Environmental factors shape cloacal bacterial assemblages in great tit Parus major and blue tit P. caeruleus nestlings

Despite their potential ecological and evolutionary importance, factors shaping the composition of bacterial communities in wild vertebrate populations remain poorly understood. The goal of this study was to examine the relative contributions of environmental factors and genetic factors (e.g. species and common origin) to the variation of cloacal bacterial assemblages in wild bird nestlings. We conducted a partial cross-fostering experiment with two passerine species, the great tit Parus major and the blue tit P. caeruleus, sharing similar habitats and breeding biology. Nestlings of the two species were exchanged four days after hatching and cloacal bacteria were sampled nine days later. The structure of cloacal bacterial communities was determined by Ribosomal Intergenic Spacer Analysis. Our results showed that each nestling displayed a unique bacterial community. Furthermore, nestlings raised in the same nest shared significantly similar bacterial communities. The similarity of bacterial community was higher among heterospecific siblings raised within the same nest than between biological siblings raised in separate nests. Effects of common origin between species could not be detected and, if present, were dominated by nest-based short-term environmental effects. Our results show that growth conditions within nests and individually based endogenous factors have significant effects on cloacal bacteria assemblages and could affect post-fledging condition.     

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.     

Speckled eggs: water-loss and incubation behaviour in the great tit Parus major

Many small passerine birds worldwide lay white eggs speckled with red, brown and black protoporphyrin pigment spots (maculation). Unlike some patterns of avian eggshell pigmentation which clearly serve a crypsis or signalling function, the ubiquity of maculation among passerines suggests that its origins lie in another function, not specific to any particular ecological or behavioural group. Elsewhere, we have presented evidence that protoporphyrin pigments serve a structural function related to eggshell thickness and calcium availability: eggshell maculation in the great tit Parus major increases with decreasing soil calcium levels, pigments demarcate thinner areas of shell, and both the pigment intensity and distribution are related to shell thickness. Here we show that maculation also affects the rate of water loss from the egg during incubation (≈ Mass Loss per Day or MLD, which is critical to egg viability), but not that of unincubated eggs. We also demonstrate, both by observation and experiment, that the effect of female incubation behaviour on MLD compensates in some way for variation in egg characteristics, and that differences between females in the degree of such compensation are related to differences in clutch maculation. Our results suggest that, while a principal function of maculation in this species may be to strengthen the eggshell, it may also reduce eggshell permeability when large amounts of pigment are used, and that this necessitates a behavioural adjustment from the female during incubation. We discuss these findings and make further testable predictions from our model.     

Intensity of nest defence is related to offspring sex ratio in the great tit Parus major

Nest-defence behaviour of passerines is a form of parental investment. Parents are selected, therefore, to vary the intensity of their nest defence with respect to the value of their offspring. Great tit, Parus major, males were tested for their defence response to both a nest predator and playback of a great tit chick distress call. The results from the two trials were similar; males gave more alarm calls and made more perch changes if they had larger broods and if they had a greater proportion of sons in their brood. This is the first evidence for a relationship between nest-defence intensity and offspring sex ratio. Paternal quality, size, age and condition, lay date and chick condition did not significantly influence any of the measured nest-defence parameters.     

Egg quality and parental ornamentation in the blue tit Parus caeruleus

Components of avian egg quality often show remarkable variation between females and yet the causes of variation in this fitness-related trait remain poorly understood. We investigated the egg size and yolk carotenoid investment of blue tit Parus caeruleus females in relation to multiple parental traits. We show that females produce eggs with more colourful yolks when mated to ultraviolet (UV) attractive males. Since yolk carotenoids may enhance offspring fitness, but may be available to females in limiting amounts, the correlation we found suggests adaptive egg quality adjustment in response to mate attractiveness, but an experimental approach is required to rule out alternative explanations. Yolk colour was also correlated with laying date and yolk mass. Though the effect of laying date could not be explained by ambient temperature during egg formation, it suggests a proximate constraint of general carotenoid availability on yolk composition. Egg size was not affected by male attractiveness; however, females with a brighter crown produced larger eggs, suggesting that crown coloration might indicate individual quality in females too.     

No effect of parental quality or extrapair paternity on brood sex ratio in the blue tit (Parus caeruleus)

Sex allocation theory predicts that parents should manipulatebrood sex ratio in order to maximise the combined reproductivevalue of their progeny. Females mating with high quality malesshould, therefore, be expected to produce brood sex ratiosbiased towards sons, as male offspring would receive a relativelygreater advantage from inheritance of their father's characteristicsthan would their female siblings. Furthermore, it has been suggested that sex allocation in chicks fathered through extrapair fertilizations should also be biased towards sons. Contraryto these predictions, we found no evidence that the distributionof sex ratios in a sample of 1483 chicks from 154 broods ofblue tits (Parus caeruleus) deviated significantly from thatof a binomial distribution around an even sex ratio. In addition,we found no significant effect on brood sex ratio of the individualquality of either parent as indicated by their biometrics, feather mite loads, time of breeding, or parental survival. This suggeststhat females in our population were either unable to manipulateoffspring sex allocation or did not do so because selectionpressures were not strong enough to produce a significant shiftaway from random sex allocation. The paternity of 986 chicks from 103 broods was determined using DNA microsatellite typing.Extrapair males sired 115 chicks (11.7%) from 41 broods (39.8%).There was no significant effect of paternity (within-pair versusextrapair) on the sex of individual offspring. We suggest that,in addition to the weakness of selection pressures, the possiblemechanisms responsible for the allocation of sex may not besufficiently accurate to control offspring sex at the levelof the individual egg.     

Offspring sex ratio is related to male body size in the great tit (Parus major)

Sex allocation theory predicts that the allocation of resourcesto male and female function should depend on potential fitnessgain realized through investment in either sex. In the greattit (Parus major), a monogamous passerine bird, male resourceholdingpotential (RHP) and fertilization success both depend on malebody size (e.g., tarsus length) and plumage traits (e.g., breaststripe size). It is predicted that the proportion of sons ina brood should increase both with male body size and plumage traits,assuming that these traits show a father—offspring correlation. Thiswas confirmed in our study: the proportion of sons in the brood increasedsignificantly with male tarsus length and also, though not significantly,with the size of the breast stripe. A sex ratio bias in relationto male tarsus length was already present in the eggs because(1) the bias was similar among broods with and without mortalitybefore the nestlings' sex was determined, and (2) the bias remainedsignificant when the proportion of sons in the clutch was conservativelyestimated, assuming that differential mortality before sex determinationcaused the bias. The bias was still present among recruits.The assumption of a father—offspring correlation was confirmedfor tarsus length. Given that both RHP and fertilization successof male great tits depend on body size, and size of father andoffspring is correlated, the sex ratio bias may be adaptive.     

High individual repeatability and population differentiation in stable isotope ratios in winter-grown collared flycatcher Ficedula albicollis feathers

For migrants, we often lack complete information of their spatial distribution year round. Here, we used stable carbon, nitrogen and hydrogen isotope ratios extracted from feathers grown at the wintering sites of the long-distance migratory collared flycatcher Ficedula albicollis , to study how individuals from different breeding populations are distributed at the wintering sites. A sub-sample of birds was also sampled in two consecutive years to test for the repeatability of isotope ratios. Birds from the same breeding populations had more similar isotope ratios compared to birds from other nearby populations (10–100 km apart). Furthermore, isotope repeatability within individuals was high, implying that the observed pattern of isotope variation is consistent between years. We put forward two hypotheses for these patterns; 1) strong wintering site philopatry and migratory connectivity, suggesting that migratory connectivity may potentially be found on a much smaller spatial scale than previously considered, and 2) consistent interpopulation differentiation of feeding ecology at their wintering site.     

Eggshell coloration in relation to male ornamentation, female condition and egg quality in the collared flycatcher Ficedula albicollis

The blue-green eggshell pigment biliverdin is an efficient antioxidant, the deposition of which may be costly and act as a handicap to the female's antioxidant capacity. Thus, it is hypothesized that there exists a positive correlation between the degree of eggshell coloration and the condition and health state of the mother. In this study, we measured the blue-green chroma of collared flycatcher Ficedula albicollis eggs, and investigated the associations between egg colour and parental characteristics, laying date, clutch size and laying order. We also analysed the possible correlations between egg size, yolk carotenoid and antibody levels and eggshell pigmentation. Our results revealed no significant relationships between egg colour and female age, morphological and serological variables. Blue-green colour intensity of eggs increased throughout the breeding season, which could be the result of either a proximate constraint or an adaptive strategy. We found a positive association between egg colour saturation and yolk lutein concentration. This relationship could be the result of the seasonal improvement of caterpillar supply, which could influence the deposition of both components into the egg. Furthermore, we found that females mated to lower-quality males produced eggs with higher biliverdin pigmentation. It is possible that deposition of biliverdin is correlated to some unmeasured aspect of egg quality that could enhance the viability of offspring of lower-quality fathers. We found that intensity of blue-green coloration increased with laying order between the first and fifth eggs.     

Sex-dependent risk taking in the collared flycatcher, Ficedula albicollis, when exposed to a predator at the nestling stage

An increased mortality rate is a cost of parental care, and can be high during the provisioning phase of altricial nestlings. When a parent stops feeding the nestlings temporarily after seeing a predator, it can reduce its own predation risk, but the suspension of parental care may also reduce its offspring's chances of surviving. We modelled this situation by exposing a stuffed sparrowhawk near collared flycatcher nests and removing it when both parents had seen it. We measured the time (return time) between the removal and when each parent entered the nestbox. The parents' risk taking and the return time are assumed to be inversely related. We studied which brood variables the parents take into account when deciding how much risk they are willing to take during the provisioning period. Males took more risk for older and better-quality nestlings and earlier broods. The females' behaviour was opposite to that of the males: they took significantly less risk for older and better-quality offspring and visited the nestbox later for earlier broods. The males' behaviour supported the reproductive value hypothesis, that risk taking is related to brood value and survival chances, whereas the females' behaviour supported the harm to offspring hypothesis, that risk taking is related to the broods' vulnerability. Copyright 2000 The Association for the Study of Animal Behaviour.     

Laying order and offspring sex in blue tits Parus caeruleus

According to sex allocation theory, females may benefit from a differential investment in sons and daughters, where fitness returns from male and female offspring vary. One way in which investment may be differentiated is by assigning male and female offspring to eggs of different sizes, and/or eggs laid early or late in the laying order. Here, we investigate whether such sex-specific adjustments occur in blue tits Parus caeruleus . We found that the proportion of males changed non-linearly with laying order showing the increase in early-laid eggs. Egg size increased with laying order, but this pattern differed between female and male eggs. Female eggs increased initially in size and became smaller later in the laying sequence, while male eggs exhibited constant increase in mass. However, egg size did not differ between genders. Egg size and laying order was found to interact in moulding sex of the embryo. Thus, females seem to simultaneously adjust the sex of the offspring in relation to their investments into eggs and the position of the egg in the laying sequence.     

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.     

Is variation in brood sex ratios adaptive in the great tit (Parus major)?

Life-history theory predicts skewed offspring sex ratios ina range of situations in which the costs and benefits of producingthe two sexes differ. In recent years, many studies have demonstratedbiased sex ratios in a variety of bird species. However, manyof these investigations have been based on small sample sizes,on data from a single year, or both. Using a recently developedpolymerase chain reaction-based molecular DNA technique, 912great tit (Parus major) nestlings from 118 broods in 5 differentyears were sexed. As found in a number of previous studieson the same species, there were significant predictors of offspringsex ratio in individual years. However, there were no consistenttrends across years, and none of the measured variables significantlypredicted sex ratio over all years combined. Furthermore, broodsex ratio of the population did not depart from the expectedbinomial distribution. Although there are theoretical advantagesto manipulating the sex ratio in this and other species, thephysiological mechanism by which it is achieved in birds remainsobscure. We argue that data from several years are needed toconfirm whether facultative sex ratio manipulation is a consistentbreeding strategy used by birds.     

Egg shell quality,clutch size and hatching success of the great tit (Parus major) and the pied flycatcher (Ficedula hypoleuca) in an air pollution gradient

Egg shell thickness, egg volume, clutch size and hatching success of Parus major and Ficedula hypoleuca were studied at 14 study sites around a copper smelter complex in Harjavalta, south-west Finland, in 1991–1993. In 1991–1992 unhatched eggs were collected to measure shell quality. F. hypoleuca was more susceptible to pollutants than P. major, the response of which was weaker in all aspects studied. Egg shells of F. hypoleuca were about 17% thinner and eggs were about 8% smaller in volume near the factory than at a distance of 10 km. The clutch size of F. hypoleuca was significantly smaller and hatching success markedly lower at a study site next to the factory complex than at all other sites. In P. major, variation in shell thickness and egg volume was not significantly related to the distance from the pollution source. Clutch size and hatching success of P. major did not significantly differ among study sites, although the trend in hatching success was in the same direction as in F. hypoleuca. Clutches of both species contained less shell material and both species had more nests without eggs near the factory than further away. The surface structure of the eggshells was studied by scanning electron microscope. Especially in F. hypoleuca, the egg shell surface was more rough and porous near the factory. The roles of Ca and heavy metals in shell thinning are discussed.     

Endocrine phenotype,reproductive success and survival in the great tit,Parus major

A central goal in evolutionary ecology is to characterize and identify selection patterns on the optimal phenotype in different environments. Physiological traits, such as hormonal responses, provide important mechanisms by which individuals can adapt to fluctuating environmental conditions. It is therefore expected that selection shapes hormonal traits, but the strength and the direction of selection on plastic hormonal signals are still under investigation. Here, we determined whether, and in which way, selection is acting on the hormones corticosterone and prolactin by characterizing endocrine phenotypes and their relationship with fitness in free‐living great tits, Parus major. We quantified variation in circulating concentrations of baseline and stress‐induced corticosterone and in prolactin during the prebreeding (March) and the breeding season (May) for two consecutive years, and correlated these with reproductive success (yearly fledgling number) and overwinter survival in female and male individuals. In both years, individuals with high baseline corticosterone concentrations in March had the highest yearly fledgling numbers; while in May, individuals with low baseline corticosterone had the highest yearly reproductive success. Likewise, individuals that displayed strong seasonal plasticity in baseline corticosterone concentrations (high in March and low in May) had the highest reproductive success in each year. Prolactin concentrations were not related to reproductive success, but were positively correlated to the proximity to lay. Between‐year plasticity in stress‐induced corticosterone concentrations of males was related to yearly variation in food abundance, but not to overall reproductive success. These findings suggest that seasonally alternating directional selection is operating on baseline corticosterone concentrations in both sexes. The observed between‐year consistency in selection patterns indicates that a one‐time hormone sample in a given season can allow the prediction of individual fitness.     

Fat reserves and perceived predation risk in the great tit, Parus major

The fat reserves of small birds are built up daily as insurance against starvation. They are believed to reflect a trade-off between the risks of starvation and predation such that in situations of high predation risk birds are expected either to reduce their fat reserves in response to mass-dependent predation risk or to increase them in response to foraging interruptions. We assessed the effect on fat reserves of experimentally altering the perceived (but not the actual) risk of predation of wild great tits at a winter feeding site. The perceived predation risk was alternated between 'safe' and 'risky'. Increasing the perceived risk of predation involved 'swooping' a model sparrowhawk over the feeder at four unpredictable times each day using a remote mechanism We produce evidence that the experiment was suceessfull in altering the perceived risk of predation. As predicted from the hypothesis of mass-dependent predation risk, great tits (Parus major) carried significantly reduced fat reserves during the 'risky' treatment. Furthermore, dominant individuals were able to reduce their reserves more than subordinates. As birds returned to feeders within seconds after a predator 'attack', the reduction in fat reserves cannot be attributed to an interruption in feeding.     

Inheritance and variation in eggshell patterning in the great tit Parus major

The inheritance of patterns on avian eggshells is central to understanding the evolution of traits such as egg mimicry (e.g. in cuckoos). Yet little is known about the inheritance, or indeed function, of eggshell patterns. It has long been believed that the evolution of eggshell pattern mimicry required that patterns be determined by genes situated on the female-specific W chromosome. However, it has never been demonstrated for any bird that egg pattern traits (rather than ground colour) are female sex linked, or indeed that they are inherited. We studied the inheritance of three measures of egg-pigment patterns in a wild great tit population. Egg patterns were female specific but unrelated to female attributes such as age or condition and showed only weak environmental effects. Eggs of daughters resembled those of both their mothers and maternal grandmothers, but not of their paternal grandmothers. We conclude that this is the first demonstration of female sex-linked inheritance of avian eggshell patterning, so raising the probability that such a system operates in egg mimics and their hosts.     

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.