Body Mass Of Fledglings Research Articles

Food availability aligns with contrasting demographics in populations of an at-risk songbird.

Golden-winged Warblers (Vermivora chrysoptera) have become rare across much of their historic breeding range and response to conservation efforts is variable. Evidence from several recent studies suggests that breeding output is a primary driver explaining responses to conservation and it is hypothesized that differences in food availability may be driving breeding output disparity between two subpopulations of the warbler's Appalachian breeding range. Herein, we studied two subpopulations: central Pennsylvania ("central subpopulation"), where breeding productivity is relatively low, and eastern Pennsylvania ("eastern subpopulation"), where breeding productivity is relatively high. To test the food-availability hypothesis in this system, we measured density of caterpillars, plasma lipid metabolites (triglycerides [TRIG; fat deposition] and glycerol [GLYC; fat breakdown]), body mass of adults males, and acquired body mass data for fledglings at 38 sites managed for nesting habitat. Consistent with our prediction, leaf-roller caterpillar density, the group upon which Golden-winged Warblers specialize, was 45× lower in the central subpopulation than the eastern subpopulation. TRIG concentrations were highest within the eastern subpopulation during breeding grounds arrival. The change in TRIG concentrations from the breeding-grounds-arrival stage to the nestling-rearing stage was subpopulation dependent: TRIG decreased in the eastern subpopulation and was constant in the central subpopulation, resulting in similar concentrations during the nestling-rearing stage. Furthermore, GLYC concentrations were higher in the eastern subpopulation, which suggests greater energy demands in this region. Despite this, adult male warblers in the eastern subpopulation maintained a higher average body mass. Finally, fledgling body mass was 16% greater in the eastern subpopulation than the central subpopulation before and after fledging. Collectively, our results suggest that poor breeding success of Golden-winged Warblers in the central subpopulation could be driven by lower availability of primary prey during the breeding season (leaf-roller caterpillars), and this, in turn, limits their response to conservation efforts.

The interplay among breeding timing, brood size, food quantity, and nestling growth rate in the Eurasian eagle owl (Bubo bubo)

Abstract Optimal clutch size theory predicts a trade-off between egg number and fledgling body mass, potentially affecting their survival rate. Yet, only a few studies have directly linked clutch or brood size to energy (food), nestling growth rate, and timing of breeding. We studied the relationship among all these variables in the Eurasian eagle owl. Of 150 eggs laid in 48 nests, 119 hatched and 117 nestlings fledged, indicating low nestling mortality. The number of eggs, hatchlings and fledglings (possibly indicating territory quality) decreased with ordinal date. The total amount of food brought to the nest increased with brood size, but the average amount per nestling decreased. Nestling growth rate (and thus probably fledgling body mass) was negatively correlated with brood size. A trade-off between brood size and fledgling survival may lessen the benefit of occupying food-rich territories. However, we suggest that fledglings from large broods, despite their relatively low body mass, may have higher survival rates owing to their relatively early fledging. This may be due to: (1) higher prey abundance in late spring compared with late summer and (2) a longer period before entering the cold season. Variation in brood size may therefore represent a set of strategies linked to prey abundance.

Combined influence of food availability and agricultural intensification on a declining aerial insectivore

AbstractAerial insectivores show worldwide population declines coinciding with shifts in agricultural practices. Increasing reliance on certain agricultural practices is thought to have led to an overall reduction in insect abundance that negatively affects aerial insectivore fitness. The relationship between prey availability and the fitness of insectivores may thus vary with the extent of agricultural intensity. It is therefore imperative to quantify the strength and direction of these associations. Here we used data from an 11‐year study monitoring the breeding of Tree Swallows (Tachycineta bicolor) and the availability of Diptera (their main prey) across a gradient of agricultural intensification in southern Québec, Canada. This gradient was characterized by a shift in agricultural production, whereby landscapes composed of forage and pastures represented less agro‐intensive landscapes and those focusing on large‐scale arable row crop monocultures, such as corn (Zea mays) or soybean (Glycine max) that are innately associated with significant mechanization and agro‐chemical inputs, represented more agro‐intensive landscapes. We evaluated the landscape characteristics affecting prey availability and how this relationship influences the fledging success, duration of the nestling period, fledgling body mass, and wing length as these variables are known to influence the population dynamics of this species. Diptera availability was greatest within predominately forested landscapes, while within landscapes dominated by agriculture, it was marginally greater in less agro‐intensive areas. Of the measured fitness and body condition proxies, both fledging success and nestling body mass were positively related to prey availability. The impact of prey availability varied across the agricultural gradient as fledging success improved with increasing prey levels within forage landscapes yet declined in more agro‐intensive landscapes. Finally, after accounting for prey availability, fledging success was lowest, nestling periods were the longest, and wing length of fledglings were shortest within more agro‐intensive landscapes. Our results highlight the interacting roles that aerial insect availability and agricultural intensification have on the fitness of aerial insectivores, and by extension how food availability may interact with other aspects of breeding habitats to influence the population dynamics of predators.

Flexible growth and body mass predict physiological condition at fledging in the synchronously breeding European starling, Sturnus vulgaris.

Recent studies have reported beneficial carryover effects of juvenile development that predict interspecific survival differences at independence. Yet, traits relating to body size (i.e. morphological traits) have proven to be unreliable predictors of juvenile survival within species. Exploring individual variation of growth trajectories and how they covary with physiology could reveal species-specific developmental modes which have implications for our assessments of juvenile quality. Here, we investigated morphological development of European starlings (Sturnus vulgaris) approaching fledging in relation to three components of physiological condition at independence: aerobic capacity, energy state and oxidative status. We found evidence of flexible mass and wing growth which independently covaried with fledgling energy state and aerobic capacity, respectively. By comparison, tarsus and wing length at fledging were unrelated to any physiological trait, while mass was positively associated with principal component scores that comprised aerobic capacity and energy state. Thus, flexible growth trajectories were consistent with ‘developmental plasticity’: adaptive pre-fledging mass recession and compensatory wing growth, which seemingly came at a physiological cost, while fledgling body mass positively reflected overall physiological condition. This highlights how patterns of growth and absolute size may differently reflect fledgling physiology, potentially leading to variable relationships between morphological traits and juvenile fitness.

The Grey-backed Shrike parents adopt brood survival strategy in both the egg and nestling phases

BackgroundGreat diversity exists in the parenting pattern of altricial birds, which has long been considered as an adaptive response to specific environmental conditions but not to their life-history style.MethodsWe examined the egg-laying and nestling-raising pattern of the Grey-backed Shrike (Lanius tephronotus) that breeds only once a year on the Tibetan Plateau. We compared the dietary composition to that of its sympatric competitor, the Brown-cheeked Laughing Thrush (Trochalopteron henrici) that breeds twice a year.ResultsFemale Grey-backed Shrikes produced a fixed clutch size of five, with increasing egg size by their laying sequence. The last offspring in the brood is disadvantageous in the size hierarchy because it hatches later. However, they had the largest fledgling body mass. These findings indicate that Grey-backed Shrikes adopt the brood survival strategy in both the egg and nestling phases. Moreover, males and females exhibit no sexual division in providing parental care as they made an equal contribution to the total amount of food delivered to their brood. This parenting pattern of Grey-backed Shrikes, as well as their dietary items, differ significantly from those of the Brown-cheeked Laughing Thrush.ConclusionsWe suggest that the differentiation in life-history style between sympatric competitors, rather than a behavioral response to specific environmental conditions, plays a decisive role in driving avian parenting strategy diversification.

Trophic match/mismatch and reproduction of the pied flycatcher Ficedula hypoleuca in a metal-polluted area.

In polluted areas, birds can suffer from changes in diet composition and inferior food quality. It is unknown if pollution can desynchronize the phenology of birds and their prey, resulting in a mismatch between food demand and availability. For 2 years, we studied seasonal changes in the biomass of leaf-eating invertebrates and the timing of reproduction and breeding success of an insectivorous bird, the pied flycatcher, in an area heavily polluted by the Middle Ural copper smelter and in an unpolluted control area. Seasonal variations in herbivore biomass were relatively synchronous in polluted and unpolluted areas, whereas birds started breeding later in the polluted area. In the year with an earlier spring, the herbivore peak was early and short, resulting in lower food availability for birds feeding nestlings. The greater the mismatch between food demand and availability the higher was the frequency of nests containing perished nestlings and the lower the body mass of fledglings. Our data did not support the hypothesis that the detrimental effect of the trophic mismatch on birds is greater in the polluted area than in the unpolluted one. Nevertheless, delayed breeding in the pied flycatcher in polluted areas suggests a higher probability of mistimed bird reproduction in the years with a short period of food abundance. Thus, the synchrony of phenology of birds and their prey is an important pollution-related factor that should be taken into account when analyzing the effects of pollution on birds.

Contrasting sensitivity of nestling and fledgling Barn Swallow Hirundo rustica body mass to local weather conditions

Local weather can influence the growth and development of young birds either indirectly, by modifying prey availability, or directly, by affecting energetic trade‐offs. Such effects can have lasting implications for life history traits, but the nature of these effets may vary with the developmental stage of the birds, and over timescales from days to weeks. We examined the interactive effects of temperature, rainfall and wind speed on the mass of nestling and fledgling Barn Swallows Hirundo rustica both on the day of capture and averaging weather across the time since hatching. At the daily timescale, nestling mass was negatively correlated with temperature, but the strength of this association depended on the level of rainfall and wind speed; nestlings were typically heavier on dry or windy days, and the negative effect of temperature was strongest under calm or wet conditions. At the early lifetime timescale (i.e. from hatching to pre‐fledging), nestling mass was negatively correlated with temperature at low wind speed. Fledgling body mass was less sensitive to weather; the only weather effect evident was a negative correlation with temperature at the daily scale under high rainfall that became slightly positive under low rainfall. These changes are consistent with weather effects on the availability and distribution of insects within the landscape (e.g. causing high concentrations of flying insects) and with the effects of weather variation on nest microclimate. These results together demonstrate the impacts of weather on chick growth, over immediate (daily) and longer term (nestling/fledgling lifetime) timescales. This shows that sensitivity to local weather conditions varies across the early lifetime of young birds (nestling–fledgling stages) and illustrates the mechanisms by which larger scale (climate) variations influence the body condition of individuals.

Epizootics and sanitary regulations drive long-term changes in fledgling body condition of a threatened vulture

Abstract Epizootics and deliberate changes in policies affecting the environment may affect large groups of species and the functioning of entire ecosystems. Although these effects often overlap in time, their simultaneous effect is rarely examined despite their importance as causes of current biodiversity loss. Here, based on the monitoring of an Egyptian vulture (Neophron percnopterus) population over thirty-three years (1986–2018), we increase our knowledge about the effects of anthropogenic-induced changes in food availability, both direct (sanitary policies limiting livestock carcass disposal) and indirect (a wild rabbit epizootic), on brood size and body condition of fledglings at nests. We compared the body mass of fledglings of broods with one chick (Single) and two chicks (within which we distinguished First and Second-hatched). The mass of Second-hatched chicks decreased after the plummet in rabbit populations (in the year 1990) and the regulations limiting carcass disposal (2005), reaching minimum values during the period with lowest food availability (i.e. 2005–2013). Recent sanitary legislation allowing carcass disposal by farmers coincides with a slight recovery in the observed body masses. Overall, this study shows that environmental changes of disparate origin can have synergistic effects on individual condition. Conservation of endangered vultures will require multi-targeted conservation plans aimed at ensuring nutritional requirements, in addition to detailed long-term monitoring, in order to detect obscure/masked drivers that affect body condition of fledglings.

Effects of extra‐pair paternity and maternity on the provisioning strategies of the Azure‐winged Magpie Cyanopica cyanus

Altricial birds show enormous intraspecific diversity in their provisioning strategies, in terms of both the provisioning rate and the amount of food delivered per feeding bout. Extra‐pair copulations (EPCs), which result in either extra‐pair paternity (EPP) or maternity (EPM), provide an opportunity to demonstrate why provisioning strategies vary among individuals. Because EPP‐cuckolded males and EPM‐cuckolded females must raise unrelated young, whereas EPM‐cuckolded males and EPP‐cuckolded females need not, we hypothesized that the first two categories of breeders would reduce parental effort, whereas the latter two categories of breeders would increase parental effort. We tested this hypothesis in the Azure‐winged Magpie Cyanopica cyanus by comparing the number and body mass of fledglings, provisioning rates and food amount delivered per feeding bout between EPP‐ and EPM‐cuckolded breeders and faithful breeders. We found that (1) the number of fledglings did not differ significantly between cuckolded and faithful breeders, and (2) fledglings raised by EPM‐cuckolded males and EPP‐cuckolded females did not differ from faithful breeders, whereas fledglings raised by EPP‐cuckolded males and EPM‐cuckolded females were significantly smaller than those raised by faithful breeders. Compared with faithful breeders, cuckolded breeders increased parental efforts in nests that contained no unrelated young; hence, their loss in parentage may be compensated for by the enhanced quality of fledglings. In nests that contained unrelated young, cuckolded breeders did not reduce parental efforts; hence, their own offspring would not starve and could survive in competition with mixed brood‐mates. Our findings suggest that the differences in parentage created by EPP and EPM scenarios and the potential fitness return of raising a brood for the cuckolded breeders can explain the intraspecific variation in provisioning strategies of altricial birds.

Egg investment in response to helper presence in cooperatively breeding Tibetan ground tits

Life‐history theory predicts a trade‐off between current and future reproduction to maximize lifetime fitness. In cooperatively breeding species, where offspring care is shared between breeders and helpers, helper presence may influence the female breeders’ egg investment, and consequently, survival and future reproductive success. For example, female breeders may reduce egg investment in response to helper presence if this reduction is compensated by helpers during provisioning. Alternatively, female breeders may increase egg investment in response to helper presence if helpers allow the breeders to raise more or higher quality offspring successfully. In the facultatively cooperative‐breeding Tibetan ground tit Pseudopodoces humilis, previous studies found that helpers improve total nestling provisioning rates and fledgling recruitment, but have no apparent effects on the number and body mass of fledglings produced, while breeders with helpers show reduced provisioning rates and higher survival. Here, we investigated whether some of these effects may be explained by female breeders reducing their investment in eggs in response to helper presence. In addition, we investigated whether egg investment is associated with the female breeder's future fitness. Our results showed that helper presence had no effect on the female breeders’ egg investment, and that egg investment was not associated with breeder survival and reproductive success. Our findings suggest that the responses of breeders to helping should be investigated throughout the breeding cycle, because the conclusions regarding the breeders’ adjustment of reproductive investment in response to being helped may depend on which stage of the breeding cycle is considered.

Feather content of porphyrins in Eurasian eagle owl (Bubo bubo) fledglings depends on body condition and breeding site quality.

Porphyrins are pigments produced in most animal cells during the synthesis of heme, but their importance for external coloration is unclear. Owls (Order Strigiformes) are among the few animals that accumulate porphyrins in the integument, where it could serve as a means of signaling. Here we hypothesized that the porphyrin content of feathers may depend on body condition and breeding site quality in Eurasian eagle owl (Bubo bubo) fledglings and, thus, constitute amplifiers of the quality of the area where they are born. Using high-performance liquid chromatography, we found 2 porphyrins (protoporphyrin IX and coproporphyrin III) in the body feathers of 19 eagle owl fledglings from 7 breeding territories. Coproporphyrin III, but not protoporphyrin IX feather concentration, was positively associated with the body mass of fledglings and with the quality of the breeding sites where they were reared with respect to food quality and availability. As coproporphyrin III is produced under oxidative stress, we suggest that good breeding sites may lead to fledglings in good condition. This, in turn, may make fledglings induce a certain level of free radical and coproporphyrin III production to signal to conspecifics their site-mediated capacity to cope with oxidative stress. This is the first time that porphyrin content in the integument has been found to be related to individual quality, opening a new scenario for studying evolution of animal coloration.

Offspring fitness and the optimal propagule size in a fluctuating environment

Propagule size is an important maternal effect on offspring fitness and phenotype in birds and other oviparous animals. The performance of propagules often increases with size, but a fluctuating environment may introduce temporal variation in the optimal phenotype. Understanding these mechanisms will provide novel insights into the eco‐evolutionary dynamics of life history strategies in parental reproductive investment. We investigated the interaction between propagule size (measured as egg volume) and environmental conditions on offspring mortality and phenotype in a Norwegian house sparrow population. Increased propagule size reduced offspring mortality in early life, with more pronounced effects under heavy precipitation. However, the optimal propagule size for low offspring mortality until recruitment shifted from large to small as temperature increased. Propagule size had no significant effect on fledgling body mass and tarsus length. These results reveal a potential for eco‐evolutionary dynamics in propagule size, as populations adapt to fluctuating environmental conditions. The ultimate outcome of this dynamic process will also depend on variation in parental fitness and tradeoffs with other life‐history traits, particularly clutch size.

Small patches of broadleaf trees influence nest-site selection and reproductive performance of two tit species (Paridae) in a Japanese cedar plantation

ABSTRACTConifer plantations generally provide less food for woodland birds such as tit species than do broadleaf or mixed forests. We examined whether small patches of broadleaf (thus, caterpillar-rich) trees in a Japanese cedar (Cryptomeria japonica) plantation could improve the breeding success of coal tits (Periparus ater) and varied tits (Poecile varius) using nest boxes in the plantation. We evaluated the relationships between the distribution of the broadleaf trees (the minimum distance from each nest box to the nearest patch and the total area of broadleaf patches within 25, 50, 100, and 200 m from each nest box) and breeding performance (clutch size, number, and body mass of fledglings, fledging success, fledge rate, and nestling period). Coal tits selected nest boxes regardless of the distribution of broadleaf patches. By contrast, varied tits nested preferentially within 50 m of broadleaf patches and, within 50 m, favored boxes near a larger total area of broadleaf patches than coal tits. The breeding performance of both species was not affected by the distribution of patches; however, the clutch size of varied tits (but not coal tits) tended to decline in nests ≥50 m from broadleaf patches, and there were no large clutches in such positions. The broadleaf trees might have improved the habitat quality, particularly for varied tits, by providing food resources. Thus, broadleaf patches as foraging sites and nest boxes as breeding sites could both be critical to the breeding success of woodland birds in conifer plantations.

Caterpillar biomass depends on temperature and precipitation, but does not affect bird reproduction

Complex changes in phenological events appear as temperatures are increasing: In deciduous forests bud burst, hatching of herbivorous caterpillars, egg laying and nestling time of birds when feeding chicks on caterpillars, may differentially shift into early season and alter synchronization. If timing of bird reproduction has to match with short periods of food availability, phenological mismatch could negatively affect reproductive success. Using a unique empirical approach along an altitudinal temperature gradient, we firstly asked whether besides temperature, also precipitation and leaf phenology interplay and affect caterpillar biomass, since impacts of rainfall on caterpillars have been largely neglected so far. Secondly, we asked whether abundance of caterpillars and thereby body mass of great tit nestlings, which are mainly fed with caterpillars, vary along the altitudinal temperature gradient. We demonstrated that next to temperature also precipitation and leaf phenology affected caterpillar biomass. In our beech forest, even along altitudes, caterpillars were available throughout the great tit breeding season but in highly variable amounts. Our findings revealed that although timing of leaf phenology and great tit breeding season were delayed with decreasing temperature, caterpillars occurred synchronously and were not delayed according to altitude. However, altitude negatively affected caterpillar biomass, but body mass of fledglings at high altitude sites was not affected by lower amounts of caterpillar biomass. This might be partially outweighed by larger territory sizes in great tits.

Barn Owl (Tyto alba) breeding biology in relation to breeding season climate

Winter weather has a strong influence on Barn Owl (Tyto alba) breeding biology. Here, we analyzed the impacts of weather conditions on reproductive performance during the breeding season using data collected over 22 years in a Swiss Barn Owl population. Variations in rain and temperature during the breeding season played an important role in within-year variation in Barn Owl reproduction. An increase in rainfall during the period from 4 to 2 weeks preceding egg laying had a positive effect on clutch size. In contrast, fledgling body mass was negatively influenced by rainfall during the 24 h preceding the measurements. Finally, ambient temperature during the rearing period was positively associated with brood size at fledging. In conclusion, weather conditions during the breeding season place constraints on Barn Owl reproduction.

Nest Site Preparation and Reproductive Output of the European Starling (Sturnus Vulgaris)

Hole-nesting birds are faced with the problem of dealing with the presence of nests from previous seasons. In a few species, specific nest site preparation behaviour has evolved, whereby birds remove part or all of old nest and only later build a new one. Such behaviour has not been studied in any great detail. European Starlings ( Sturnus vulgaris) were allowed to breed in nestboxes that were empty or contained old nests from a previous year. The material of old nests was marked so it was possible to evaluate the amount of material that was removed by the bird itself. Additional observations were carried out to study the temporal aspect of such nest site preparation. Birds started old nest removal soon after arrival at the breeding location, around one month before egg laying. Birds with the highest cleaning effort, i.e. removed a larger part of the old nest, began laying eggs later, laid fewer eggs and brought food to nestlings less frequently. Besides wing length, there was no relationship between cleaning efforts and fledgling body mass nor tarsus length. Nest site preparation did not decrease the load of fleas in nests. Time and energy invested in the removal of an old nest may negatively affect the entire reproductive cycle. Other hypotheses connected with nest site preparation and low reproductive output are discussed.

Estimating fluctuating selection in age‐structured populations

In age-structured populations, viability and fecundity selection of varying strength may occur in different age classes. On the basis of an original idea by Fisher of weighting individuals by their reproductive value, we show that the combined effect of selection on traits at different ages acts through the individual reproductive value defined as the stochastic contribution of an individual to the total reproductive value of the population the following year. The selection differential is a weighted sum of age-specific differentials that are the covariances between the phenotype and the age-specific relative fitness defined by the individual reproductive value. This enables estimation of weak selection on a multivariate quantitative character in populations with no density regulation by combinations of age-specific linear regressions of individual reproductive values on the traits. Demographic stochasticity produces random variation in fitness components in finite samples of individuals and affects the statistical inference of the temporal average directional selection as well as the magnitude of fluctuating selection. Uncertainties in parameter estimates and test power depend strongly on the demographic stochasticity. Large demographic variance results in large uncertainties in yearly estimates of selection that complicates detection of significant fluctuating selection. The method is illustrated by an analysis of age-specific selection in house sparrows on a fitness-related two-dimensional morphological trait, tarsus length and body mass of fledglings.

Maternal effects on offspring Igs and egg size in relation to natural and experimentally improved food supply

1. Maternal effects have been suggested to function as a mechanism for transgenerational plasticity, in which the environment experienced by the mother is translated into the phenotype of the offspring. In birds and other oviparous vertebrates where early development is within the egg, mothers may be able to improve the viability prospects of their offspring at hatching by priming eggs with immunological and nutritional components. 2. We studied how resource availability affects maternal investment in offspring by feeding Ural owl (Strix uralensis, Pall.) females prior to egg-laying in 3 years of dramatically different natural food conditions. 3. Supplementary feeding prior to laying increased body mass and the level of Igs of females measured at clutch completion. Supplementary fed Ural owl females laid larger eggs than control females, and had offspring with higher levels of Igs at hatching compared to offspring of control females. 4. We found variation in maternal allocation of resources to the eggs in response to environmental conditions: during a year of rapidly declining food abundance, maternal Igs in hatchlings were higher, whereas egg size was smaller compared to years with a more stable food supply. 5. Egg size had a positive effect on offspring body mass at fledging, whereas Igs at hatching did not affect Igs at fledging. 6. We conclude that maternal body condition and maternal Igs, as well as hatchling Igs and egg size are limited by food resources during egg production. Hatchlings rely on maternally derived Igs and, hence, our results suggest that mothers with high levels of Igs passively transfer higher Igs levels to their eggs instead of active manipulation of Igs levels in eggs. Ural owl egg size appears to be highly sensitive to short-term changes in food abundance, with important consequences for nestling growth.

Fitness Consequences of Increased Testosterone Levels in Female Spotless Starlings

The role of testosterone in female fitness is poorly known in most vertebrates. It has been proposed that the dynamics of female testosterone may represent a correlated response to selection acting on male testosterone. However, several costs and benefits of enhancing the circulating levels of testosterone in females have been documented in a number of species, suggesting that female testosterone may be the product of direct selection. Despite this, no study has tried to quantify the long-term fitness consequences of manipulating female testosterone. We report the results of an experiment in which we implanted females with testosterone (T-females) and recorded the consequences of this manipulation on investment and reproductive success throughout their lifetime in relation to control females (C-females). Our results show that T-females bred more years in the colony than C-females. The onset of laying was delayed in T-females compared with C-females, but this effect decreased in the years following manipulation. The T-females laid fewer eggs per year, but eggs were heavier than those laid by C-females. The T-females fed nestlings at a lower rate than C-females in the 3 years in which feeding rates were measured and raised fewer fledglings than C-females in the manipulation year. However, since T-females accomplished more breeding attempts than C-females, as a result of their longer stay in the colony, there were no significant differences between the lifetime reproductive success of T- and C-females. There were no overall differences in body size of nestlings raised by T- and C-females, but the body mass of fledglings reached a peak 2-3 years after manipulation for C-females, a tendency not recorded in offspring of T-females. Also, young that were fledged in broods of C-females tended to recruit into the study colony more frequently than those fledged in T-females' nests. All these results indicate that our testosterone manipulation improved the ability of females to acquire and maintain a breeding site but interfered with their reproductive biology and parental investment. Thus, our main conclusion is that under the ecological conditions of our experiment, the addition of exogenous testosterone reduced female fitness. However, differences in population density and female-female competition are likely to alter the fitness landscape of this trait and change the adaptiveness of increased testosterone as a component of female reproductive strategies.

Sex-specific development of cell-mediated immunity under experimentally altered rearing conditions in blue tit nestlings

In birds, poor rearing conditions usually have negative effects on T-cell-mediated immune response. However, earlier studies demonstrate that fitness-related traits such as body mass may show sex-specific patterns when subject to alteration of rearing conditions. Therefore, to investigate whether deterioration of rearing conditions influences the development of immune function differently in male and female nestlings, we performed brood size manipulation experiments on blue tit (Parus caeruleus) nestlings. To alter rearing conditions, some broods were increased by three nestlings soon after hatching, while other broods were left non-manipulated. Immune response was assessed as a hypersensitivity reaction to phytohaemagglutinin in 11-day-old nestlings. Additionally, we studied the consequences of brood size manipulation for fledgling body mass and tarsus length. The enlargement of brood size had different effects on the cellular immune responses of male and female nestlings, with males being more negatively affected than their female nest-mates. Sex-specific effects of poor rearing conditions were also recorded for tarsus length, such that tarsus growth was more retarded in female than in male nestlings. We discuss the effects of deterioration of rearing conditions on sex-specific development of cell-mediated immunity with respect to sexual dimorphism of size and developmental strategies in male and female nestlings.