The function of eggshell pigmentation has long been the subject of much speculation. Most of the proposed hypotheses have been based on a signalling function. According to the recently proposed structural–function hypothesis, birds use the pigment protoporphyrin to strengthen the eggshell in response to dietary calcium (Ca) shortage. The aim of the study reported here was to test this hypothesis in a Ca-provisioning experiment involving Great Tit Parus major populations breeding in Ca-poor coniferous and Ca-rich deciduous Estonian forests, respectively, over a 2-year period. Ca-supplemented birds laid eggs with ‘darker’ pigmentation than birds in the unsupplemented (control) group, which is at odds with the structural–function hypothesis. Although eggs were smaller and had thinner shells in Ca-poor habitat, eggshell maculation did not differ between the habitats. Eggshells were more pigmented in the year when they were also thinner, but their thickness was not related to their maculation within each year. Although the results of our study neither prove nor disprove the structural–function hypothesis, they do suggest that the relationship between eggshell pigmentation and Ca availability cannot be explained simply by this hypothesis.

The role of natural selection in shaping adaptive trait differentiation in natural populations has long been recognized. Determining its molecular basis, however, remains a challenge. Here, we search for signals of selection in candidate genes for colour and its perception in a passerine bird. Pied flycatcher plumage varies geographically in both its structural and pigment-based properties. Both characteristics appear to be shaped by selection. A single-locus outlier test revealed 2 of 14 loci to show significantly elevated signals of divergence. The first of these, the follistatin gene, is expressed in the developing feather bud and is found in pathways with genes that determine the structure of feathers and may thus be important in generating variation in structural colouration. The second is a gene potentially underlying the ability to detect this variation: SWS1 opsin. These two loci were most differentiated in two Spanish pied flycatcher populations, which are also among the populations that have the highest UV reflectance. The follistatin and SWS1 opsin genes thus provide strong candidates for future investigations on the molecular basis of adaptively significant traits and their co-evolution.

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.

The effect of habitat heterogeneity on animal behaviour and reproduction has recently captured serious attention in population and conservation biology. The great tit (Parus major) is a facultative double-brooded species that prefers deciduous forests as breeding habitats. However, it is able to reproduce in managed coniferous forests where nest boxes are provided. During 1999-2002, we measured various reproductive parameters of great tits, including double breeding, in a heterogeneous habitat complex consisting of both deciduous and coniferous forests. Probability of laying a second clutch after the hatch of the first clutch did not differ significantly between habitats. However, clutch size was allocated between two successive breeding attempts more equally in coniferous than in deciduous forests. To our knowledge, this is the first time that such a difference in seasonal breeding patterns between adjacent habitats has been demonstrated in birds. However, the total number of offspring fledged per pair did not differ significantly between habitats. Possible proximate and ultimate causes of the observed habitat differences are discussed. We suggest that habitat-specific allocation of reproductive investment between successive breeding attempts plays an important role in optimizing breeding tactics by facultative multiple-brooded bird species in a heterogeneous environment, potentially serving as a useful mechanism facilitating their adaptation to novel habitats.

Several bird species breeding in acidified habitats face serious problems caused by lowered availability of calcium-rich foods. Moreover, recent studies have revealed that calcium shortage causes phenotypic variation in certain reproductive parameters of passerines even in non-acidified habitats. However, to date, no clear evidence has been found of the limiting effect of calcium deficiency on the reproductive output of birds breeding in such habitats. In this paper, we present experimental evidence from a long-term study that calcium shortage may indeed limit reproductive output of passerines that breed in non-acidified habitats. We carried out calcium-supplementation experiments with great tits Parus major breeding in coniferous and deciduous woodlands in Estonia. Calcium-supplemented birds laid more eggs, tended larger broods and raised more fledglings with longer tarsi compared with control birds. Calcium shortage constrained reproduction of tits not only in spring, but also in mid-summer, during second breeding attempts. Moreover, no significant differences were detected between deciduous and coniferous habitats with respect to the effect of calcium supplements on reproductive success.