Sex ratio of Parus major and P. caeruleus broods depends on parental condition and habitat quality

Brood sex ratio was studied in 88 families of Parus caeruleus (blue tit) and 95 families of P. major (great tit) in deciduous and mixed forest habitats differing in food availability. As a food specialist, the blue tit is expected to be more sensitive to the nutritional differences between the habitats than a food generalist such as the great tit. A shift of brood sex ratio towards males was detected for great tits in the high quality habitat, but there was no significant impact of parental condition or the number of nestlings. In contrast, brood sex ratio of blue tits was not affected by habitat quality. In blue tits, male condition correlated positively with a male-biased sex ratio. Habitat quality, however, affected the body mass differences of male and female blue tit siblings, and nestlings developed differently. The low quality habitat had a negative effect on the sexual dimorphism of siblings in male-biased broods, and the condition of offspring was bad. Nevertheless, sexual dimorphism cannot explain the differences between great and blue tits with respect to the correlation of sex ratio and individual condition.     

Selection for fast and slow exploration affects baseline and stress-induced corticosterone excretion in Great tit nestlings, Parus major

In nestlings, glucocorticoid (GC) secretion has short-term and long-term f itness consequences. For example, short-time elevations trigger begging activity, whereas chronically elevated GC levels impair body condition, growth and cognitive abilities. Despite a growing body of literature on personality traits, the effects of selection for fast and slow exploration on GC secretion have received little attention. We compared baseline and stress-induced hypothalamic–pituitary–adrenal (HPA) axis activity of hand-reared great tit nestlings of lines selected for fast and slow exploration. Nestling droppings were collected under three conditions: control, test (following handling stress, day 14 after hatching) and the following day. The concentrations of excreted immunoreactive corticosterone metabolites (CM) were determined via an enzyme immunoassay. We also observed nestlings' begging behaviour. CM differed significantly between the lines. Nestlings of the fast line excreted lower CM than slow-line birds. In response to handling stress, nestlings excreted significantly higher concentrations of CM than during the control and on the day after handling. Sex and begging activity were not related to CM levels. Under the control condition, but not after handling, males begged significantly more often than females. In both lines, adults excreted significantly less CM compared to nestlings. Both nestlings and adults of the slow line produced higher baseline CM values than fast-line birds. Fast-line nestlings excreted lower baseline CM than nestlings of a wild population not selected for fast or slow exploration. Slow-line nestlings did not. Our results show that selection on the basis of exploratory behaviour affected HPA axis reactivity.     

Selection for fast and slow exploration affects baseline and stress-induced corticosterone excretion in Great tit nestlings,Parus major

In nestlings, glucocorticoid (GC) secretion has short-term and long-term f itness consequences. For example, short-time elevations trigger begging activity, whereas chronically elevated GC levels impair body condition, growth and cognitive abilities. Despite a growing body of literature on personality traits, the effects of selection for fast and slow exploration on GC secretion have received little attention. We compared baseline and stress-induced hypothalamic–pituitary–adrenal (HPA) axis activity of hand-reared great tit nestlings of lines selected for fast and slow exploration. Nestling droppings were collected under three conditions: control, test (following handling stress, day 14 after hatching) and the following day. The concentrations of excreted immunoreactive corticosterone metabolites (CM) were determined via an enzyme immunoassay. We also observed nestlings' begging behaviour. CM differed significantly between the lines. Nestlings of the fast line excreted lower CM than slow-line birds. In response to handling stress, nestlings excreted significantly higher concentrations of CM than during the control and on the day after handling. Sex and begging activity were not related to CM levels. Under the control condition, but not after handling, males begged significantly more often than females. In both lines, adults excreted significantly less CM compared to nestlings. Both nestlings and adults of the slow line produced higher baseline CM values than fast-line birds. Fast-line nestlings excreted lower baseline CM than nestlings of a wild population not selected for fast or slow exploration. Slow-line nestlings did not. Our results show that selection on the basis of exploratory behaviour affected HPA axis reactivity.     

Habitat fragmentation influences nestling growth in Mediterranean blue and great tits

In patchy forest areas, the size of the forest patch where birds breed has a strong influence on their breeding success. However, the proximate effects contributing to lowering the breeding success in small forest patches remain unclear; and a shortage of crucial resources in those forest patches has been suggested to account in some degree for this failure. With the aim to further investigate this issue, we have monitored the breeding cycle of blue and great tits in three ‘large’ forest patches (ranging between 26.5 and 29.6 ha) and twelve ‘small’ forest patches (ranging between 1.1 and 2.1 ha) in a Mediterranean area in central Spain, during three years (2011–2013). We also recorded the nestling diet inside the nest-boxes with the aid of handy-cams. Only males significantly differed between forest patch size categories; being on average younger and with better body condition in small patches for great and blue tits respectively. Reproductive traits did not vary between forest patch size categories, but the body condition of blue tit nestlings and the size of great tit nestlings did, being significantly better and larger respectively in large forest patches. The recruitment rate of blue tit nestlings was also higher in large patches. Regarding nestling diet, blue tits did not differ but great tits did, delivering a larger amount of caterpillars in large forest patches. Most variation in the reproductive traits occurred between years, probably due to annual differences in environmental conditions. This study suggests that food supply could be limiting the breeding success of birds above all in small patches, but also in large patches under particular environmental conditions.     

Is the blue tit falling into an ecological trap in Argentine ant invaded forests?

Because insectivorous birds must evaluate resources for reproduction before settling into a breeding habitat, they can fall into an ecological trap if informative cues about habitat suitability become dissociated from their actual yield. Given their potential to affect ecological networks, invasive ant species are potential candidates for triggering such ecological traps. We combined observational and experimental approaches to examine whether the variation in food supply for nestlings resulting from the invasion of the Argentine ant, Linepithema humile, had any influence on the breeding ecology of the blue tit, Cyanistes caeruleus, an insectivorous foliage-gleaner. We investigated the effects of the ant invasion on breeding performance (nesting success, clutch size, brood size and breeding success) and offspring quality (body size and condition, developmental stability and plumage colour) in replicated Mediterranean forest areas over a period of 3 years. There was no evidence that the reduction in caterpillar availability resulting from the invasion had a concurrent negative effect on the blue tit’s ability to successfully rear nestlings in optimal conditions, at least as measured here. Although the raw figures suggest an increased level of nutritional stress in blue tits breeding in invaded forests, the data analyses showed no significant alterations in terms of productivity or offspring fitness. The reproductive performance of the blue tit has been shown to be remarkably resilient to the Argentine ant-mediated food shortage, either because the prey reduction following the invasion did not reach a critical threshold or because of compensatory activity by the progenitors. We cannot conclusively reject an ecological trap triggered by the ant invasion on blue tits, since neither fledgling recruitment nor the prospective survival of parents were assessed. Even though we could not confirm short-term consequences of the Argentine ant invasion on blue tit reproductive fitness, the long-term bottom-up effects of the invasion remain unknown and should not be ruled out.     

Teleconnections and local weather orchestrate the reproduction of tit species in the Carpathian Basin

Variation in climatic conditions is an important driving force of ecological processes. Populations are under selection to respond to climatic changes with respect to phenology of the annual cycle (e.g. breeding, migration) and life‐history. As teleconnections can reflect climate on a global scale, the responses of terrestrial animals are often investigated in relation to the El Niño‐Southern Oscillation and North Atlantic Oscillation. However, investigation of other teleconnections and local climate is often neglected. In this study, we examined over a 33‐year period the relationships between four teleconnections (El Niño‐Southern Oscillation, North Atlantic Oscillation, Arctic Oscillation, East Atlantic Pattern), local weather parameters (temperature and precipitation) and reproduction in great tits Parus major and blue tits Cyanistes caeruleus in the Carpathian Basin, Hungary. Furthermore, we explored how annual variations in the timing of food availability were correlated with breeding performance. In both species, annual laying date was negatively associated with the Arctic Oscillation. The date of peak abundance of caterpillars was negatively associated with local temperatures in December–January, while laying date was negatively related to January–March temperature. We found that date of peak abundance of caterpillars and laying date of great tits advanced, while in blue tits clutch size decreased over the decades but laying date did not advance. The results suggest that weather conditions during the months that preceded the breeding season, as well as temporally more distant winter conditions, were connected to breeding date. Our results highlight that phenological synchronization to food availability was different between the two tit species, namely it was disrupted in blue tits only. Additionally, the results suggest that in order to find the climatic drivers of the phenological changes of organisms, we should analyze a broader range of global meteorological parameters.     

Plumage colour in nestling blue tits: sexual dichromatism,condition dependence and genetic effects

Sexual-selection theory assumes that there are costs associated with ornamental plumage coloration. While pigment-based ornaments have repeatedly been shown to be condition dependent, this has been more difficult to demonstrate for structural colours. We present evidence for condition dependence of both types of plumage colour in nestling blue tits (Parus caeruleus). Using reflectance spectrometry, we show that blue tit nestlings are sexually dichromatic, with males having more chromatic (more 'saturated') and ultraviolet (UV)-shifted tail coloration and more chromatic yellow breast coloration. The sexual dimorphism in nestling tail coloration is qualitatively similar to that of chick-feeding adults from the same population. By contrast, the breast plumage of adult birds is not sexually dichromatic in terms of chroma. In nestlings, the chroma of both tail and breast feathers is positively associated with condition (body mass on day 14). The UV/blue hue of the tail feathers is influenced by paternally inherited genes, as indicated by a maternal half-sibling comparison. We conclude that the expression of both carotenoid-based and structural coloration seems to be condition dependent in blue tit nestlings, and that there are additional genetic effects on the hue of the UV/blue tail feathers. The signalling or other functions of sexual dichromatism in nestlings remain obscure. Our study shows that nestling blue tits are suitable model organisms for the study of ontogenetic costs and heritability of both carotenoid-based and structural colour in birds.     

Does weather affect biting fly abundance in avian nests?

Environmental factors may strongly affect avian‐biting fly interactions in different ways because insects are heterothermic organisms that depend greatly on environmental variables to activate their metabolism and behaviour. We studied the effects of weather on both blackfly (Simuliidae) and biting midge Culicoides (Ceratopogonidae) abundance in nests of three passerine species: blue tits Cyanistes caeruleus, great tits Parus major and pied flycatchers Ficedula hypoleuca, breeding in the same area. We controlled for different host‐related factors (hatching date, brood size and host species). Blackfly abundance was negatively related to minimum temperature. In addition, blackfly and biting midge abundances were negatively affected by wind speed measured at 07.00 h, but blackfly abundance was positively associated to wind speed at 18.00 h. We found higher blackfly and biting midge abundances in nests with larger broods breeding later in the season, and significantly higher biting midge abundance in pied flycatcher nests as compared to tit nests. These results represent, to our knowledge, the first report of both environmental and host‐related effects on haematophagous fly abundance in the nests of wild hole‐nesting birds.     

Long‐term population dynamics of a migrant bird suggests interaction of climate change and competition with resident species

The separation of abiotic and biotic factors affecting populations and communities is an important step in understanding how climate change can influence ecological processes, but quantifying their relative contribution to community changes is a challenge. We assessed the effect of temperature and species interactions on the population dynamics of a forest bird community with a hierarchical dynamic population model in a Bayesian framework. We used a long‐term time‐series (1956–2012) of four secondary cavity‐nesting birds with similar food and nesting requirements but different migration habits, to analyse the effects of the four species population size and the local weather fluctuations on each species’ population dynamics. We found clear evidence of a negative effect of two resident species (blue tit and great tit) on a long‐distance migrant (pied flycatcher). Among the residents we only found a competition effect of the great tit on the marsh tit. The birds showed opposite responses to weather: the pied flycatcher favoured colder springs whereas the blue tit and great tit favoured warmer springs. Although alternative mechanisms cannot be ruled out, our results suggest that the resident species (blue tit and great tit) could adjust to increasing spring temperature while the migrant species (pied flycatcher) could not, leading progressively to the exclusion of the pied flycatcher from the area. These results point out the potential role of competitive interactions by providing insightful clues, call for refined research, and support recent efforts to include population dynamics in species distribution models.     

Early learning affects social dominance: interspecifically cross-fostered tits become subdominant

Social dominance influences the outcome of competitive interactionsover limited resources, and may hence be important for individualfitness. Theory thus predicts that its heritability will below and that non-genetic determinants of dominance should prevail.In this field experiment we reciprocally cross-fostered greattits (Parus major) to blue tits (Parus caeruleus) to investigatethe impact of early social experience on dominance status incompetition over food during winter. Controlling for potentialeffects of age, size, sex and site-related dominance, we showthat cross-fostered birds of both species were subdominant toconspecific immigrants, while controls originating from unmanipulatedbroods were dominant to conspecific immigrants. Furthermore,blue tits reared by blue tit parents but with at least one greattit broodmate had lower dominance status relative to conspecificimmigrants than did controls. Although great tits generallydominated blue tits, cross-fostered birds of both species initiatedmarginally more fights against the other species than did theirrespective controls, suggesting faulty species recognition.Since both social parents and broodmates strongly influencethe dominance behavior of offspring later in life, we concludethat social conditions experienced at an early age are crucialfor the determination of subsequent social dominance.     

Sexual imprinting and the origin of obligate brood parasitism in birds

We discuss two pathways along which obligate brood parasitism (OBP) may evolve and examine some of the critical steps that must be passed by letting great tits Parus major be reared by blue tits Parus caeruleus in a field experiment. The cross-fostered chicks survived well in blue tit nests, but their local recruitment and reproductive success was much lower than that of controls. The effect was strongest when great tits grew up with siblings of the host species rather than with conspecific siblings in blue tit nests. The low success seemed to be caused by misimprinting because the cross-fostered birds behaved like blue tits in several aspects (species association, alarm calls, and aggressive response by resident females to caged intruders). Some birds of both sexes were apparently so strongly imprinted that they did not attract or accept a social mate of their own species. We conclude that imprinting may be necessary for OBP to evolve in birds because the parasite must be attracted to the nests of the host species to add eggs and thereby continue the parasitic life cycle. However, strong imprinting may also prevent OBP from occurring if parasitic offspring seek a mate from the host species.     

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.     

Environmental and not maternal effects determine variation in offspring phenotypes in a passerine bird

Maternal and environmental effects can profoundly influence offspring phenotypes, independent of genetic effects. Within avian broods, both the asymmetric post‐hatching environment created by hatching asynchrony and the differential maternal investment through the laying sequence have important consequences for individual nestlings in terms of the allocation of resources to body structures with different contributions to fitness. The purpose of this study was to evaluate the relative importance of post‐hatching environmental and maternal effects in generating variation in offspring phenotypes. First, an observational study showed that within blue tit, Cyanistes caeruleus, broods, late‐hatched nestlings allocated resources to tarsus development, maintained mass gain and head‐bill growth and directed resources away from the development of fourth primary feathers. Second, a hatching order manipulation experiment resulted in nestlings from first‐laid eggs hatching last, thereby allowing comparison with both late and early‐hatched nestlings. Experimental nestlings had growth patterns which were closer to late‐hatched nestlings, suggesting that within‐brood growth patterns are determined by post‐hatching environmental effects. Therefore, we conclude that post‐hatching environmental effects play an important role in generating variation in offspring phenotypes.     

Effects of nestling condition on UV plumage traits in blue tits: an experimental approach

Intraspecific sexual and social communications are among themost important factors shaping costly color traits in birds.Condition capture models assume that only animals in superiorcondition can develop and maintain a colorful plumage. Althoughthere is good evidence that carotenoid-based components of plumagecolors show condition dependence, the situation is more controversialwith the underlying UV-reflecting structural component. We conducteda brood size manipulation in blue tits (Parus caeruleus) toinvestigate condition-dependent effects on plumage colorationin male and female offspring. Carotenoid chroma and UV reflectanceof the yellow breast plumage showed condition-dependent expressionin male and female fledglings. However, only males that wereraised in reduced broods had higher UV reflectance in the UV/bluetail feathers, whereas female tail coloration did not differbetween treatments. Our data suggest that there is a sex-specificeffect on the blue but not the yellow plumage and that thisis related to differences in the signaling function of bothplumage traits. Although sexual selection may already act onmale nestlings to develop colorful tail feathers for the nextbreeding season, the UV/yellow breast feathers are molted duringthe postjuvenile molt, and their signaling value is likely tobe important for both sexes during the extended postfledglingphase.     

Adaptive inter-population differences in blue tit life-history traits on Corsica

Few studies of natural populations have investigated how phenotypic variation across populations relates to key factors in the environment and landscape structure. In the blue tits of southern France, inter-population differences in reproductive life-history traits (e.g. laying date and clutch size) are small, whatever the timing of maximum caterpillar availability, a key factor for offspring survival in tits. These small differences are attributed to gene flow between local populations occupying different habitat types. In contrast, in blue tits on the island of Corsica, we noted large differences in reproductive life-history traits between two populations, where each population is synchronized with the peak-date of caterpillar abundance. These occur over a short geographical distance (25km). Considering our study within a framework of long-term population studies in tits, our results support the hypothesis that different blue tit populations on Corsica show adaptive differences in life-history traits, and suggest that landscape structure at a small spatial scale can have profound effects on adaptive between-population differentiation in life-history traits that are closely linked with fitness.     

Maternal Condition but Not Corticosterone Is Linked to Offspring Sex Ratio in a Passerine Bird

There is evidence of offspring sex ratio adjustment in a range of species, but the potential mechanisms remain largely unknown. Elevated maternal corticosterone (CORT) is associated with factors that can favour brood sex ratio adjustment, such as reduced maternal condition, food availability and partner attractiveness. Therefore, the steroid hormone has been suggested to play a key role in sex ratio manipulation. However, despite correlative and causal evidence CORT is linked to sex ratio manipulation in some avian species, the timing of adjustment varies between studies. Consequently, whether CORT is consistently involved in sex-ratio adjustment, and how the hormone acts as a mechanism for this adjustment remains unclear. Here we measured maternal baseline CORT and body condition in free-living blue tits (Cyanistes caeruleus) over three years and related these factors to brood sex ratio and nestling quality. In addition, a non-invasive technique was employed to experimentally elevate maternal CORT during egg laying, and its effects upon sex ratio and nestling quality were measured. We found that maternal CORT was not correlated with brood sex ratio, but mothers with elevated CORT fledged lighter offspring. Also, experimental elevation of maternal CORT did not influence brood sex ratio or nestling quality. In one year, mothers in superior body condition produced male biased broods, and maternal condition was positively correlated with both nestling mass and growth rate in all years. Unlike previous studies maternal condition was not correlated with maternal CORT. This study provides evidence that maternal condition is linked to brood sex ratio manipulation in blue tits. However, maternal baseline CORT may not be the mechanistic link between the maternal condition and sex ratio adjustment. Overall, this study serves to highlight the complexity of sex ratio adjustment in birds and the difficulties associated with identifying sex biasing mechanisms.     

Asymmetric competition between two tit species: a reciprocal removal experiment

1. In a 5-year field experiment, competition for food was tested between great tit ( Parus major L.) and blue tit ( P. caeruleus L.), two common hole-nesters in Central Europe. Experimental ('allopatric') populations of both species were created during the breeding seasons by preventing the nesting or egg laying of one of the competing species.
2. An asymmetric relationship was found between the two tits; blue tits were more successful in the competitive interaction. Detectable effects were found only in nestling condition. Great tits raised lighter nestlings when breeding sympatrically with blue tits.
3. A possible mechanism is suggested that could be responsible for the different competitive abilities of the two species; blue tits are more effective in utilizing the most abundant size categories of caterpillar food supply than great tits.     

Immunocompetence and its costs during development: an experimental study in blue tit nestlings

The allocation into T-cell-mediated immunocompetence was experimentally increased in 68 out of 139 nestlings by supplementary feeding methionine to half of the nestlings in 15 blue tit nests. Methionine-supplemented nestlings had an increased T-cell-mediated immunocompetence, but a reduced growth compared with control siblings. Nestlings that had low initial weights and nestlings that were supplemented with methionine had an increased mortality risk. The investment that nestling blue tits make in immunocompetence appears to be tightly controlled by survival costs paid through a trade-off between immunocompetence and growth.     

Climate change can alter competitive relationships between resident and migratory birds

Climate change could affect resource competition between resident and migratory bird species by changing the interval between their onsets of breeding or by altering their population densities. We studied interspecific nest-hole competition between resident great tits and migrant pied flycatchers in South-Western Finland over the past five decades (1953-2005). We found that appearance of fatal take-over trials, the cases where a pied flycatcher tried to take over a great tit nest but was killed by the tit, increased with a reduced interspecific laying date interval and with increasing densities of both tits and flycatchers. The probability of pied flycatchers taking over great tit nests increased with the density of pied flycatchers. Laying dates of the great tit and pied flycatcher are affected by the temperatures of different time periods, and divergent changes in these temperatures could consequently modify their competitive interactions. Densities are a result of reproductive success and survival, which can be affected by separate climatic factors in the resident great tit and trans-Saharan migrant pied flycatcher. On these bases we conclude that climate change has a great potential to alter the competitive balance between these two species.     

REPRODUCTIVE COST,AGE-SPECIFIC SURVIVAL AND A COMPARISON OF THE REPRODUCTIVE STRATEGY IN TWO EUROPEAN TITS (GENUS PARUS)

Theoretical analyses of optimal reproductive rates usually assume a trade-off between offspring production and parental survival. This study verified a survival cost for willow tit males; nonbreeding males survived better than males attending a brood. Theory also predicts a smaller clutch size in birds that are less successful in transforming reproductive investments into mature offspring. As predicted, we found that crested tits, suffering a higher nest predation rate, laid smaller clutches than willow tits. The generally lower survival rate of willow tit adults may largely be attributed to their higher reproductive commitment (larger willow tit clutch size), because no significant interspecific survival difference remained between nonbreeding males. Finally, in willow tits we found a positive correlation between average clutch size and juvenile survival rate (density-dependent) the ensuing year, suggesting that willow tits may adjust clutch size in response to changing survival prospects for their young by using the breeding density as a cue.