Tropical birds usually lay smaller clutches and are less likely to initiate a second brood than their temperate-zone relatives. This reduction in annual fecundity is generally explained as an adaptation either to higher rates of nest predation or to a more limited food supply concurrent with higher adult survival in the tropics. However, the physiological parameters associated with lower annual fecundity in tropical birds have not been well investigated. We compared the annual fecundity, behaviour and a number of physiological parameters of stonechat parents feeding £edged juveniles in territories with and without ¢scal shrikes, a predator on adult and £edged birds. Stonechat pairs in territories with shrikes were less likely to initiate a second brood and delayed successive broods compared to pairs in territories without shrikes. After £edging of their young, males showed a greater propensity than females to initiate distraction calls after a human intrusion into their territory and, therefore, invested more in the defence of their young. In territories with shrikes stonechat males had higher initial plasma corticosterone levels and lower body conditions than males in territories without shrikes, suggesting that they were chronically stressed. In contrast, the females from both types of territory had low initial plasma corticosterone levels. We conclude that shrike presence might account for the delay in initiation of a second brood and the reduction in the tendency to initiate a second brood. Whether these e¡ects are mediated by the elevated levels of corticosterone remains to be demonstrated.

Species which have settled in urban environments are exposed to different conditions from their wild conspecifics. A previous comparative study of an urban and a forest-living European blackbird population had revealed a three weeks earlier onset of gonadal growth in urban individuals. These physiological adjustments are either the result of genetic differences that have evolved during the urbanization process, or of phenotypic flexibility resulting from the bird's exposure to the different environmental conditions of town or forest. To identify which of these two mechanisms causes the differences in reproductive timing, hand-reared birds originating from the urban and the forest populations were kept in identical conditions. The substantial differences in the timing of reproduction between urban and forest birds known from the field did not persist under laboratory conditions, indicating that temporal differences in reproductive timing between these two populations are mainly a result of phenotypic flexibility. Nevertheless, urban males initiated plasma luteinizing hormone (LH) secretion and testicular development earlier than forest males in their first reproductive season. Moreover, plasma LH concentration and follicle size declined earlier in urban females than in forest females, suggesting that genetic differences are also involved and might contribute to the variations in the timing of reproduction in the wild.

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Neuroplasticity in the vocal control system of songbirds is strongly inf luenced by seasonal f luctuations in circulating testosterone. These seasonally plastic telencephalic structures are implicated in the learning and production of song in songbirds. The role of the indoleamine melatonin in seasonal adaptations in birds has remained unclear. In this experiment, European starlings were castrated to remove the neuromodulating activity of gonadal steroids and were exposed to different photoperiods to induce reproductive states characteristic of different seasonal conditions. Long days increased the volume of the song-control nucleus high vocal center compared with its volume on short days. Exogenous melatonin attenuated the long-day-induced volumetric increase in high vocal center and also decreased the volume of another song-control nucleus, area X. This effect was observed regardless of reproductive state. To our knowledge, this is the first direct evidence of a role for melatonin in functional plasticity within the central nervous system of vertebrates.European starlings (Sturnus vulgaris) are highly photoperiodic (1). Reproductive activity occurs in the spring as day length is increasing (i.e., photostimulation occurs), but is subsequently curtailed by the onset of photorefractoriness during exposure to long day lengths. During the onset of photorefractoriness, the hypothalamo-pituitary-gonadal (HPG) axis becomes inactive and the gonads regress (2). Starlings remain refractory to long day lengths until short days are experienced in the winter; the HPG axis slowly becomes responsive again in the absence of a long day photostimulus, in preparation for increasing day length and consequent full reproductive activity in the spring. It must be noted that these centrally mediated different reproductive states are not mediated by seasonal changes in gonadal steroids and still occur at the levels of the hypothalamus and the pituitary gland, even in the absence of gonads (2). Coincident with changes in reproductive activity, seasonal neuroplasticity now documented in several species of oscine songbirds occurs within discrete telencephalic nuclei that are involved in song learning and production (3-7). Increases in the volumes of these song-control nuclei largely depend on seasonal increases in circulating testosterone (T) and its metabolites (8-10) that are directly related to the annual reproductive cycles of these birds (11). These seasonal changes in volumes of the song-control nuclei are associated with changes in cell size and cell number in various song-control nuclei (12).Recent studies suggest that there are gonad-and Tindependent seasonal changes in the volumes of song nuclei (13-15). To date, it has been unclear what factors might be contributing to these T-independent neuronal changes. A complex suite of physiological events occurs during the onset of photorefractoriness (2). These include changes in circulating concentrations of photoperiodically controlled hormones other than T. There are also alt...

2005. Underlying physiological control of reproduction in urban and forest-dwelling European blackbirds Turdus merula. Á/ J. Avian Biol. 36: 295 Á/305.The development and the continual expansion of urban areas have not only destroyed natural habitats, but also have drastically changed the environmental and ecological conditions of these areas. Consequently, species that have settled in these new manmade ecosystems are exposed to considerable alternations in environmental conditions compared to their 'wild' conspecifics. To understand the impact of human-induced environmental changes on life history events such as reproduction, we compared the timing of the reproductive season and its underlying endocrine control in free-living European blackbirds Turdus merula inhabiting urban and nearby forest areas. Body mass, fat score, gonadal size, luteinizing hormone (LH), testosterone (T), and estradiol (E 2 ) were measured. Urban blackbirds developed their gonads approximately three weeks earlier than forest birds, whereas the timing of gonadal regression did not differ. There are several factors (e.g. artificial light, temperature, food availability, and social cues) which may have caused the differences in the temporal organization of gonadal growth between the urban and forest-living populations. The advanced gonadal development of urban blackbirds did not coincide with an earlier secretion of reproductive hormones. In contrast, urban males had lower plasma LH and T levels during testicular growth than forest males. Differences in social interactions and environmental conditions may explain the contrast of gonadal development and the timing of hormone secretion between urban and forest blackbirds.

Little is known about to what extent the sensitivity of the hypothalamic^pituitary^adrenal (HPA) axis may be state dependent and vary in the same species between environments. Here we tested whether the faecal corticosteroid concentrations of matrilineal adult female spotted hyenas are in£uenced by social and reproductive status in adjacent ecosystems and whether they vary between periods with and without social stress. Females in the Serengeti National Park frequently become socially subordinate intruders in other hyena territories by undertaking long-distance foraging trips to migratory herds, whereas in the Ngorongoro Crater they usually forage inside their own small territories on resident prey. The faecal corticosteroid concentrations in Serengeti females were signi¢cantly higher than in Ngorongoro females. Energy expenditure by lactation is exceptionally high in spotted hyenas and this may be re£ected in their corticosteroid levels. The faecal corticosteroid levels in both populations were higher in lactating than in non-lactating females. During periods of social stability, faecal corticosteroid concentrations increased in non-lactating females but not in lactating females as social status declined. Lactating Serengeti females had signi¢cantly higher faecal corticosteroid concentrations during periods with acute severe social stress than during periods without, indicating that the HPA axis is sensitive to social stimuli even in lactating females. So far few studies have used non-invasive monitoring methods for assessing social stress in freeranging animals. This study demonstrates for the ¢rst time, to the authors' knowledge, that corticosteroid concentrations may di¡er between periods with and without social stress for a free-ranging female mammal and that the modulating e¡ect of social status may depend on reproductive status.

This study presents the daily rhythm of melatonin secretion throughout one year in a bird from the northern hemisphere, the Svalbard ptarmigan (Lagopus mutus hyperboreus), which lives naturally at 76-80 degrees N. Eight Svalbard ptarmigan were caged outdoors at 70 degrees N and blood sampled throughout one day each month for 13 months. At this latitude, daylight is continuous between May and August, but there is a short period of civil twilight around noon from late November to mid January. There was no daily rhythm in plasma melatonin in May-July. Plasma melatonin levels varied significantly throughout the day in all other months of the year, with the nighttime increase reflecting the duration of darkness. The highest mean plasma concentration occurred at midnight in March (110.1 +/- 16.5 pg/ml) and represented the annual peak in estimated daily production. Around the winter solstice, melatonin levels were significantly reduced at noon and elevated during the nearly 18 h of consecutive darkness, and the estimated mean daily production of melatonin was significantly reduced. Thus, at the times of the year characterized by light-dark cycles, melatonin may convey information concerning the length of the day and, therefore, progression of season. The nearly undetectable low melatonin secretion in summer and the reduced amplitude and production in midwinter indicate a flexible circadian system that may reflect an important adaptation to life in the Arctic.

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