Body mass of female great tits (Parus major) at egg laying

Based on automated weighing of nest-boxes, we found that female great tits (Parus major) usually maintained their body mass during the egg-laying period. A drop in temperature (3°C or more) during laying was followed by a drop in body mass, and female body mass also depended slightly but significantly on mean temperature on the other days. Day to day change in body mass depended negatively on the actual body mass. Moreover, the repeatability of the morning body mass was high (r _I =0.90, n=24). Thus, female body mass seems to be regulated around a female specific, but temperature-dependent, level during laying. The lack of strong correlation between female average body mass and clutch size makes it improbable that body mass at laying plays a leading, proximate role in the control of clutch-size of great tits.     

Leafing phenology and timing of egg laying in great tits Parus major and blue tits P. caeruleus

To evaluate the importance of tree leafing for the start of laying and clutch size of birds, we compared the breeding phenology of great tits Parus major and blue tits P. caeruleus between one coastal and two inland sites in the same geographical region. Because of the cooling influence of the sea, trees at the coastal site were known to initiate budburst about a week later than at the inland sites. During 5 years, breeding by the tits and the leaf phenology of birch Betula pendula , and oak Quercus robur were monitored. The leaf phenology of birch and oak explained a significant part of the between-year variation in the start of egg laying in blue and great tits, respectively. The tits started laying earlier at the sites with an early budburst, i.e. normally inland. However, leaf phenology was not an absolute cue to the start of laying, since blue tits laid earlier relative to leafing at the inland site than at the coastal site, and both tit species laid eggs earlier relative to leafing during late springs. In neither species was clutch size affected by leafing phenology. However, great tit females at the coastal site consistently produced fewer eggs than did those at the inland site. No such difference was found in the blue tits. Although leafing phenology may predict the start of laying in tits, other factors also influence its timing. These factors might include other cues, or differing life-history trade-offs depending on site or general climatic factors during the spring.     

Chick development in free-living great tits Parus major in relation to calcium availability and egg composition

Evidence from a number of avian studies suggests that limitation of exogenous calcium (Ca) may reduce egg quality and retard nestling growth. However, it is poorly understood whether reduction in chick growth in Ca-poor areas is due to insufficient intake of dietary Ca or caused by maternal Ca limitation mediated through subtle changes in composition of eggs. In this study, we provide new evidence that Ca availability during egg formation may indeed affect egg composition and influence chick development of the great tit Parus major at early developmental stages. Ca-supplemented birds breeding in base-poor pine forests produced eggs with elevated yolk Ca concentration compared with controls, while no such effect of supplementation was detected in case of eggshell thickness. Nestling tarsus length in the first half of the nestling period was positively influenced by both yolk dry mass and yolk Ca concentration. The effect of supplementary Ca did not persist throughout the nestling period; initial effects of egg components disappeared as nestlings aged. We conclude that apparently normal eggs may harbor poor nutrient and mineral conditions for chick growth. Such subtle changes in composition of eggs can depress chick growth, especially in harsh years.     

Predation by sparrowhawks favours early breeding and small broods in great tits

The selective pressures determining timing of reproduction and brood or litter size in animals remain to be clarified, despite much research. In several detailed studies of birds, selection for an early start of breeding has been demonstrated. Young born early in the season are often assumed to benefit from long experience, high dominance in flocks and/or early settlement in territories. For forest-breeding great tits (Parus major) in Sweden, predation by sparrowhawks (Accipiter nisus) was lower on fledglings hatched early than on those hatched late in the season. Hawk predation may therefore contribute to the higher success of early breeding great tits. Factors favouring early fledglings were (1) increasing food demands over the season in breeding hawks, and (2) hunting by hawks mainly in non-forest habitats when early tit fledglings left their nests. The proportion of fledglings taken was lower in broods of 2-6 than in broods of 7-12, indicating that predation favours small broods. Fledglings in large broods may be more vulnerable due to (1) a higher level of hunger and begging, and (2) higher provisioning rates by parents, which may reveal sites with fledglings to hawks. Territories of sparrowhawks were used as sample units in the statistical analysis, leading to strong inferences about predation at the landscape level.     

Clutch size and malarial parasites in female great tits

life-history models predict an evolutionary trade-off in theallocation of resources to current versus future reproduction.This corresponds, at the physiological level, to a trade-offin the allocation of resources to current reproduction or tothe immune system, which will enhance survival and thereforefuture reproduction. For clutch size, life-history models predicta positive correlation between current investment in eggs andthe subsequent parasite load. In a population of great tits,we analyzed the correlation between natural clutch size of femalesand the subsequent prevalence of Plasmodium spp., a potentiallyharmful blood parasite. Females that showed, 14 days after hatchingof the nestlings, an infection with Plasmodium had a significantlylarger clutch (9.3 eggs ± 0.5 SE, n = 18) than uninfectedfemales (8.0 eggs ± 0.2 SE, n = 80), as predicted bythe allocation trade-off. Clutch size was positively correlatedwith the prevalence of Plasmodium, but brood size 14 days afterhatching was not. This suggests that females incur higher costsduring laying the clutch than during rearing nestlings. Infectionstatus of some females changed between years, and these changeswere significantly correlated with a change in clutch size aspredicted by the trade-off. The link between reproductive effortand parasitism may represent a possible mechanism by which thecost of egg production is mediated into future survival andmay thereby be an important selective force in the shaping ofclutch size     

Energy expenditure and testosterone in free-living male yellow-pine chipmunks

The onset of mating in yellow-pine chipmunks (Tamias amoenus) follows emergence from a prolonged period of energy conservation during hibernation. Energy expenditures are greatly accelerated to meet the demands of the reproductive season. When emerging from hibernation, typical male chipmunks (breeders) have enlarged testes and a high level of plasma testosterone (T). However, certain males that do not participate in reproduction (nonbreeders) maintain small testes and low plasma T levels and emerge several weeks later than the breeders. The timing of the terminal arousal from hibernation and onset of mating are associated with increased plasma T levels. Experimental elevation of T levels in T. amoenus outside the mating season has been associated with a decrease in body mass, further suggesting an effect of T on energy balance. To test this hypothesis, we measured daily energy expenditure (DEE) in free-living, nonbreeding male chipmunks in the presence and absence of a T-implant. We also measured DEE in breeding males when endogenous T levels were high. DEE of the nonbreeders was not affected by our manipulation of plasma T, and the DEE of breeding males did not differ from that of nonbreeders. We conclude that energy expenditure on a daily basis in male yellow-pine chipmunks is not influenced by levels of T. However, on a seasonal basis, the earlier emergence from hibernation by breeding males, which appears to be influenced by T, represents an overall seasonal energy expenditure that exceeds that of nonbreeding males.     

Double-brooding and annual breeding success of great tits in urban and forest habitats

Urban areas differ from natural habitats in several environmental features that influence the characteristics of animals living there. For example, birds often start breeding seasonally earlier and fledge fewer offspring per brood in cities than in natural habitats. However, longer breeding seasons in cities may increase the frequency of double-brooding in urban compared with nonurban populations, thus potentially increasing urban birds’ annual reproductive output and resulting in lower habitat difference in reproductive success than estimated by studies focusing on first clutches only. In this study, we investigated 2 urban and 2 forests great tit Parus major populations from 2013 to 2019. We compared the probability of double-brooding and the total number of annually fledged chicks per female between urban and forest habitats, while controlling for the effects of potentially confounding variables. There was a trend for a higher probability of double-brooding in urban (44% of females) than in forest populations (36%), although this was not consistent between the 2 urban sites. Females produced significantly fewer fledglings annually in the cities than in the forest sites, and this difference was present both within single- and double-brooded females. Furthermore, double-brooded urban females produced a similar number of fledglings per season as single-brooded forest females. These results indicate that double-brooding increases the reproductive success of female great tits in both habitats, but urban females cannot effectively compensate in this way for their lower reproductive output per brood. However, other mechanisms like increased post-fledging survival can mitigate habitat differences in reproductive success.     

Effects of experimentally increased egg production on female body condition and laying dates in the great skua Stercorarius skua

We investigated the effects of increased egg production on body condition as well as on measures of reproductive performance in great skuas, Stercorarius skua, over two subsequent years. We experimentally increased egg production from the normal two to six eggs. Six eggs might also be produced under natural circumstances after repeated clutch loss. After the production of the last egg we measured: (i) body mass, (ii) pectoral muscle, and (iii) haematocrit, total red blood cell count and mean corpuscular volume, as indicators of body condition. We took the same measurements of control females who had produced the normal clutch of two eggs. The measurements were repeated one year after the manipulation, and survival, laying dates, clutch sizes and hatching success were recorded for up to three consecutive years. After producing six eggs, females were lighter, had smaller pectoral muscles and lower haematological values than control females. Hatching success of eggs was significantly reduced. Even one year after the experiment there were still differences in body condition. Annual survival was not affected by the manipulations, although there was an indication that survival costs depended on whether chicks were raised after the increased egg production. While pair bonds and egg sizes were not affected in the post‐experimental year, females started breeding significantly later than in the previous year. Two years after the experiment laying dates had advanced again and were not different from those of control females. This pattern of maintaining survival and egg sizes, but delaying breeding in the post‐experimental year was found for two independent groups of females which had both been subjected to increased egg production. These results present evidence that increased egg production can have long‐term effects on female body condition and aspects of reproduction. However, although present, the costs of extra eggs appear to have been relatively small in the great skua in comparison to the two other bird species for which inter‐annual effects have been reported.     

Climate change and breeding parameters of great and blue tits throughout the western Palaearctic

Increasing evidence suggests that climate change has consequences on avian breeding phenology. Here, variations in laying date and clutch size of great tit Parus major and blue tit Parus caeruleus within and between breeding populations through the western Palaearctic are examined in relation to climatic fluctuations, measured by the winter North Atlantic Oscillation (NAO) index. Within and across breeding sites, laying date was related to winter‐NAO index such that great and blue tit females lay earlier after warmer, moister winters (positive values of winter NAO‐index). The present study shows that for most populations there is an advancement of laying date, but the rate of change with respect to NAO significantly differed geographically across the western Palaearctic and did not differ between species. However, clutch size of great and blue tits was not affected by climatic fluctuations, presumably because the whole season is being shifted, but not in relation to food supplies. These combined analyses for the two species controlled for potentially confounding variables such as latitude, longitude, elevation and habitat of each study site.     

Energy expenditure increases during the active season in the small,free-living hibernator Muscardinus avellanarius

Little is known about strategies employed by small mammals to reduce energy expenditure during the summer. To understand whether ambient conditions impact euthermic energy demands in a small free-living hibernator, we measured metabolic rate of hazel dormice (Muscardinus avellanarius) in the field. Furthermore, we aimed to reveal which variables influence torpor use. Our results show that hazel dormice altered euthermic energy expenditure during summer but not as expected as a response to environmental conditions. Euthermic resting metabolic rate was lowest directly after emergence from hibernation and increased by about 95% until the end of August. A considerable part of this increase was presumably caused by the changing influence of gender and rain on energy demands during different months, variation in food quality and quantity, and reversible size changes of organs that had been atrophied during hibernation. Torpor use in hazel dormice occurred more frequently when it was colder, earlier during the day, and in lighter individuals. Torpor was used routinely in males and non-reproductive females. We show that torpor is used more frequently than previously suggested by studies that only used visual proof of torpor use by surveying nest boxes.     

The costs of egg production and incubation in great tits (Parus major).

The costs of egg production and incubation may have a crucial effect on avian reproductive decisions, such as clutch size and the timing of reproduction. We carried out a brood-size enlargement experiment on the great tit (Parus major), in which the birds had to lay and incubate extra eggs (full costs), only incubate extra eggs (free eggs) or did not pay any extra cost (free chicks) in obtaining a larger brood. We used female fitness (half the recruits produced plus female survival) as a fitness measure because it is the female which pays the costs of egg production and incubation, and because clutch size is under female control. Female fitness decreased with increasing costs (fitness of free chicks females is higher than that of free eggs females which is higher than that of full costs females). These fitness differences were due to differences in female survival rather than in the number of recruits produced. This is the first time that the costs of egg production and incubation have been estimated using such a complete fitness measure, including, as our measure does, the local survival to the following year of both the female and her offspring. Our results emphasize that reproductive decisions cannot be understood without taking egg production and incubation costs into account.     

Effects of Enterococcus faecalis on egg production,egg quality and caecal microbiota of hens during the late laying period

This study was conducted to determine the effects of diet supplementation of laying hens with Enterococcus faecalis (EF) on egg production, egg quality and caecal microbiota. A total of 360 Hy-Line Brown laying hens (72 weeks old) were divided into three groups with four replicates of 30 birds each. The laying hens were fed with the basal diet (Control), the basal diet + 3.75 · 10⁸ cfu EF/kg (Group I) or the basal diet + 7.5 · 10⁸ cfu EF/kg (Group II). The experiment lasted for 45 d. Eggs and caecal samples were collected at the end of the experiment. Results showed that dietary supplementation with EF did not affect the average daily egg weight, cracked egg rate, mortality and egg quality. However, EF supplementation caused a significantly increased laying rate and decreased feed/egg ratio (p < 0.05). The differences in caecal microbiota between Group II and the Control were significant. The relative abundance of Verrucomicrobia and Cyanobacteria at the phylum level, Rikenellaceae, Christensenellaceae and Veillonellaceae at the family level, and the Faecalibacterium, Christensenellaceae R-7 group and Eubacterium coprostanoligenes group at the genus level changed significantly in Group II compared with the Control (p < 0.05). In conclusion, the tested dietary supplementations with EF improved product performance and affected the caecal microbial community structure of laying hens during the late laying period.     

Partial incubation during egg laying reduces eggshell microbial loads in a temperate‐breeding passerine

Incubation prior to clutch completion may be adaptive if it maintains egg viability by inhibiting eggshell microbial growth, thus reducing the likelihood that the embryo becomes infected. To test this hypothesis, we examined the effect of partial incubation during egg laying on eggshell microbial loads in eastern bluebirds Sialia sialis breeding at a temperate‐zone site. We sampled eggshell microbes prior to and following four days of exposure to either partial incubation during the laying period or ambient environmental conditions without incubation (experimental eggs). Microbial colony counts declined significantly for eggs left in the nest during the laying period but did not vary significantly for eggs exposed to ambient conditions. Initial microbial loads were more similar to those previously reported from tropical than temperate environments, and microbes from potentially pathogenic groups were detected on 88% of first‐laid eggs on the day of laying. Egg viability was maintained when eggs were held indoors for four days without incubation but declined sharply thereafter. Our results suggest that partial incubation during egg laying may enhance egg viability in eastern bluebirds by reducing eggshell microbial loads; these effects appear stronger than those usually reported from the temperate zone.     

Weight change and the body reserves of female blue tits, Parus caeruleus, during the breeding season

A female blue tit lays a large clutch that is commonly heavier than her own body weight. Body reserves cannot provide for the entire clutch and little is known of the importance of these reserves during the breeding season. This study reports on weight changes in adult female blue tits during the breeding season, observed by the use of electronic balances. In addition, data from dissecting 67 adult female blue tits, at different stages of the breeding cycle, were used to investigate the changes in body weight. There was a rapid increase in body weight during a period of 4–5 days before the first egg was laid, which was explained mostly by the development of the oviduct and the production of an egg on the final day. Body weight remained constant during the laying period, before decreasing by approximately one gram after clutch completion, owing to the absence of an egg and the absorption of the oviduct. Body reserves increased during the day after clutch completion, were maintained throughout the incubation period and were mobilized during the first five days of the nestling period. Breeding female blue tits appear to deposit small body reserves to act as a short-term buffer against adverse conditions or to support the brooding phase.     

Hematological parameters in brood-rearing great tits in relation to habitat, multiple breeding and sex

Very little is known about the causes and correlates for variation of individual condition in the wild. However, such knowledge is essential for understanding the mechanisms that mediate environmental effects to populations. We studied the variation of several hematological condition indices (hematocrit, albumin, globulin and triglyceride concentrations, albumin/globulin ratio, lymphocyte and heterophile concentrations and heterophile/lymphocyte ratio) and body mass in brood-rearing great tits (Parus major) in relation to habitat, multiple breeding and gender. Although great tits prefer deciduous forest to coniferous forests, individuals breeding in coniferous forests tended to be in a superior health state than those breeding in deciduous habitat. We suggest that this difference in adult condition can be caused by differences in breeding densities between habitats. Although there was some variation in condition indices between breeding attempts, none of these parameters measured at the end of the first breeding attempt predicted the probability of double breeding. We also found that females were in poorer condition and probably more stressed than males, both during the first and the second breeding attempt. These findings demonstrate that hematological parameters can be used to assess spatial and temporal variation of individual condition in the wild.     

The relation between dominance and exploratory behavior is context-dependent in wild great tits

Individual differences in personality affect behavior in novelor challenging situations. Personality traits may be subjectto selection because they affect the ability to dominate others.We investigated whether dominance rank at feeding tables inwinter correlated with a heritable personality trait (as measuredby exploratory behavior in a novel environment) in a naturalpopulation of great tits, Parus major. We provided clumped resourcesat feeding tables and calculated linear dominance hierarchieson the basis of observations between dyads of color-ringed individuals,and we used an experimental procedure to measure individualexploratory behavior of these birds. We show that fast-exploringterritorial males had higher dominance ranks than did slow-exploringterritorial males in two out of three samples, and that dominancerelated negatively to the distance between the site of observationand the territory. In contrast, fast-exploring nonterritorialjuveniles had lower dominance ranks than did slow-exploringnonterritorial juveniles, implying that the relation betweendominance and personality is context-dependent in the wild.We discuss how these patterns in dominance can explain earlierreported effects of avian personality on natal dispersal andfitness.     

Immune challenge of female great tits at nests affects provisioning and body conditions of their offspring

The trade-off between animal’s parental reproductive effort and survival is still poorly understood. Parental allocation between the workload during breeding attempts and the parents’ own body conditions can be assessed through the offspring quality. Here, I questioned whether the immune responsiveness of female great tits may be considered as a mediator of this trade-off. Specifically, I tested whether (1) the parental reproductive effort decreases, (2) the food composition provided to chicks changes, and (3) whether the nestling immunocompetence and body mass decrease after experimental immunisation. Two populations of great tit Parus major occupying nest boxes were studied in Niepo?omice Forest and Krzyszkowice Forest (Southern Poland) in 2011 and 2012, respectively. Three days after hatching, half of the females were challenged with sheep red blood cells (SRBC), while other females were injected with phosphate-buffered saline PBS (control). Six days later, food provided by the parents was collected from nestlings. After another 2 days, the offspring’s body mass was measured and wing web swelling in response to an additional phytohaemagglutinin (PHA) injection. In both years, immunocompetence and in 2012 also body mass in the offspring of SRBC-immunised mothers were lower than in control nestlings, indicating a cost of mounting the immune response in the female. Six days after the start of the female treatment, the number of caterpillars and the volume of food items provided by parents to chicks were higher, whereas the number of spiders was lower in nests with SRBC treatment than in control ones. This might be explained by compensational parental feeding after recovery from the inflammation of a female. Thus, the trade-off between parental effort and survival of parents is mediated by the costs incurred for their immunity and can be assessed by the amount and quality of food provided to the nestlings and the offspring condition.