We investigated whether the variation in T‐cell‐mediated immune function of blue tit nestlings affected their fledgling success and the probability of local survival. We studied the relationship between immune function and survival under two rearing conditions: control, unmanipulated, and experimentally enlarged broods. Brood enlargement had negative effects on nestling immune response. Immune response was positively related to fledgling success and it predicted the probability of local recruitment. However, the relationship between immune response and the probability of recruitment was significantly positive only among control broods and nonsignificant among enlarged broods. The effect of immune response on the recruitment probability was not affected by variation in body mass. Our study suggests that selection for immune responsiveness seems to be weak or even absent under unfavourable rearing conditions as simulated by brood size enlargement. Therefore, year‐to‐year environmental variation and environmental heterogeneity may constrain evolution towards higher immune responsiveness.

Telomeres have recently been suggested to play important role in ageing and are considered to be a reliable ageing biomarkers. The life history theory predicts that costs of reproduction should be expressed in terms of accelerated senescence, and some empirical studies do confirm such presumption. Thus, a link between reproductive effort and telomere dynamics should be anticipated. Recent studies have indeed demonstrated that reproduction may trigger telomere loss, but actual impact of reproductive effort has not received adequate attention in experimental studies. Here, we experimentally manipulated reproductive effort by increasing the brood size in the wild blue tit (Cyanistes caeruleus). We show that parents attending enlarged broods experienced larger yearly telomere decay in comparison to control birds attending unaltered broods. In addition, we demonstrate that the change in telomere length differs between sexes, but this effect was independent from our treatment. To our knowledge, this is the first experimental study in the wild revealing that telomere dynamics may be linked to reproductive effort. Thus, telomere shortening may constitute one of the potential proximate mechanisms mediating the costs of reproduction.

Some bird species place in their nests fresh green plant materials, e.g. leaves, sprigs or branches of herbs, shrubs and trees, which are not a part of the basic nest structure. These additional materials are often characterised by a high content of volatile secondary metabolites and constitute a non-random, small fraction of plants available in the habitat. Several non-mutually exclusive hypotheses have been proposed to explain the function of green material in avian nests with three of them attracting the most attention. The courtship hypothesis proposes that green material is used by males in order to attract females to nesting sites. The nest protection hypothesis posits that birds place in the nest green material rich in volatile compounds to reduce the abundance of parasites and pathogens which, in turn, should mitigate their negative impact on the host. According to the drug hypothesis green material directly positively affects the health and development of nestlings, e.g. through stimulation of some components of the nestlings' immune system. Here, we present an overview of hypotheses explaining the phenomenon of green material in avian nests with a thorough description of three most frequently tested hypotheses and suggest the directions for future studies.

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.

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.

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Environmental conditions are key drivers of life-history evolution, and the urban environment is an extreme form of land-use readily inhabited by avian wildlife, whose life-history variation in such altered environment is still poorly understood. Recently, the study of environmental variables associated with urban living-which include shifts in temperature, light, noise or food availability-has attracted increased attention. Another environmental axis that sets the urban space at odds relative to natural habitats is high human abundance, yet very little is known about its effect on avian fitness. We developed a protocol to quantify human presence by performing repeated counts of humans on the ground within a 15 m radius of nestboxes monitored in two centrally-located study areas of a European capital city. In parallel, a GIS-based approach was used to infer nestbox distance to the nearest path and road. Multiple counts of human presence around each nestbox yielded moderate to high repeatabilities (0.6 ≤ r ≤ 0.8) while requiring considerable resources time-and people-wise. In contrast, GIS-based estimates of nestbox distance to paths and roads were time efficient and generated highly repeatable results. The effects of (i) human presence around each nestbox, (ii) nestbox distance to the nearest path and (iii) nestbox distance to the nearest road were tested on reproductive traits of blue tits Cyanistes caeruleus and great tits Parus major breeding in two urban sites. Human presence did not influence blue tit or great tit life-history traits and reproductive success, suggesting reproductive habituation to humans in an urban landscape. In contrast, nestbox distance to roads shortened incubation time in great tits while nestbox distance to paths increased incubation time in blue tits. Moreover, blue tit offspring 2 weeks after hatching were lighter closer to roads. Our study confirms the reliability of a field protocol capturing human presence around multiple fixed locations that can be easily implemented in either urban or rural landscapes. At the same time, it appears that when applied to two urban sites where habituation to humans might have occurred, it is infrastructural networks rather than human presence per se that played a greater role in tit reproductive trait variation.

Introduction Rapid environmental change driven by urbanization offers a unique insight into the adaptive potential of urban‐dwelling organisms. Urban‐driven phenotypic differentiation is increasingly often demonstrated, but the impact of urbanization (here modelled as the percentage of impervious surface (ISA) around each nestbox) on offspring developmental rates and subsequent survival remains poorly understood. Furthermore, the role of selection on urban‐driven phenotypic divergence was rarely investigated to date. Methods and Results Data on nestling development and body mass were analysed in a gradient of urbanization set in Warsaw, Poland, in two passerine species: great tits (Parus major) and blue tits (Cyanistes caeruleus). Increasing levels of impervious surface area (ISA) delayed the age of fastest growth in blue tits. Nestling body mass was also negatively affected by increasing ISA 5 and 10 days after hatching in great tits, and 10 and 15 days in blue tits, respectively. High levels of ISA also increased nestling mortality 5 and 10 days after hatching in both species. An analysis of selection differentials performed for two levels of urbanization (low and high ISA) revealed a positive association between mass at day 2 and survival at fledging. Discussion This study confirms the considerable negative impact of imperviousness—a proxy for urbanization level—on offspring development, body mass and survival, and highlights increased selection on avian mass at hatching in a high ISA environment.