimulated males with females increase plasma testosterone levels sooner than do individually housed photostimulated males. Peak plasma T levels are similar in both groups, although peak levels are maintained longer in males housed with females. The gonadal cycle is similarly affected; males in the presence of females have earlier gonadal recrudescence and maintain mature gonads longer than do photostimulated males without females. Plasma corticosterone levels increase in the unpaired males, suggesting that removal of social cues is stressful for these birds. Free-living paired males have significantly higher plasma testosterone levels than do unpaired/unknown males early in the season, when social relationships are being established; the levels are similar thereafter. There is no difference between the two groups in testicular maturation rates; nor do they differ in plasma corticosterone levels at any time of the season. These results suggest that social stimuli are important in modulating the secretion of testosterone in males early in the season when pairing occurs, and possibly late in the season as well, probably to prevent termination of breeding prior to that of females.     

Plasma levels of prolactin and gonadal steroids in relation to multiple-brooding and renesting in free-living populations of the song sparrow, Melospiza melodia

Plasma levels of testosterone (T), estradiol (E2), and prolactin were measured in samples collected from free-living song sparrows, Melospiza melodia. In males, plasma levels of T were elevated early in the season when territories were established and when females laid the first clutch of eggs. Thereafter, T levels declined and remained low throughout the remainder of the breeding cycle. However, if the first brood was lost to a predator, or by experimental removal of the nest, plasma levels of T increased as renesting, to replace the clutch, occurred. Circulating levels of prolactin in males began to rise during the egg-laying stage of the first brood, reached a maximum toward the end of the incubation stage, remained elevated until breeding was terminated, and then declined throughout the moulting stage to basal values in October. Prolactin levels remained high throughout the breeding season irrespective of whether a brood was raised successfully or whether the nest was lost and renesting occurred. In females, plasma levels of E2 were elevated prior to the egg-laying stage for each brood as is typical of multiple-brooded species. However, prolactin titers rose dramatically during egg-laying for the first clutch (slightly higher than in males) and were maximal by onset of incubation. Only females of this species incubate, although males do feed young. As in males, plasma prolactin in females remained high between broods and during experimentally induced renesting, and then declined to basal by the end of the moult stage in October. These data suggest that there are no differences in the temporal patterns of prolactin concentrations in blood between multiple-brooding and renesting. In a separate experiment, captive male song sparrows were transferred from a short day to artificial long days (18L 6D) and a control group was maintained on 9L 15D. In the long-day group, prolactin levels rose abruptly over the first 20 days, as the testes developed, and remained high well into postnuptial moult after the gonads had regressed. Prolactin remained basal in the control group. These data suggest that the temporal pattern of circulating prolactin levels throughout the breeding season is regulated at least partly by changing photoperiod. However, nonphotoperiodic factors are also important since photoperiodically induced increases in prolactin are significantly less than those seen in free-living individuals. These differences may be related to parental behavior.     

Hormone profiles of obligate avian brood parasites during the breeding season

The ultimate explanations for avian brood parasitism have been studied intensively as a model system for coevolution, but little is known about the proximate mechanisms, for example hormonal regulation, underlying brood parasitic behaviour. In this study, we explored seasonal hormone profiles in two brood parasitic Cuculus species breeding in the Republic of Korea. As brood parasites have relatively simple breeding stages without incubation and provisioning, we predicted that during the breeding season individuals would exhibit similar levels of testosterone (T) and stress‐induced corticosterone (CORT), hormones that are known to be closely related to the transition of breeding stages. We also assessed how these hormone profiles were associated with traits such as body size and sex. Overall, male cuckoos showed similarly high T levels throughout the breeding season, as predicted, but individual variation became greater as the season progressed. Individual CORT levels tended to decrease as the season progressed, although the decrease was significant only in Common Cuckoos Cuculus canorus. We also found that male Lesser Cuckoos Cuculus poliocephalus showed a much higher level of T than females, as expected, but this sexual difference was not observed in Common Cuckoos. Our results suggest that the seasonal hormone profiles of avian brood parasites are likely to be similar to typical hormone profiles expected for non‐brood parasites during the breeding season. This may suggest that not only the breeding cycle but also other factors such as social interaction may be affected by hormonal changes. Further studies are needed to fully understand the proximate mechanism of avian brood parasitism.     

Impacts of the season and reproductive status on fecal reproductive and adrenocortical steroid metabolites in zoo Cuban crocodiles (Crocodylus rhombifer)

Conservation strategies for crocodilians often include captive breeding to create stable assurance populations. Evaluating adrenal and gonadal hormone patterns can provide animal managers with data to more effectively monitor animal welfare and reproductive status. This study evaluated the effects of season (breeding, nesting, or off), sex (male and female), and reproductive status of females (egg-laying/housed with a male or non-laying/housed solo) on concentrations of fecal glucocorticoid metabolite (FGM), fecal androgen metabolite (FAM), and fecal progestogen metabolite (FPM) in seven Cuban crocodiles, Crocodylus rhombifer, at the Smithsonian's National Zoological Park. Overall, seasonal changes in FGM and FPM concentrations were only observed in egg-laying females; FGM and FPM concentrations were both higher during the nesting season compared to the breeding and off seasons. Seasonal changes in FAM concentrations were only observed in males; males had higher FAM concentrations during the breeding and nesting seasons compared to the off season. Future studies investigating the use of fecal hormone metabolites in crocodilians are necessary to understand differences between individuals and species, to further elucidate the interactions between hormones and environmental factors, such as social housing, and to develop long-term datasets for the management of this species.     

Patterns of lipid storage and mobilisation in the female green sea turtle ( Chelonia mydas)

Reproductive data from southern Queensland indicate that vitellogenesis in female Chelonia mydas takes approximately 8 months and is followed by a migration to a breeding area. At Heron Island, females lay multiple clutches over approximately 3 months. To investigate how females mobilise and store lipid during the breeding season we collected plasma, yolk, and fat tissue samples from females at a variety of stages during the nesting season. In breeding females, concentrations of plasma triglyceride increased seasonally. They reached peak concentrations during vitellogenesis and courtship, remained high throughout the nesting season, and then declined to a nadir after the last clutch. Plasma protein concentration increased throughout the breeding season, peaking following the last clutch for the season. Yolk lipids were highest during courtship and were similar throughout the nesting season, suggesting that uptake of lipid by ovarian follicles is completed prior to the beginning of the nesting season. Plasma triglyceride decreases in females with prolonged periods of unsuccessful nesting, and total lipid levels in adipose tissue and follicle yolks were significantly lower in atretic females. It appears that: (1) endogenous energy reserves can be reduced by stochastic environmental events (such as those reducing nesting success), and (2) a metabolic shift signalling the end of the nesting season is characterised by a drop in plasma triglycerides and slight increase in total plasma protein.     

Hormonal and metabolic responses to nesting activities in the green turtle, Chelonia mydas

We examined the association between green turtle nesting activities and plasma profiles of hormones that are widely implicated in aspects of heightened metabolism and energy regulation; epinephrine (EPI), norepinephrine (NE) and corticosterone. In conjunction, we examined plasma profiles of glucose and lactate to infer metabolic processes associated with green turtle nesting behaviour. Finally, because these hormones are also involved in mediating behaviour and physiology associated with stressful situations, we examined the effect of a stressor encountered during nesting, physical disturbance, on hormone levels. Plasma profiles of epinephrine, norepinephrine and corticosterone were not significantly altered across different stages of nesting. Plasma glucose and lactate both exhibited significant increases related to nesting activity; glucose increased dramatically during the emergence stage of nesting before stabilizing, and lactate levels continued to increase throughout the nesting process. There was no significant association between plasma hormones and glucose. For female turtles that abandoned nesting activities due to competition for nest space, there was no significant difference in plasma levels of epinephrine, norepinephrine and corticosterone compared to females that persisted with nesting activities. Overall, while distinct metabolic changes took place in nesting females, there was little association in profiles of hormones typically considered important for regulating heightened metabolism and nesting activity. This disassociation could arise because hormonal action may be altered in breeding female green turtles to facilitate reproductive processes.     

Patterns of breeding behaviour and plasma levels of hormones in a free-living population of Pied flycatchers, Ficedula hypoleuca

Plasma levels of luteinizing hormone (LH) and steroid hormones were measured by radioimmunoassay in a series of plasma samples collected from a free-living population of Pied flycatchers, Ficedula hypoleuca. Both males and females have similar circulating levels of LH and testosterone when they arrive in the breeding area. However, as territories and pair bonds are established, and as courtship and nest-building ensue, the plasma levels of testosterone in males become elevated over those of females, although a lesser peak also occurs in females. As the egg laying phase and incubation begin, plasma levels of LH and testosterone decline sharply in both sexes. On the other hand polyterritorial males, that establish secondary territories, maintain higher levels of LH and testosterone than males remaining in the home-territory, at least until they return to the home-territory, usually at the middle part of the incubation period. By this time polyterritorial males also have basal levels of these hormones. Curiously, females on home-territories show a small, but significant, rise of circulating LH and testosterone at the time the males return.
In males, plasma levels of corticosterone are elevated from the time of arrival on the breeding grounds until the end of the nest-building period. Thereafter corticosterone levels decline in males staying on the home-territory but remain elevated in polyterritorial males until the latter return to the home-territory. In contrast, circulating levels of corticosterone in females are only elevated for brief period during egg laying.     

Seasonal changes in thyroid-gonadal interactions in the edible dormouse,Glis glis

Summary This study was conducted to determine changes in thyroid-gonadal interaction in the edible dormouse during the phase of the annual cycle that corresponds to the end of the breeding season (from June to September). We evaluated intra-hypothalamic luteinizing hormone-releasing hormone (LHRH) content, and plasma concentrations of luteinizing hormone (LH), testosterone, thyroid-stimulating hormone (TSH) and thyroxine (T4) in three groups of dormice: (1) controls; (2) dormice receiving sufficient T4 supplementation to maintain June levels in control animals until September, thus counteracting the seasonal reduction of T4 that normally begins in July; and (3) thyroidectomized dormice. The effect of thyroidectomy was only detectable in June, when plasma T4 concentration in the control group was maximal, and consisted of a significant decrease in plasma testosterone levels. This provides strong support for the hypothesis that thyroid function positively influences gonadal function during the breeding season. The T4 supplementation resulted in a decrease in hypothalamic LHRH concentration, suggesting that an increased LHRH release led to the observed stimulated hypophyseal secretion of LH in June and September and the increased circulating testosterone levels in September. There was no detectable effect in July and August. These results show that thyroid axis activation of the hypothalamic-pituitary-gonadal system is only possible during certain phases of the annual cycle, particularly evidenced here during the breeding season. They also reinforce our conclusions drawn from the thyroidectomy results. Conversely, the summer testicular regression which normally occurs after the breeding season is no longer controlled by plasma T4 levels. Even though the sensitivity of the gonadal axis to the thyroid axis appears to reappear at the end of the summer, results of previous studies indicate that this resumption is only temporary.Abbreviations LH luteinizing hormone - LHRH luteinizing hormone-releasing hormone - RIA radioimmunoassay - T4 thyroxine - TSH thyroid-stimulating hormone     

Nonbreeder birds at colonies display qualitatively similar seasonal mass change patterns as breeders

The difficulty in studying nonbreeding birds means that little is known about them or their resource requirements, despite forming a large and significant component of a population. One way to assess food requirements is to examine changes in body mass, because it indicates the amount of food acquired. In terms of body mass changes, our expectation is that nonbreeders will either (a) be in poorer condition than the breeders which potentially explains why they do not breed or (b) remain at a stable higher mass as they are unconstrained by the physiological costs associated with rearing chicks. Here, we interrogate body mass datasets of breeding and nonbreeding birds of two penguin species to assess these predictions and determine whether differences in mass exist between these two groups throughout the breeding season. The first dataset is from a wild Adélie penguin population, where bird mass was recorded automatically and breeding status determined from a resighting program. A second population of captive gentoo penguins were weighed regularly each breeding season. We demonstrate that although there were times in each year when breeders were heavier than their nonbreeding counterparts for both populations, the mass changes showed qualitatively similar patterns throughout the breeding season irrespective of breeding status. Heavier breeders at times during the breeding season are not unexpected but the overall similar pattern of mass change irrespective of breeding status is in contrast to expectations. It appears that breeding status per se and the constraints that breeding places on birds are not the only driver of changes in mass throughout the breeding season and, although not explicitly studied here, the role of hormones in driving changes in appetite could be key to explain these results. These results present a significant step toward understanding food requirements of nonbreeders in avian populations.     

Non-migratory stonechats show seasonal changes in the hormonal regulation of non-seasonal territorial aggression

In many birds and mammals, male territorial aggression is modulated by elevated circulating concentrations of the steroid hormone testosterone (T) during the breeding season. However, many species are territorial also during the non-breeding season, when plasma T levels are basal. The endocrine control of non-breeding territorial aggression differs considerably between species, and previous studies on wintering birds suggest differences between migratory and resident species. We investigated the endocrine modulation of territorial aggression during the breeding and non-breeding season in a resident population of European stonechats (Saxicola torquata rubicola). We recorded the aggressive response to a simulated territorial intrusion in spring and winter. Then, we compared the territorial aggression between seasons and in an experiment in which we blocked the androgenic and estrogenic action of T. We found no difference in the aggressive response between the breeding and the non-breeding season. However, similarly to what is found in migratory stonechats, the hormonal treatment decreased aggressive behaviors in resident males in the breeding season, whereas no effects were recorded in the non-breeding season. When we compared the aggressive responses of untreated birds with those obtained from migratory populations in a previous study, we found that territorial aggression of resident males was lower than that of migratory males during the breeding season. Our results show that in a resident population of stonechats T and/or its metabolites control territorial aggression in the breeding but not in the non-breeding season. In addition, our study supports the hypothesis that migratory status does modulate the intensity of aggressive behavior.     

Plasma mammalian leptin analogue predicts reproductive phenology,but not reproductive output in a capital‐income breeding seaduck

To invest in energetically demanding life history stages, individuals require a substantial amount of resources. Physiological traits, particularly those related to energetics, can be useful for examining variation in life history decisions and trade‐offs because they result from individual responses to environmental variation. Leptin is a protein hormone found in mammals that is proportional to the amount of endogenous fat stores within an individual. Recently, researchers have confirmed that a mammalian leptin analogue (MLA), based on the mammalian sequence of leptin, is present with associated receptors and proteins in avian species, with an inhibitory effect on foraging and body mass gain at high circulating levels. While MLA has been both quantified and manipulated in avian species, little is currently known regarding whether plasma MLA in wild‐living species and individuals is associated with key reproductive decisions. We quantified plasma MLA in wild, Arctic‐nesting female common eiders (Somateria mollissima) at arrival on the breeding grounds and followed them to determine subsequent breeding propensity, and reproductive phenology, investment, and success. Common eiders are capital‐income breeding birds that require the accumulation of substantial fat stores to initiate laying and successfully complete incubation. We found that females with lower plasma MLA initiated breeding earlier and in a shorter period of time. However, we found no links between plasma MLA levels and breeding propensity, clutch size, or reproductive success. Although little is still known about plasma MLA, based on these results and its role in influencing foraging behaviors and condition gain, plasma MLA appears to be closely linked to reproductive timing and is therefore likely to underlie trade‐offs surrounding life history decisions.     

Temporal Variation in Decisions about Parental Care in Bluegill, Lepomis macrochirus

Parental investment theory states that an individual will trade‐off present and future reproductive potential to maximize lifetime reproductive success. Only when parental care is costly in terms of reduced future reproductive potential should individuals be sensitive to changes in the value of current offspring and adjust their care. Here, we examine temporal variation in parental care decision‐making in bluegill (Lepomis macrochirus), in which care is provided by males called ‘parentals’. Previous research has shown that parentals that nest early in the breeding season are in higher energetic condition than those that nest later, and early nesting males appear not to pay an opportunity cost to their care in terms of reduced future reproductive potential. Early nesting males also may have higher paternity in their broods than later nesting males. To examine the parental care decisions made by early and mid‐season nesting parentals, we experimentally reduced males’ perceived paternity by swapping eggs between nests. We found that experimental males that nested early in the breeding season adjusted their brood defence behaviour similarly to control males, which had sham egg swaps performed. Conversely, experimental males that nested mid‐season significantly decreased their brood defence behaviour after the manipulation as compared with control males. Thus, unlike mid‐season nesting males, early nesting males appear relatively insensitive to changes in brood value (paternity), possibly because early nesting males pay little cost in terms of reduced future reproductive potential to providing full care or because these males have a predisposition to high paternity.     

Context-dependent effects of castration and testosterone treatment on song in male European starlings

Most seasonally breeding songbirds display dramatic seasonal fluctuations in plasma testosterone (T) levels and mate attraction behaviors, including song. However, males of some songbird species, such as the European starling (Sturnus vulgaris), continue to sing at high levels after the breeding season, when T levels are basal. In male starlings song during the breeding season functions mainly to attract mates, whereas song during the nonbreeding season appears unrelated to reproduction. This suggests that song produced in a context unrelated to female courtship, unlike song directed toward females, is not regulated by plasma T. In captive males housed in large outdoor aviaries we explored the relationship between plasma T and song produced during the breeding season within and outside a courtship context. This was achieved by determining the effects of castration and subsequent T treatment on song and mate attraction behaviors in both the presence and the absence of a female. Compared to intact males, castrated males did not show reduced song activity in the absence of a female for at least 6 months after the operation, strongly suggesting that the expression of noncourtship song is not regulated by plasma T. Likewise, we found that experimentally elevating T levels in castrated males did not affect noncourtship song rates. However, control castrated males receiving empty implants tended to show reduced noncourtship song rates after implantation. This may have been due to a suppressive effect caused by the presence of the T-implanted castrated males in the same aviary. In contrast, courtship singing was clearly controlled by plasma T: it was abolished by castration and restored by subsequent T replacement when males were housed both individually and in a group situation. High plasma levels of T also appeared necessary for the activation of three other behavioral traits critical for mate attraction, namely, nesthole occupancy, spending time (singing) in a nesthole, and carrying green nesting material into a nesthole.     

Changes in numbers of large ovarian follicles, plasma luteinizing hormone and estradiol-17beta concentrations and egg production figures in farmed ostriches throughout the year

In this study we described and analysed changes in the numbers of large ovarian follicles (diameter 6.1-9.0 cm) and in the plasma concentrations of luteinizing hormone (LH) and estradiol-17beta (E(2)beta) in relation to individual egg production figures of farmed ostriches (Struthio camelus spp.) throughout one year. Ultrasound scanning and blood sampling for plasma hormone analysis were performed in 9 hens on a monthly basis during the breeding season and in two periods of the non-breeding season. Our data demonstrated that: (1) large follicles were detected and LH concentrations were elevated already 1 month before first ovipositions of the egg production season took place; (2) E(2)beta concentrations increased as soon as the egg production season started; (3) numbers of large follicles, LH and E(2)beta concentrations were elevated during the entire egg production season; and that (4) numbers of large follicles, LH and E(2)beta concentrations decreased simultaneous with or following the last ovipositions of the egg production season. By comparing these parameters during the egg production season with their pre-and post-seasonal values, significant differences were found in the numbers of large follicles and E(2)beta concentrations between the pre-seasonal, seasonal and post-seasonal period; while LH concentrations were significantly different between the seasonal and post-seasonal period. In conclusion, our data demonstrate that changes in numbers of large follicles and in concentrations of LH and E(2)beta closely parallel individual egg production figures and provide some new cues that egg production in ostriches is confined to a marked reproductive season. Moreover, our data provide indications that mechanism, initiating, maintaining and terminating the egg production season in farmed breeding ostriches are quite similar to those already known for other seasonal breeding bird species.     

Seasonal variations in plasma luteinizing hormone levels of gonadectomized male ground squirrels (Spermophilus lateralis)

Plasma luteinizing hormone (LH) levels were determined at monthly intervals in intact and orchidectomized ground squirrels maintained in a constant 14L:10D photoperiod at a temperature of 23 +/- 2 degrees C. Animals were orchidectomized or sham operated at different ages and at different stages of the annual reproductive cycle. LH levels were elevated in orchidectomized as compared to intact males within 1 month, and often within 48 h, after orchidectomies performed either before or after the normal breeding season. LH levels tended to remain chronically elevated in long-term orchidectomized males. Although negative feedback regulation of LH by the testes was evident at most stages of the reproductive cycle, effects of orchidectomy on LH were most pronounced during the nonbreeding season when plasma LH titers of intact males were at their annual nadirs, and less pronounced or absent during the breeding season when levels were normally elevated in intact males. For most of the year male gonadal hormones exert negative feedback on LH release. We hypothesize that under the influence of a circannual timing mechanism, sensitivity to steroid feedback is reduced as the breeding season approaches and LH secretion increases despite the attendant rise in plasma androgen levels. These results contrast with data for female squirrels in which there is a seasonally restricted and characteristically brief elevation of plasma LH after ovariectomy; in females, but not in males, onset and termination of LH secretion appear to be regulated by a circannual clock that functions independently of gonadal secretions.     

Impact of Nesting Mortality on Avian Breeding Phenology: A Case Study on the Red-Backed Shrike (Lanius collurio)

The seasonal timing of avian reproduction is supposed primarily to coincide with favourable feeding conditions. Long-term changes in avian breeding phenology are thus mostly scrutinized in relation to climatic factors and matching of the food supplies, while the role of nesting mortality is largely unexplored. Here we show that higher seasonal mean daily mortality rate leads to a shift in the distribution of breeding times of the successful nests to later dates in an an open-nesting passerine bird, the red-backed shrike Lanius collurio. The effect appeared to be strong enough to enhance or counteract the influence of climatic factors and breeding density on the inter-annual variation in mean hatching dates. Moreover, the seasonal distribution of reproductive output was shifted to larger, or smaller, broods early in the season when the nesting mortality increased, or decreased, respectively, during the season. We suggest that population level changes in timing of breeding caused by a general advancement of spring and of the food supplies might be altered by the seasonality in nesting mortality. Hence, we argue that consideration of nesting mortality is of major importance for understanding long-term trends in avian phenology, particularly in species capable of renesting.     

Prolactin secretion in a brood parasite, the brown-headed cowbird, Molothrus ater

Blood samples were collected from a free-living population of male and female brown-headed cowbirds, Molothrus ater , during the breeding season and plasma levels of prolactin were measured by radioimmunoassay. Prolactin concentrations in both sexes increased significantly in mid to late May; peak levels were reached by June and were maintained throughout June and into July. Prolactin levels in juveniles were significantly lower than in adults but were still elevated. Prolactin has been implicated in parental care in several avian species; however, cowbirds are brood parasites with complete absence of parental behaviour, suggesting that prolactin must function in another capacity in cowbirds, or that cowbirds have developed target tissue insensitivity to this hormone.