Current spring warming as a driver of selection on reproductive timing in a wild passerine.

Evolutionary adaptation as a response to climate change is expected for fitness-related traits affected by climate and exhibiting genetic variance. Although the relationship between warmer spring temperature and earlier timing of reproduction is well documented, quantifications and predictions of the impact of global warming on natural selection acting on phenology in wild populations remain rare. If global warming affects fitness in a similar way across individuals within a population, or if fitness consequences are independent of phenotypic variation in key-adaptive traits, then no evolutionary response is expected for these traits. Here, we quantified the selection pressures acting on laying date during a 24-year monitoring of blue tits in southern Mediterranean France, a hot spot of climate warming. We explored the temporal fluctuation in annual selection gradients and we determined its temperature-related drivers. We first investigated the month-specific warming since 1970 in our study site and tested its influence on selection pressures, using a model averaging approach. Then, we quantified the selection strength associated with temperature anomalies experienced by the blue tit population. We found that natural selection acting on laying date significantly fluctuated both in magnitude and in sign across years. After identifying a significant warming in spring and summer, we showed that warmer daily maximum temperatures in April were significantly associated with stronger selection pressures for reproductive timing. Our results indicated an increase in the strength of selection by 46% for every +1°C anomaly. Our results confirm the general assumption that recent climate change translates into strong selection favouring earlier breeders in passerine birds. Our findings also suggest that differences in fitness among individuals varying in their breeding phenology increase with climate warming. Such climate-driven influence on the strength of directional selection acting on laying date could favour an adaptive response in this trait, since it is heritable.

Nest design in a changing world: great tit Parus major nests from a Mediterranean city environment as a case study.

Investigations of urbanization effects on birds have focused mainly on breeding traits expressed after the nest-building stage (e.g. first-egg date, clutch size, breeding success, and offspring characteristics). Urban studies largely ignored how and why the aspects of nest building might be associated with the degree of urbanization. As urban environments are expected to present novel environmental changes relative to rural environments, it is important to evaluate how nest-building behavior is impacted by vegetation modifications associated with urbanization. To examine nest design in a Mediterranean city environment, we allowed urban great tits (Parus major) to breed in nest boxes in areas that differed in local vegetation cover. We found that different measures of nest size or mass were not associated with vegetation cover. In particular, nests located adjacent to streets with lower vegetation cover were not smaller or lighter than nests in parks with higher vegetation cover. Nests adjacent to streets contained more pine needles than nests in parks. In addition, in nests adjacent to streets, nests from boxes attached to pine trees contained more pine needles than nests from boxes attached to other trees. We suggest that urban-related alterations in vegetation cover do not directly impose physical limits on nest size in species that are opportunistic in the selection of nesting material. However, nest composition as reflected in the use of pine needles was clearly affected by habitat type and the planted tree species present, which implies that rapid habitat change impacts nest composition. We do not exclude that urbanization might impact other aspects of nest building behaviour not covered in our study (e.g. costs of searching for nest material), and that the strengths of the associations between urbanization and nest structures might differ among study populations or species.

Variation in phenotypic plasticity and selection patterns in blue tit breeding time: between- and within-population comparisons.

1. Phenotypic plasticity, the response of individual phenotypes to their environment, can allow organisms to cope with spatio-temporal variation in environmental conditions. Recent studies have shown that variation exists among individuals in their capacity to adjust their traits to environmental changes and that this individual plasticity can be under strong selection. Yet, little is known on the extent and ultimate causes of variation between populations and individuals in plasticity patterns. 2. In passerines, timing of breeding is a key life-history trait strongly related to fitness and is known to vary with the environment, but few studies have investigated the within-species variation in individual plasticity. 3. Here, we studied between- and within-population variation in breeding time, phenotypic plasticity and selection patterns for this trait in four Mediterranean populations of blue tits (Cyanistes caeruleus) breeding in habitats varying in structure and quality. 4. Although there was no significant warming over the course of the study, we found evidence for earlier onset of breeding in warmer years in all populations, with reduced plasticity in the less predictable environment. In two of four populations, there was significant inter-individual variation in plasticity for laying date. Interestingly, selection for earlier laying date was significant only in populations where there was no inter-individual differences in plasticity. 5. Our results show that generalization of plasticity patterns among populations of the same species might be challenging even at a small spatial scale and that the amount of within-individual variation in phenotypic plasticity may be linked to selective pressures acting on these phenotypic traits.

Spring vegetation phenology is a robust predictor of breeding date across broad landscapes: a multi-site approach using the Corsican blue tit (Cyanistes caeruleus).

The regulation of reproductive schedules is an important determinant of avian breeding success. In heterogeneous environments, the optimal breeding period may fluctuate temporally across habitats, often on a spatial scale much shorter than the average dispersal range of individuals. The synchronisation of reproductive events with the most favourable period at a given site therefore involves the integration of a suite of fine-scale environmental signals which contain information about future breeding conditions. In this study, we monitored clutch initiation date of blue tits (Cyanistes caeruleus) breeding in a wide range of environmental conditions (altitude, temperature regimes, habitat type) in Corsica (France) to understand the role of spring temperature and leafing phenology on the precise fine-tuning of egg laying on a local scale. Timing of breeding in blue tits was strongly correlated with phenology of the dominant vegetation (r(2) = 0.87). In contrast, spring temperature was not as robust a predictor of the timing of breeding, because a large part of the residual variation in egg-laying date was accounted by differences (ca. 2 weeks) in the development of the vegetation between habitat types (evergreen vs. deciduous oak forests). Female blue tits therefore appear to use the environmental variable (vegetation phenology) that is most closely linked to the future production of insect prey in order to accurately time laying over the entire spatio-temporal breeding landscape.

Aromatic plants in nests of the blue tit Cyanistes caeruleus protect chicks from bacteria.

Several bird species add fresh fragments of plants which are rich in volatile secondary compounds to their nests. It has been suggested, although never tested, that birds use fresh plants to limit the growth of nest microorganisms. On Corsica, blue tits (Cyanistes caeruleus) incorporate fresh fragments of aromatic plants into their nests. These plants do not reduce infestation by nest ectoparasites, but have been shown to improve growth and condition of chicks at fledging. To understand the mechanisms underlying such benefits, we experimentally tested the effects of these plants on the bacteria living on blue tits. Aromatic plants significantly affected the structure of bacterial communities, in particular reducing bacterial richness on nestlings. In addition, in this population where there is a strong association between bacterial density and infestation by blood-sucking Protocalliphora blow fly larvae, these plants reduced bacterial density on the most infested chicks. Aromatic plants had no significant effect on the bacteria living on adult blue tits. This study provides the first evidence that fresh plants brought to the nests by adult birds limit bacterial richness and density on their chicks.

Estimating biodiversity changes in the Camargue wetlands: An expert knowledge approach.

Mediterranean wetlands are critical strongholds for biodiversity and the provision of ecosystem functions and services; yet, they are being severely degraded by a number of socio-economic drivers and pressures, including climate change. Moreover, we still lack comprehensive understanding of the extent to which biodiversity loss in Mediterranean wetlands will accelerate change in ecosystem processes. Here, we evaluate how changes in biodiversity can alter the ecosystem of the Camargue (southern France). We collected data on species presence/absence, trends and abundance over a 40-year period by combining observations from the scholarly literature with insights derived from expert knowledge. In total, we gathered more than 1500 estimates of presence/absence, over 1400 estimates of species abundance, and about 1400 estimates of species trends for eight taxonomic groups, i.e. amphibians, reptiles, breeding birds, fish, mammals, dragonflies (odonates), orthopterans and vascular plants. Furthermore, we used information on recently arrived species and invasive species to identify compositional changes across multiple taxa. Complementing targeted literature searches with expert knowledge allowed filling important gaps regarding the status and trends of biodiversity in the Camargue. Species trend data revealed sharp population declines in amphibians, odonates and orthopterans, while birds and plants experienced an average increase in abundance between the 1970s and the 2010s. The general increasing trends of novel and invasive species is suggested as an explanation for the changing abundance of birds and plants. While the observed declines in certain taxa reflect the relative failure of the protection measures established in the Camargue, the increasing exposure to novel and invasive species reveal major changes in the community structure of the different taxonomic groups. This study is the first attempt to assess changes in biodiversity in the Camargue using an expert knowledge approach, and can help manage the uncertainties and complexities associated with rapid social-ecological change in other Mediterranean wetlands.

A new technique for ordering asymmetrical three-dimensional data sets in ecology.

The aim of this paper is to tackle the problem that arises from asymmetrical data cubes formed by two crossed factors fixed by the experimenter (factor A and factor B, e.g., sites and dates) and a factor which is not controlled for (the species). The entries of this cube are densities in species. We approach this kind of data by the comparison of patterns, that is to say by analyzing first the effect of factor B on the species-factor A pattern, and second the effect of factor A on the species-factor B pattern. The analysis of patterns instead of individual responses requires a correspondence analysis. We use a method we call Foucart's correspondence analysis to coordinate the correspondence analyses of several independent matrices of species x factor A (respectively B) type, corresponding to each modality of factor B (respectively A). Such coordination makes it possible to evaluate the effect of factor B (respectively A) on the species-factor A (respectively B) pattern. The results obtained by such a procedure are much more insightful than those resulting from a classical single correspondence analysis applied to the global matrix that is obtained by simply unrolling the data cube, juxtaposing for example the individual species x factor A matrices through modalities of factor B. This is because a single global correspondence analysis combines three effects of factors in a way that cannot be determined from factorial maps (factor A, factor B, and factor A x factor B interaction) whereas the applications of Foucart's correspondence analysis clearly discriminate two different issues. Using two data sets, we illustrate that this technique proves to be particularly powerful in the analyses of ecological convergence which include several distinct data sets and in the analyses of spatiotemporal variations of species distributions.

Interspecific variation in the relationship between clutch size, laying date and intensity of urbanization in four species of hole-nesting birds.

The increase in size of human populations in urban and agricultural areas has resulted in considerable habitat conversion globally. Such anthropogenic areas have specific environmental characteristics, which influence the physiology, life history, and population dynamics of plants and animals. For example, the date of bud burst is advanced in urban compared to nearby natural areas. In some birds, breeding success is determined by synchrony between timing of breeding and peak food abundance. Pertinently, caterpillars are an important food source for the nestlings of many bird species, and their abundance is influenced by environmental factors such as temperature and date of bud burst. Higher temperatures and advanced date of bud burst in urban areas could advance peak caterpillar abundance and thus affect breeding phenology of birds. In order to test whether laying date advance and clutch sizes decrease with the intensity of urbanization, we analyzed the timing of breeding and clutch size in relation to intensity of urbanization as a measure of human impact in 199 nest box plots across Europe, North Africa, and the Middle East (i.e., the Western Palearctic) for four species of hole-nesters: blue tits (Cyanistes caeruleus), great tits (Parus major), collared flycatchers (Ficedula albicollis), and pied flycatchers (Ficedula hypoleuca). Meanwhile, we estimated the intensity of urbanization as the density of buildings surrounding study plots measured on orthophotographs. For the four study species, the intensity of urbanization was not correlated with laying date. Clutch size in blue and great tits does not seem affected by the intensity of urbanization, while in collared and pied flycatchers it decreased with increasing intensity of urbanization. This is the first large-scale study showing a species-specific major correlation between intensity of urbanization and the ecology of breeding. The underlying mechanisms for the relationships between life history and urbanization remain to be determined. We propose that effects of food abundance or quality, temperature, noise, pollution, or disturbance by humans may on their own or in combination affect laying date and/or clutch size.

Variable responses to large-scale climate change in European Parus populations.

Spring temperatures in temperate regions have increased over the past 20 years and many organisms have responded to this increase by advancing the timing of their growth and reproduction. However, not all populations show an advancement of phenology. Understanding why some populations advance and others do not will give us insight into the possible constraints and selection pressures on the advancement of phenology. By combining two decades of data on 24 populations of tits (Parus sp.) from six European countries, we show that the phenological response to large-scale changes in spring temperature varies across a species' range, even between populations situated close to each other. We show that this variation cannot be fully explained by variation in the temperature change during the pre- and post-laying periods, as recently suggested. Instead, we find evidence for a link between rising temperatures and the frequency of second broods, which results in complex shifts in the laying dates of first clutches. Our results emphasize the need to consider links between different life-history parameters in order to predict the ecological consequences of large-scale climate changes.

The convergent trajectories of bird communities along ecological successions in european forests.

There is much more variation in the composition of bird communities in the earlier open and semi-open seral stages of ecological successions in forested landscapes of Europe than later on in preforested and forested climactic stages. The demonstration of this trend is achieved from the study of four habitat gradients, two in the mediterranean region (Provence and Corsica) and two in central Europe (Burgundy, France and Poland). A multivariate analysis has been used to illustrate the dynamics of communities along these succession. Displays of the results in bivariate space as well as an illustration of the distributional profiles of some of the most characteristic species show that: i) there is a discrimination between the two mediterranean gradients and the two medioeuropean ones and ii) each succession starts with a very distinct set of species and then the four gradients regularly converge in the last climactic stage where there is almost no discrimination between communities. These results are discussed in the light of the history of European biotas during the Pleistocene. The reason why there is more variation in species composition in the earlier seral stages than in the later forested stages are discussed according to current theories on the role of habitat selection on speciation processes.

Local specialization and maladaptation in the Mediterranean blue tit (Parus caeruleus).

Although the evolutionary interplay between gene flow and local adaptation of organisms in heterogeneous environments has been widely discussed from a theoretical point of view, few empirical studies have been designed to test predictions on the consequences of habitat patchiness on the evolution of life history traits. Using blue tits in Mediterranean habitat mosaics as a model, we defined two nested levels of habitat heterogeneity: an inter-regional level which compares two isolated landscapes (mainland, southern France vs the island of Corsica), and an intra-regional level which compares two habitat types within each landscape (deciduous vs evergreen trees). Deciduous habitats are more common than evergreen habitats on the mainland whereas the opposite is true on the island. Results suggest that: (1) on a regional scale, each population is specialized to the more common habitat, i.e. life history traits have evolved in such a way that breeding success is high; (2) in the less common habitats within each landscape, birds are clearly mistimed because they mismatch the best period of food availability, and hence their breeding success is lower; and (3) the density of the populations and the morphometry of the birds support the model of ideal despotic distribution. These results, which are supported by preliminary data on the genetic variation and gene flow of populations in the mainland landscape, are consistent with a source-sink model of population structure within each landscape. They are discussed in the framework of metapopulation theory and habitat selection models.

Feeding ecology and life history variation of the blue tit in Mediterranean deciduous and sclerophyllous habitats.

High variation in laying date and clutch size of the blue tit between a Mediterranean mixed habitat on the mainland, southern France, and a sclerophyllous habitat on the island of Corsica is hypothesized to be related to differences in the food supply. The diet of the nestlings and feeding frequencies were studied using camera nestboxes and electronic chronographs. Food items brought to the nestlings were much more diverse on Corsica than on the mainland, including many fewer caterpillars and a wider range of taxa. However, when expressed as a volume index, prey items were on average larger on Corsica than on the mainland. Feeding frequencies were significantly lower on Corsica. A good correlation was found in both habitats between laying date and the caterpillar peak date, although both the leafing development of oaks and the peak of abundance of caterpillars occurred 3 weeks later in the Corsican sclerophyllous trees than in the mainland deciduous ones. Differences in the feeding ecology of tits between the two habitats are discussed in the light of the evergreen habit, which means that only 30% of leaves are available for phyllophagous insects instead of 100% in deciduous trees. the combination of a late and low food supply in evergreen trees is the best explanation for the differences in breeding traits betwen the two populations.

High blowfly parasitic loads affect breeding success in a Mediterranean population of blue tits.

The detrimental effects of ectoparasites on the breeding success of birds have been especially well demonstrated in the case of ectoparasites that affect both chicks and their parents. Since blowfly larvae of the genus Protocalliphora attack only nestling birds, they represent a good model for testing the consequences of parasitism on nestlings. A Corsican population of blue tit suffers extremely high rates of infestation by blowflies, which are suspected to negatively affect young birds. Comparing experimentally deparasitized (treated) and naturally infested (control) broods, we showed that the attack by Protocalliphora causes anaemia and an important disturbance to the chicks. Therefore, we expected that these effects would have a negative impact on body condition and survival in the infested broods. Although we did not find any effect of treatment on fledging success, our predictions were confirmed by lower growth rate, body mass at fledging and tarsus length at fledging in the control compared with the treated group. This suggests that in this population, blowflies decrease the probability of recruitment of young blue tits.

Nestling diet variation in an insular Mediterranean population of blue tits Parus caeruleus: effects of years, territories and individuals.

Mediterranean evergreen forests of Corsica are characterized by relatively high species diversity of arthropods with low population densities. Food is never superabundant for Corsican blue tits Parus caeruleus. This study focused on the composition of the food of blue tit nestlings and especially on two main components, caterpillars and spiders. The nestling diet was studied for two years using 8-mm cameras that automatically took photographs of adult birds with food. The diet was composed of c. 50% caterpillars and c. 30% spiders. There were between-year and between-individual differences in these proportions. In both years of the study the proportion of caterpillars declined during the course of the breeding season. Individual and time effects on prey sizes were also observed. Pairs and individuals were fairly constant in the proportions of prey over the feeding period. Different food items were not brought in runs. These findings suggest that strong food limitation exists on Corsica, which can considerably influence life-history traits of the blue tit.

Is the small clutch size of a Corsican blue tit population optimal?

In an attempt to test predictions of the optimisation hypothesis of life history traits in birds, we estimated fitness consequences of brood size manipulations. Experiments were carried out over a period of 4 years in a Mediterranean population of blue tits Parus caeruleus which is confronted with a particular set of environmental constraints. Effects of brood size manipulation were investigated in relation to year-to-year variation in environmental conditions, especially caterpillar abundance. There was a strong variation in the effects of brood size manipulation depending on year. Most effects were on offspring quality (fledging mass, tarsus length). The absolute number of recruits did not significantly differ among categories (reduced, control, enlarged broods) but varied considerably among years. Females recruited from enlarged broods were of lower quality, started to breed later and laid fewer eggs than those recruited from control and reduced broods. Neither parental survival nor reproductive performances of adults in year n + 1 was affected by brood size manipulation in year n. Thus there was no evidence for a cost of reproduction in this population. Since the number of recruits did not depend on brood size manipulation (recruitment rates were higher in reduced broods), but recruits from reduced broods were of better quality compared with other groups, we conclude that adults lay a clutch that is larger than that which is predicted by the optimisation hypothesis. Producing more young could incur some penalties because offspring from large broods are of lower quality and less likely to recruit in the population. Two possible reasons why decision rules in this population seem to be suboptimal are discussed.

Physiological ecology of Mediterranean blue tits (Parus caeruleus L.): effects of ectoparasites (Protocalliphora spp.) and food abundance on metabolic capacity of nestlings.

The consequences of nest ectoparasites, such as Protocalliphora larvae, on nestling birds have been the subject of numerous studies. Despite observed reductions in mass and hematocrit of chicks from parasitized nests, no studies have found any effect of Protocalliphora on nestling survival, suggesting that fitness consequences of Protocalliphora are either weak or occur after fledging. From experiments on the metabolic performance of chicks, we found that parasitized chicks suffer from reduced thermogenic and metabolic capacities as a result of decreased mass and hematocrit. Hence, Protocalliphora may potentially affect nestling survival after fledging, when energetically costly activities such as flight and moult are undertaken. Previous studies have demonstrated an increase in parental feeding rate to compensate for the detrimental consequences of parasite infestation. We tested whether parasite effects on nestling aerobic capacity were dependent on food availability during the feeding period. Measures of caterpillar densities and experimental manipulations of parasite loads allowed us to investigate relationships among host, parasite, and environment. A positive relationship between chick aerobic and thermogenic performances and caterpillar density suggests that negative effects of parasitism may be offset by increased food availability. This study provides the first measurement of the effects of an ectoparasite on metabolic competence in wild birds and documentation of the effect of food availability on ectoparasite virulence using a quantitative measure of food abundance.

Variation in clutch size in relation to nest size in birds.

Nests are structures built to support and protect eggs and/or offspring from predators, parasites, and adverse weather conditions. Nests are mainly constructed prior to egg laying, meaning that parent birds must make decisions about nest site choice and nest building behavior before the start of egg-laying. Parent birds should be selected to choose nest sites and to build optimally sized nests, yet our current understanding of clutch size-nest size relationships is limited to small-scale studies performed over short time periods. Here, we quantified the relationship between clutch size and nest size, using an exhaustive database of 116 slope estimates based on 17,472 nests of 21 species of hole and non-hole-nesting birds. There was a significant, positive relationship between clutch size and the base area of the nest box or the nest, and this relationship did not differ significantly between open nesting and hole-nesting species. The slope of the relationship showed significant intraspecific and interspecific heterogeneity among four species of secondary hole-nesting species, but also among all 116 slope estimates. The estimated relationship between clutch size and nest box base area in study sites with more than a single size of nest box was not significantly different from the relationship using studies with only a single size of nest box. The slope of the relationship between clutch size and nest base area in different species of birds was significantly negatively related to minimum base area, and less so to maximum base area in a given study. These findings are consistent with the hypothesis that bird species have a general reaction norm reflecting the relationship between nest size and clutch size. Further, they suggest that scientists may influence the clutch size decisions of hole-nesting birds through the provisioning of nest boxes of varying sizes.

A new technique for analysing interacting factors affecting biodiversity patterns: crossed-DPCoA.

We developed an approach for analysing the effects of two crossed factors A and B on the functional, taxonomic or phylogenetic composition of communities. The methodology, known as crossed-DPCoA, defines a space where species, communities and the levels of the two factors are organised as a set of points. In this space, the Euclidean distance between two species-specific points is a measure of the (functional, taxonomic or phylogenetic) dissimilarity. The communities are positioned at the centroid of their constitutive species; and the levels of two factors at the centroid of the communities associated with them. We develop two versions for crossed-DPCoA, the first one moves the levels of factor B to the centre of the space and analyses the axes of highest variance in the coordinates of the levels of factor A. It is related to previous ordination approaches such as partial canonical correspondence analysis and partial non-symmetrical correspondence analysis. The second version projects all points on the orthogonal complement of the space generated by the principal axes of factor B. This second version should be preferred when there is an a priori suspicion that factor A and B are associated. We apply the two versions of crossed-DPCoA to analyse the phylogenetic composition of Central European and Mediterranean bird communities. Applying crossed-DPCoA on bird communities supports the hypothesis that allopatric speciation processes during the Quaternary occurred in open and patchily distributed landscapes, while the lack of geographic barriers to dispersal among forest habitats may explain the homogeneity of forest bird communities over the whole western Palaearctic. Generalizing several ordination analyses commonly used in ecology, crossed-DPCoA provides an approach for analysing the effects of crossed factors on functional, taxonomic and phylogenetic diversity, environmental and geographic structure of species niches, and more broadly the role of genetics on population structures.

Circulating corticosterone levels in breeding blue tits Parus caeruleus differ between island and mainland populations and between habitats.

Little is known about whether adaptations to an insular life also involve adaptations in basal corticosterone levels or in the adrenocortical stress response, thus being part of a genetically based island syndrome. However, differences in corticosterone between island and mainland may also be a direct phenotypic response to differences in environmental conditions or may depend on individual characteristics of the animal such as body condition or parental investment. In this paper, we investigated whether insular (Island of Corsica) and mainland (nearby Southern France) blue tits Parus caeruleus populations differed in baseline and handling-stress induced corticosterone levels during the breeding season as a response to biological changes of insular biota. We also examined whether corticosterone levels of both mainland and insular blue tits differed between birds living in two different habitats (summergreen and evergreen oak woods) that differ in food availability and whether individual characteristics affected corticosterone levels. We found (a) differences in baseline corticosterone plasma levels between Corsica and the mainland, independent of regional differences in fat scores, (b) a regional difference in the relationship between corticosterone levels and brood size, (c) a difference in the rapidity of onset of the stress response to handling between habitats, independent of region, and (d) a negative relationship between body fat stores and baseline corticosterone levels independent of region. Reduced baseline corticosterone levels on Corsica may be a component of the insular syndrome, allowing birds to be less aggressive and to enhance parental investment despite higher breeding densities. We suggest that baseline corticosterone levels are only elevated if food availability affects directly the parents. However, when conditions deteriorate unexpectedly (as mimicked by handling stress), food allocation between parents and offspring needs to be re-adjusted in favor of the parents, possibly by increased circulating corticosterone levels. The switch to self-maintenance seems to be modified by the amount of body energy stores.

Effect of human presence and handling on circulating corticosterone levels in breeding blue tits (Parus caeruleus).

Birds may react to the presence of humans with an immediate primary behavioural reaction and with physiological responses, such as the activation of the hypothalamo-pituitary-adrenal axis. This study investigates the suite of behavioural and adrenocortical responses to the presence of humans and to handling in two subspecies of blue tits Parus caeruleus, a small hole-nesting passerine, during the period of feeding their nestlings. The first aim was to investigate whether the presence of humans near their nests elicits an adrenocortical response and whether the increase in circulating corticosterone is correlated with the behavioural reaction of the birds. The second aim was to determine the time-lag between the onset of capture and handling stress and the increase in circulating corticosterone levels. The third aim was to try to explain individual variation in the adrenocortical response to handling with 9 intrinsic and extrinsic factors (sex, age, body size, measures of body condition, time of day, and date). One half of the parents showed a behavioural reaction to our presence near the nest, such as alarming, and hesitating to enter the nestbox. However, the degree of behavioural reaction before handling was not related to circulating corticosterone levels which remained low. The results confirm that primary behavioural and adrenocortical reactions to the presence of predators are independent of each other. A comparison with published findings supports the hypothesis that birds react to predators with an adrenocortical response only in a situation that is imminently life-threatening. Hence, the primary behavioural response of the bird to a predator may determine whether or not an adrenocortical response is elicited. An adrenocortical response to handling started 3 min after capture in the nest box. Individual variation in baseline corticosterone levels could be explained by subspecies and body condition (fat stores), variation in handling-induced corticosterone levels by subspecies, body condition, body size, and time of day.