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Raess, Michael (2006): Annual timing and life-history variation in free-living stonechats. Dissertation, LMU München: Fakultät für Biologie



The stonechat has one of the widest breeding distributions among the Old World passerines. It breeds under tropical, subtropical, north- and south-temperate, and sub-arctic conditions. Also its migratory strategies vary, ranging from year-round residency in areas such different as the British Isles and equatorial Africa, to long-distance migration in northern Asia. These characteristics and the fact that it can be successfully bred in captivity make the stonechat an ideal model species for studying life-history variation under varying seasonal regimes, both in the field and in the lab. In Chapter 1 I develop the conceptual framework for the study of annual timing and life-history variation in stonechats. My studies focus on free-living stonechats from two different populations, Siberian stonechats breeding in northern Kazakhstan and European stonechats breeding in central Slovakia. The taxonomic status and the basic biology of the stonechat and the characteristics of the two study sites are outlined in Chapter 2. Both sites are situated in the temperate zones and hence the local photoperiodic conditions are similar. However, due to the continental climate in Kazakhstan the summer is shorter and hotter and the precipitation lower than in Slovakia, and as a result the breeding season is about two months longer at the European site. The two populations differ in their migratory strategies (Chapter 3). The Siberian stonechats are long-distance migrants that travel up to 6500 km from their breeding to their wintering grounds and pass major ecological barriers like deserts and high mountain ranges on their way. The stonechats from eastern Europe are short-distance migrants that travel about 1500 km. Ringing recovery data suggest that a large proportion of them pass the Mediterranean Sea on their way. There are no ringing recoveries for Siberian stonechats, and information on their migratory routes is sparse and dispersed in different sources. I tried to fill this gap by reconstructing the spatio-temporal pattern of the stonechat migrations within Central Asia and southern Siberia from published passage data. Stonechats enter Central Asia in spring from the south-west, which suggests that they largely avoid the high inner-Asian mountain ranges. From there they spread northwards into their Central Asian and Siberian breeding areas. In autumn they retrace this pattern backwards, however the movement is less concerted than in spring. The passage data can be used to calculate the average speed of migration within the area and compare it to migration speed in the European stonechats. The European stonechats move at higher speeds during spring than during autumn. This is consistent with data on other European passerines. Within Central Asia, however, the stonechats seem to be constrained in their spring movements because they have to stop and wait for ameliorating local conditions. Increasing spring temperatures due to global warming may therefore have a high potential to change migratory phenology in the Siberian stonechats. Birds shut down their reproductive system during the nonbreeding seasons. Migratory birds face a potential conflict between the preparation for breeding and migration, because gonadal maturation takes several weeks. I investigated whether the migratory strategy affects the gonadal state at the time of arrival in the breeding areas (Chapter 4). Males of both populations arrive at the breeding sites with gonads that are not fully developed. Neither the gonadal state in relation to the onset of reproduction, nor the rate of development to the mature state differ in the two populations. This could indicate that the late stages of reproductive development are mainly affected by the targeted date of reproduction and/or by physiological determinants of gonadal maturation rates, whereas the migratory period has less effect. More knowledge about reproductive development along the migratory route, particularly in long-distance migrants, is required. In my study populations local breeders and passage migrants are not distinguishable on the basis of their gonadal development. However, the data in the passage migrants is more variable, which could reflect differences in reproductive timing due to different migratory destinations. Due to the local conditions at the breeding site and the temporal requirements of migration the stonechats in Kazakhstan have a shorter breeding season than their conspecifics in Slovakia (Chapter 5). The Siberian stonechats produce generally one clutch per season, but lost clutches are often replaced. European stonechats lay up to three clutches per season. The interval between clutch loss and relaying does not differ between the two populations. Breeding synchrony is significantly higher in Kazakhstan, both over the whole breeding period and when only the first clutches are considered. Overall clutch size is higher in the Siberian stonechats. Clutch size decreases from first to replacement clutches in Kazakhstan, and from first to second clutches in Slovakia. The clutch size of the third clutches in the European stonechats increases again, which may indicate that only high-quality parents initiate a third seasonal breeding attempt, or that a strategy of terminal investment is involved. Fledging success per egg is higher in Kazakhstan and does not differ between breeding attempts. In Slovakia fledging success drops markedly from the first to the later breeding attempts. Predation during the egg stage plays a greater role in the European stonechats, whereas in the Siberian stonechats predation on nestlings is more important. As a result of smaller clutch sizes later in the season, and of higher failure rates in later clutches, particularly in Slovakia, fledgling output per clutch decreases in both populations. A Kazakh breeding pair produces on average about five fledglings per season, a Slovak breeding pair about seven fledglings per season. In one study season 9% of the Kazakh males mated simultaneously with two females. Facultative polygyny has previously been reported in European, but not in Siberian stonechats. The primary clutches of polygynous males were initiated earlier, contained more eggs and produced more fledglings than the comparable first clutches of monogamous males. This may indicate that polygynous males were of superior parental quality. The secondary polygynous clutches were initiated later, contained less eggs and produced less fledglings, indicating costs in terms of reproductive success for secondary females. According to the challenge hypothesis the length of the breeding period, the degree of inter-male competition, and the degree of parental care affect the seasonal profiles of circulating androgens (Chapter 6). Male Siberian stonechats have elevated circulating androgen levels during May and June with a peak in mid-May. In the European stonechats androgens are elevated from March until the end of May, they peak in mid-April and mid-May. In both populations androgen levels are highest in males during the territorial stages and when the females are fertile, and decrease during the parenting stages. Overall androgen levels are higher in the European stonechat males, which could be explained by higher male-male competition and/or a lower degree of paternal care in this population. Male aggression during simulated territorial intrusions is high throughout the season in both populations and therefore apparently unrelated to the circulating androgen levels. In the Siberian stonechat males dihydrotestosterone (DHT) contributes relatively more to the total circulating androgens. This could be related to different roles of DHT and testosterone (T) in regulating male reproductive behaviours, or to different costs associated with high circulating levels of these androgens. Estradiol (E2) is basal throughout the season in Siberian and European stonechat males. In female stonechats the levels of circulating gonadal steroids (E2, T, DHT) are basal throughout the season. T is higher in the European stonechat females than in the Kazakh females, however there is no apparent seasonal pattern. The adrenal glucocorticoids (GCs) are steroid hormones that act in general energy metabolism and as part of the response of an organism to unpredictable threats to its physiological homeostasis (Chapter 7). Two hypotheses try to explain seasonal and individual variation in circulating GCs. The reproductive limitation hypothesis predicts that, because high GCs may cause nest desertion, GC response to stress is reduced when breeding opportunities are limited, as in the Siberian stonechats. Contrary to this predictions, overall GC levels are higher in the Siberian than in the European stonechats. Hence there seems to be no simple relationship between the potential number of breeding attempts and GC levels in stonechats. The energy mobilisation hypothesis predicts higher GC levels during periods of increased energy demand, such as breeding. The energetic costs may differ between breeding stages, at least in males. However, GC levels do not vary with breeding stage in the stonechats. GC levels decrease in Kazakhstan when the birds start to moult. Feather replacement is an energetically costly task; therefore a simple relationship between energy demand and circulating GCs is not supported. It is possible that GCs interfere with the physiology of feather replacement and are therefore reduced during this period. Time-dependent mortality, the degree of sibling competition, and internal constraints on growth have been discussed as the major factors affecting the evolution of developmental rates (Chapter 8). The length of the breeding season may also affect juvenile development, particularly in migrants, where the juveniles have to gain a certain level of maturity to meet the demands of the autumn movements. Incubation periods are slightly shorter in Kazakhstan than in Slovakia, indicating that embryonic development may proceed at a higher rate in the Siberian stonechats. Postnatal increase rate in body size, but not in wing length, is also higher in the Siberian stonechats. These measures of postnatal growth are not affected by the hatching date. Runt nestlings, which are at a competitive disadvantage because they have hatched later than their nestmates, are initially heavier and bigger but loose this head start during the growth period due to lower growth rates. Postjuvenile moult is initiated very early in the life of Siberian stonechats. as in their captive conspecifics. It is shifted forward by a few days in the late-hatched chicks of replacement clutches, however the effect of hatch date on the onset of moult is much lower than in captive European stonechats. The period from the start of incubation until the end of postjuvenile moult lasts about half as long in the Siberian stonechats than in the European stonechats, mainly due to differences in the onset and duration of moult between the populations. Siberian stonechats lay only one clutch per season, but lost clutches are often replaced (Chapter 9). Normal breeders (those birds that raised their first clutch successfully) initiate postnuptial moult shortly after their young have left the nest. Late breeders (birds with replacement clutches) moult later than normal breeders. However, they initiate moult earlier in relation to the age of their offspring than normal breeders. As a result, they overlap breeding and moult more than normal breeders. Simultaneous reproduction and feather replacement is thought to be costly and therefore generally avoided. This is not always possible in time-constrained breeders. In the Siberian stonechats the degree of moult-breeding overlap increases the later the offspring hatches in the season. Scarce data suggests that late breeding males initiate moult earlier than their female partners. This may imply that they reduce their share in parental care, as has been found in other species. Postponing moult of body feathers, which serves mainly in insulation, may have less severe consequences in a migratory species, than postponing wing moult. Late breeders postpone body moult more than wing moult. Siberian stonechats in Kazakhstan and European stonechats in Slowakia are closely related and breed both in the north-temperate zones in the same latitude and under similar photoperiodic conditions. However, due to the different local climatic conditions, the migratory distance and the length of the breeding season differ. This brings about consistent differences between the two populations in the migratory behaviour, the breeding performance, the hormonal regulation of reproduction, the hormonal response to environmental challenges, and the juvenile development. Because the Siberian and Slovak stonechat populations are closely related, a divergent genetic background and/or species-specific physiological constraints probably play a minor role in creating these differences. They rather reflect differences in annual timing, which affect the trade-off between current and future reproductive success in different life-history stages. The climatic changes that are observed in recent years have been associated with changing migratory and reproductive schedules and shifts in species’ distribution ranges. The example of the stonechats shows that a multitude of systemic changes is required to change the annual cycle. The success of an organism in a changing environment will depend on its ability to successfully integrate all these physiological and behavioural alterations.