Vallo Tilgar

Providing nest boxes for hole-nesting birds – Does habitat matter?

Nest boxes are a popular management tool to increase nest site availability for hole-nesting birds, but biological consequences of this technique in different habitats are poorly studied. In our study area in southwestern Estonia, nest boxes for small passerines were set up in deciduous and coniferous woods. Great tits Parus major preferred the food-rich deciduous habitat for breeding, as judged by higher nest-box occupation, earlier egg-laying and larger clutches and eggs. However, in coniferous habitat more and heavier young fledged per nest, and the return rate of both fledglings and adults was higher. We propose two mutually non-exclusive explanations, both related to the maladaptive outcome of the provision of nest boxes: (i) in the preferred habitat, nest boxes caused a supra-optimal breeding density leading to an ecological trap; (ii) boxes drastically improved the non-preferred habitat, but birds were unable to exploit the breeding habitat fully. One should be careful in providing large numbers of artificial nest sites in preferred habitats. Sometimes it would be more preferable to improve less favourable habitats by removing critical constraints.

Geographical variation in egg mass and egg content in a passerine bird.

Reproductive, phenotypic and life-history traits in many animal and plant taxa show geographic variation, indicating spatial variation in selection regimes. Maternal deposition to avian eggs, such as hormones, antibodies and antioxidants, critically affect development of the offspring, with long-lasting effects on the phenotype and fitness. Little is however known about large-scale geographical patterns of variation in maternal deposition to eggs. We studied geographical variation in egg components of a passerine bird, the pied flycatcher (Ficedula hypoleuca), by collecting samples from 16 populations and measuring egg and yolk mass, albumen lysozyme activity, yolk immunoglobulins, yolk androgens and yolk total carotenoids. We found significant variation among populations in most egg components, but ca. 90% of the variation was among individuals within populations. Population however explained 40% of the variation in carotenoid levels. In contrast to our hypothesis, we found geographical trends only in carotenoids, but not in any of the other egg components. Our results thus suggest high within-population variation and leave little scope for local adaptation and genetic differentiation in deposition of different egg components. The role of these maternally-derived resources in evolutionary change should be further investigated.

Effect of calcium availability and habitat quality on reproduction in Pied Flycatcher Ficedula hypoleuca and Great Tit Parus major

Recent studies have demonstrated that wild birds breeding in acidified areas may have difficulties in obtaining sufficient calcium for their eggshells, and that a shortage of land snails may be the cause. Here we present experimental evidence that calcium deficiency may affect reproductive traits also in birds breeding in non-acidified but naturally base-poor forest habitats. The study was conducted in Estonia in 1995 and 1996. We compared egg characteristics and other life-history traits of the Pied Flycatcher Ficedula hypoleuca and the Great Tit Parus major in two habitats differing in natural availability of calcium (base-poor pine forests and base-rich deciduous forests). In both species, egg volume was smaller in the poor than the rich habitat. Eggshell thickness also tended to be less in the poor habitat in 1996, but the difference was significant only in the Pied Flycatcher. An experimental supplementation of calcium was performed in the habitats to determine to what extent differences between rich and poor habitats might be caused by differences in calcium availability. Egg volume and shell thickness were positively affected by calcium supplementation in the Pied Flycatcher. The same tendency was observed in the Great Tit in 1996, although not significant. However, there was no effect of calcium provisioning on hatching success, nest desertion rate and number or quality of fledglings. Our results show that calcium limitation may occur in wild birds on naturally calcium-poor soils. Birds breeding in such habitats may be vulnerable to possible increases in acid deposition.

Hematological Health State Indices Predict Local Survival in a Small Passerine Bird, the Great Tit (Parus major)

In birds, it has been shown that reproductive effort may impair parental condition, while the relation of different condition indices to subsequent survival is still poorly understood. In this study, we measured body mass and various hematological condition indices in breeding great tits in relation to local survival. Number and quality of nestlings and the occurrence of second broods, potentially reflecting parents’ breeding effort, were also considered in analyses. The great tits, both male and female, that returned the following year had had a higher albumin/globulin ratio, lower plasma globulin concentration, and a lower heterophile/lymphocyte ratio during breeding in the preceding year, compared to those who did not return. Surviving males (but not females) also had had a higher level of circulating lymphocytes, compared to nonsurvivors. There was no correlation between breeding effort and survival. We conclude that better immunological state and lower stress in great tits during breeding were positively related to their survival probability.

Low Reproductive Success of Great Tits in the Preferred Habitat: A Role of Food Availability

Abstract: Habitat-related variation has been revealed in the seasonal reproductive patterns of great tits inhabiting mosaics of deciduous woodland fragments and managed coniferous forests. The number and quality of offspring tend to be higher in the non-preferred coniferous habitat compared with the preferred deciduous habitat. We explored whether these patterns can be explained by variation in food abundance and/or parental provisioning ability. Frass fall seemed not to be a reliable measure of the seasonal dynamics of nestling feeding conditions, although it is widely used for this purpose. No habitat-related differences were found in the tendency of parents to increase provisioning frequency in response to increased hunger levels of nestlings, suggesting that provisioning frequency as such is not a limiting factor for nestling growth. Higher feeding rates in deciduous habitat were associated with lower proportions of high-quality food items among prey delivered to offspring. These findings confirm that relatively high nestling feeding rates in birds may reflect the low quality of available food rather than the quality of parental care or an abundance of food in the environment. The results also indicate that deciduous forest habitat, though preferred by tits, may sometimes provide poorer brood-rearing conditions than the non-preferred, coniferous habitat. We suggest that the great tits preference for deciduous habitat, which presumably evolved in more southern regions, may be maladaptive in northern regions where deciduous woods are mainly young, secondary stands of alder and birch. Nomenclature: Tutin et al., 1993; Dickinson, 2003.

Evolutionary signals of selection on cognition from the great tit genome and methylome

For over 50 years, the great tit (Parus major) has been a model species for research in evolutionary, ecological and behavioural research; in particular, learning and cognition have been intensively studied. Here, to provide further insight into the molecular mechanisms behind these important traits, we de novo assemble a great tit reference genome and whole-genome re-sequence another 29 individuals from across Europe. We show an overrepresentation of genes related to neuronal functions, learning and cognition in regions under positive selection, as well as increased CpG methylation in these regions. In addition, great tit neuronal non-CpG methylation patterns are very similar to those observed in mammals, suggesting a universal role in neuronal epigenetic regulation which can affect learning-, memory- and experience-induced plasticity. The high-quality great tit genome assembly will play an instrumental role in furthering the integration of ecological, evolutionary, behavioural and genomic approaches in this model species.

Long-term consequences of early ontogeny in free-living Great Tits Parus major

Environmental factors during early development may have profound effects on subsequent life-history traits in many bird species. In wild birds, sex-specific effects of early ontogeny on natal dispersal and future reproduction are not well understood. The objective of this work was to determine whether hatching date and pre-fledging mass and condition of free-living Great Tits Parus major have any subsequent effect on individuals’ natal dispersal and reproductive performance at first breeding. Both males and females dispersed longer distances in coniferous than in deciduous forests, while dispersal was condition-dependent only in males (heavier as nestlings dispersed farther). In females, mass and condition at pre-fledging stage correlated significantly with clutch size, but not with subsequent reproductive performance as measured by fledging success or offspring quality. In contrast, heavier males as nestlings had higher future fledging success and heavier offspring in their broods compared with those in worse condition as nestlings. The hatching date of female as well as male parents was the only parental parameter related to the number of eggs hatched at first breeding. These results indicate that pre-fledging mass and condition predict subsequent fitness components in this bird species. We suggest that sex-specific relationships between a disperser’s condition and its selectivity with respect to breeding habitat and subsequent performance need to be considered in future models of life-history evolution.

Calcium availability affects bone growth in nestlings of free‐living great tits (Parus major), as detected by plasma alkaline phosphatase

Recent studies have revealed that calcium limitation of avian reproduction may be a widespread phenomenon, affecting both egg properties and chick development. The effect of calcium shortage on the final body size of fledglings is usually rather weak, possibly owing to compensatory growth. Achieving full skeletal size is not, however, a reliable indication of complete ossification and it is possible that chicks with similar tarsus lengths are in different stages of skeletal development. We hypothesized that measuring plasma activity of the bone alkaline phosphatase (ALP), a highly specific marker for bone calcification, may reveal subtle developmental differences in full-grown fledglings, having experienced different levels of calcium availability during growth. In two seasons, a number of pairs of great tit Parus major were provided with calcium-rich material during the nestling period, while others were not supplemented (controls). While no significant differences in size of fledglings were detected between groups, bone-ALP activity at the pre-fledging stage was lower in the calcium-provided nestlings than in the control nestlings. This may indicate that supplemented chicks had completed the rapid phase of bone formation, but this process was delayed in controls. Measuring ALP as a marker of skeletal development expands our knowledge of how delayed skeletal development of chicks can result in protraction of the nestling period, thereby reducing the breeding success of adult birds.

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.

Sibling growth patterns in great tits: does increased selection on last-hatched chicks favour an asynchronous hatching strategy?

In birds, competitive abilities of siblings in relation to their sex and the magnitude of hatching delay are still poorly understood. We compared the sex-specific growth of the last-hatched, competitively disadvantaged chicks with that of synchronously hatched chicks in two successive years. Sons exhibited higher growth rates than daughters in a year with delayed onset of breeding, and this sex-related difference was more pronounced among the asynchronously hatched chicks. Females apparently do not selectively allocate more resources to the last-laid eggs because neonatal body mass and the growth rate of asynchronously hatched chicks did not differ between years, despite the fact that in one of the years, asynchronous chicks hatched from replaced eggs taken randomly from other nests and not from eggs laid last by the incubating female. Among chicks that survived, moderately asynchronous siblings grew at a lower rate than synchronous ones, whereas no difference in mass gain was revealed between synchronous and strongly asynchronous siblings irrespective of their contrasting competitive abilities. We suggest that selection of the fittest asynchronous chicks may improve the overall quality of asynchronous broods, thus favouring the maintenance of asynchronous hatching strategies in variable environments.