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Female birds are exceptionally consistent in laying rather uniform and individually specific eggs, so that variation in eggs of individual females is low, while variation among females is high. Repeatability is a population-level parameter used in quantitative genetics to quantify the level of intraindividual consistency in relation to interindividual variation in phenotypic traits, in this case, the egg traits. The statistical measure of repeatability is intraclass correlation of particular egg traits clustered in clutches. This study is based on a long-term dataset on repeatabilities of the length, breadth, volume and shape of eggs of Blue Tits Cyanistes caeruleus investigated in two ecologically varying sites (an urban parkland versus a mature deciduous forest) around the city of Łodź, central Poland in 1999–2013. Coefficients of repeatability of egg length, breadth, volume and shape in the study populations of Blue Tits showed some variation between years and study sites, but were generally very high, on average 0.7 or more, and did not significantly differ between the traits within years. The observed differences between repeatabilities do not seem to be strongly associated with year-specific weather indicators for the first half of April. Only coefficients of variation in mean daily temperature influenced repeatabilities of egg length and breadth, whereas effects of mean temperature were non-significant. Repeatability of egg length in the forest and breadth in both sites increased with increasing variation in daily temperature, whereas repeatability of egg length in the urban park site showed a reverse relation with variation in temperature. Year-site-specific mean laying dates, clutch sizes and egg volumes, considered as indicators of breeding conditions, tended to negatively affect the repeatabilities. However, in general, repeatabilities of different egg traits were not consistent in their variation between years and sites, which suggests an important role of stochastic factors in shaping this variation. External factors that are able to force females to diversify traits of eggs in a particular laying sequence do not seem to directly result from the environmental conditions prevailing during a spring period critical for laying eggs in a population in a given year (the first half of April), but seem to be linked to more subtle aspects of variation in the conditions for egg laying, as shown by links with the mean values of basic breeding parameters.

Partial mortality in broods of altricial passerines results from various reasons, including deficiency of resources, inadequate parental care, diseases and other external factors. In this study we test two hypotheses concerning the phenomenon of partial mortality in nestling altricial passerines: (1) non-adaptive starvation hypothesis based on the assumption that partial brood losses are caused by starvation and undernourishment and has no adaptive value, and (2) adaptive starvation hypothesis assuming that losses from starvation might be elements of an adaptive strategy of reducing surplus nestlings in broods that are over-large. To test the above hypotheses we divided broods of two tit species Great Tit Parus major and Blue Tit Cyanistes caeruleus into two categories: broods with incomplete, less than 100% fledging success and broods with complete 100% fledging success. Subsequently, we compared physiological performance of nestlings between brood categories using blood hemoglobin concentratio...

Birds have specific habitat requirements during the breeding period. The Blue Tit Cyanistes caeruleus is a species evolutionarily associated with forest areas that prefers deciduous and mixed forests, whilst its breeding in urban areas is a relatively recent phenomenon. Long-term data (2002–2015) on reproductive performance (number of hatchlings, hatching success, number of fledglings and fledging success (fledging success as the number of fledglings in relation to the number of hatchlings)) were quantified for two Blue Tit populations in two, floristically and structurally contrasting areas (a mature deciduous forest and an urban parkland) in central Poland. The principal aim of this study is to see whether the habitat type, year or the food availability affect the breeding success of Blue Tits. Forest Blue Tits produced significantly more hatchlings (9.82 ± 2.64 (SD) in the forest vs.9.17 ± 2.16 in the parkland) and fledglings (9.18 ± 2.84 in the forest vs. 8.14 ± 2.68 in the parkland) than urban Blue Tits. The number of fledglings was positively correlated with the number of hatchlings in both study areas. Both forest and urban Blue Tits shared a similar hatching success (85.9 % in the parkland and 85.5 % in the forest), while the fledging success was significantly higher in the forest (83.4 % in the parkland and 86.1 % in the forest). The amount of caterpillar frassfall was also studied (caterpillars are the optimal food for nestlings) at both study areas and it suggested that caterpillars were more abundant in the forest than in the parkland (the maximum amount of frassfall, averaged 0.21 ± 0.11 g frass/m2/day in the urban parkland and 0.59 ± 0.50 g frass/m2/day in the forest in 2003–2015). In the forest area, the mean number of fledglings tended to be related to the amount of frassfall but in the parkland, this relation was non-significant. The long-term dynamics of fledging success in our study sites seems to be mutually independent. Thus low fledging success in the forest site does not mean similarly low fledging success in the urban parkland site and vice versa. We suggest that food availability is one of key drivers of differences in the tits breeding success between both studied habitats.

Urbanization affects the ecological and behavioral traits of various species of animals, including birds.We present results concerning long-term fluctuations in breeding densities of nest-box populations of the Blue Tit Cyanistes caeruleus and the Great Tit Parus major in two, structurally and floristically contrasting types of habitat (an urban parkland and a rich deciduous forest) located 10 km apart, in central Poland. This study was conducted in 1999–2012 in the parkland site and in 2002–2012 in the forest site. We found a strong correlation of year-to-year changes in breeding densities of Great Tits between the parkland site and the forest site and a lack of such a correlation in Blue Tits. Breeding densities of Great Tits were much higher in the parkland than in the forest area every year during the study period. Annual changes in breeding densities were not correlated between the species studied. The North Atlantic Oscillation Index (NAO-winter index) tended to influence the density dynami...

Sequentially laid eggs of small songbirds are built from the current income of resources and therefore can be subject to different environmental influences on the short-term and long-term time scales. In this paper we study variation in egg length, breadth, volume and shape in Pied Flycatchers Ficedula hypoleuca in Łodź, central Poland, during 2002–2012, in relation to climatic and habitat-related factors. We found a significant decreasing trend in egg length, breadth and volume across the study years and a slight tendency for eggs to be a little more elongated in a forest site than in an urban parkland site. Even though mean monthly temperature in May is known to have increased since the 1970s in the area of Łodź, the average ambient temperature during the time of formation of eggs in particular clutches tended to decrease, with increasing variability at the same time, in 2002–2012. Egg size traits showed a positive association with average temperature and a negative relation with variability i...

Egg size is a basic species-specific life-history trait in birds which influences offspring performance through the nestling stage and, therefore, fitness. Shells of snails are the principal source of calcium for female parids and some other passerines during egg formation. Shortages in the availability of snail shells may lead to laying smaller eggs with thinner shells. In urban green spaces that require protection against pests, the numbers of snails may become greatly reduced in response to molluscicide treatments. The use of a molluscicide treatment against pest slugs in the Botanic Garden, Łodź, Poland, in 2013 and 2014 provided a unique quasi-experimental setting to study the effect of molluscicide on both non-target snails and Blue Tit Cyanistes caeruleus eggs. Using a forest study site where no treatment was applied as a control area, we tested for effects of the molluscicide treatment in the Botanic Garden on the density of snails and on the egg volume of Blue Tits. We found that after the molluscicide treatment a dramatic drop in snail numbers occurred, which likely restricted the availability of snail shells for egg laying Blue Tits in the Botanic Garden. Correspondingly, egg sizes significantly decreased in the Garden, while they stayed constant over time in the forest. Eggs in bigger clutches were more strongly affected by the molluscicide treatment than eggs in smaller clutches. Our quasi-experimental study suggests that Blue Tits are sensitive to poor availability of snail shells. This is manifested by laying smaller eggs, which require less calcium to be incorporated in the eggshell, in response to a dramatic drop in the availability of snail shells. This result is expected to raise awareness of potential indirect and inadvertent effects of park pest management on biota other than molluscan pests.

Enduring changes in the surrounding environment cause long-term stress in birds, which can affect the activity of the immune system and lead to changes in heterophil-to-lymphocyte ratios (H:L). The aim of this paper is to investigate if the H:L ratios of male and female Great Tits Parus major during the first breeding attempt differ between two sites of dissimilar habitats – an urban parkland site and a deciduous forest site. In the final model we found a difference between sites, while separate submodels for the study sites showed that females had higher H:L ratio than males in the urban parkland site, with the opposite result being found in the forest site. We suppose that the pattern of variation in H:L ratios we found may result from greater parental expenditures of females as compared to males. Such difference in the H:L ratio between habitats could be the reason for the substantial difference in trophic conditions between the forest and park areas during the breeding of tits.

Physiological functions of growing nestlings are thought to be traded-off in relation to rearing conditions, with the resulting physiological state of fledglings having important long-lasting consequences for their fitness. By manipulating brood size up and down, and, separately, by supplying additional food (mealworms — larvae of Tenebrio molitor) we tested if alterations of the rearing conditions would influence nestling performance in Blue Tits Cyanistes caeruleus and Great Tits Parus major. Brood size manipulation affected body mass, heterophil-to-lymphocyte ratio (H:L) and fledging probability in both species and the level of triglycerides in nestling Great Tits. Extra food supply influenced only fledging probability, with no other effect on indicators of nestling performance. An effect on nestling body mass and a lack of effect on cell-mediated immune response in the brood-size experiment suggest that nestlings in enlarged broods sacrificed growth to maintain immunity. In general, effects ...

High metabolic rates of birds demand an efficient oxygen transport system; this is ultimately based on the oxygen carrying capacity of haemoglobin. Therefore patterns of variation in blood haemoglobin content of wild birds are an important aspect of functional ecology. In this paper we report results concerning variation in haemoglobin con- centration in the blood of adult Blue Tits Cyanistes caeruleus and Great Tits Parus major during the nestling rearing peri- od (days 7-15 of nestlings life) of first broods in central Poland in 2003-2009. The most important findings of this study are: (i) average haemoglobin concentration in blood differs between Blue Tits and Great Tits, with higher values in Blue Tits; (ii) males differ from females in both these species, with higher values in females, and (iii) there is also significant variation among years, with parallel tendencies for both species. We explain the patterns of haemoglobin content vari- ation in adult tits by differences in metabolic demands for oxygen transported by blood. The demands are higher for the smaller-bodied species (Blue Tits), heavier working sex (females) and in years with worse physical and trophic con- ditions, though only non-significant relationships with weather conditions (temperature and rainfall) or food availabil- ity (measured by frassfall) were found during our long-term study.

Wild passerines, especially tits, utilize snail shells as the main source of calcium necessary for laying females to construct egg shells. This research found that the two study areas, representing two habitat types — a mature decid- uous forest and a human-disturbed parkland — are inhabited by different snail assemblages: both species richness and density are much higher in the parkland than in the forest. This means that less calcium is available to female tits in the forest than in the parkland, which could result in calcium limitation in the former habitat. Egg size traits, i.e. volume, length and breadth, in the Blue Tit show a consistent long-term pattern of variation that reflects the pattern of calcium availability: egg trait values are higher in the parkland than in the woodland. No habitat-related variation in egg size traits was found in the Great Tits. We suggest that the lack of a relation between Great Tit egg characteristics and snail availability results from the higher ecological plasticity of this species in comparison with the Blue Tit, including its abil- ity to exploit alternative sources of calcium.

Producing high quality offspring of good physiological performance, able to survive to independence and, then, to reproductive maturity is a major component of life history strategies. The ability of nestling altricial birds to develop a good physiological condition depends to a large extent on the amount and quality of food provided by par- ents, as well as other aspects of parental care. We hypothesized that experimental changes to the original brood size should affect both parental Blue Tits and their offspring, resulting in corresponding changes in the body condition of the nestlings. Over two breeding seasons, using two habitat sites, we conducted an experiment with two manipulative treatments applied to broods of three-day-old nestlings — the reduction or enlargement of broods by three nestlings, and one non-manipulative control treatment. Our aim was to test whether the experiment would affect a number of different measures of nestling condition: blood concentrations of hemoglobin and glucose, heterophil-to-lymphocyte ratio and morphometric condition index, all being analyzed when the nestlings were 13 days old. We found no effect in the case of hemoglobin, despite the fact that it had previously been shown to be sensitive to large-scale differences in trophic conditions between habitats and years and to the experimental removal of nest parasites. All the remaining variables, i.e. heterophil-to-lymphocyte ratio, glucose concentration and morphometric condition index, responded to the experimental treatments, showing different but reasonable patterns of variation. We suggest that an experimental increase in brood size definitely hinders the development of nestling physiological condition, but even an experimen- tal reduction of broods can affect some physiological indicators (glucose), probably because of readjustments in the feeding rate.