Nestling diet variation in an insular Mediterranean population of blue tits Parus caeruleus: effects of years, territories and individuals.

Mediterranean evergreen forests of Corsica are characterized by relatively high species diversity of arthropods with low population densities. Food is never superabundant for Corsican blue tits Parus caeruleus. This study focused on the composition of the food of blue tit nestlings and especially on two main components, caterpillars and spiders. The nestling diet was studied for two years using 8-mm cameras that automatically took photographs of adult birds with food. The diet was composed of c. 50% caterpillars and c. 30% spiders. There were between-year and between-individual differences in these proportions. In both years of the study the proportion of caterpillars declined during the course of the breeding season. Individual and time effects on prey sizes were also observed. Pairs and individuals were fairly constant in the proportions of prey over the feeding period. Different food items were not brought in runs. These findings suggest that strong food limitation exists on Corsica, which can considerably influence life-history traits of the blue tit.

Linking individual foraging behavior and population spatial distribution in patchy environments: a field example with Mediterranean blue tits.

In spite of recent theoretical interest, few field studies have addressed the links between individual behavioral decisions and population distribution. This work analyzes the foraging behavior of individuals and the spatial distribution of a population of blue tits (Parus caeruleus) just before the main breeding season, when blue tit foraging was not affected by central-place or flocking behaviors. The study was carried out in open holm oak Quercus ilex woodlands (dehesas) that are patchy for canopy-foraging birds because of the scattered arrangement of trees. Residence time on each tree was not correlated either with previous flight time or with prey abundance in trees. Flight distances between trees were larger than average distances estimated in random samples of holm oaks taken close to foraging birds. Trees were not selected by birds on the basis of their expected energy costs and rewards. Bird abundance was not related to food availability in trees or to tree size across dehesas. However, bird abundance was strongly correlated with tree density and with the availability of tree holes for nesting, to the extent that the proportion of tits matched the proportions of both tree abundance and hole abundance across study plots. Overall, neither the behavior of individuals nor the distribution of the population of blue tits corresponded with food resources, which appeared superabundant; instead, tits appeared to behave and be distributed according to the distribution of structural resources such as trees and tree holes for nesting.

Seasonal variation in reproductive success and post-nuptial moult of blue tits in southern Europe: an experimental study.

Post-nuptial moult and reproductive success were studied in relation to timing of breeding in blue tits, Parus caeruleus, breeding in southern Europe. A group of experimentally delayed pairs was created by removing first clutches, thereby inducing late repeat clutches. Reproductive success and post-nuptial moult of delayed pairs were compared with both control pairs that bred early and unmanipulated late-breeding pairs. Delayed pairs fledged fewer young and with a lower body mass than control pairs. However, the number of fledged young and fledgling mass did not differ between delayed and late-breeding pairs. These results were more consistent with the date hypothesis, and it is concluded that the timing of breeding and reproductive success may be causally related in the blue tit. This study reveals a harmful effect of relaying on female body mass at the end of the nestling period. Therefore, females apparently pay the costs of relaying, since a reduction in body mass during the nestling period may be accompanied by a lowered survival probability. Delayed and late-breeding males often began moulting while still feeding young, but neither control males nor females from the three study groups did so. These results support the view that timing-related energy constraints on breeding may be important causes of a seasonal decline in reproductive success at different latitudes.

MHC class II B diversity in blue tits: a preliminary study.

In this study, we partly characterize major histocompatibility complex (MHC) class II B in the blue tit (Cyanistes caeruleus). A total of 22 individuals from three different European locations: Spain, The Netherlands, and Sweden were screened for MHC allelic diversity. The MHC genes were investigated using both PCR-based methods and unamplified genomic DNA with restriction fragment length polymorphism (RFLP) and southern blots. A total of 13 different exon 2 sequences were obtained independently from DNA and/or RNA, thus confirming gene transcription and likely functionality of the genes. Nine out of 13 alleles were found in more than one country, and two alleles appeared in all countries. Positive selection was detected in the region coding for the peptide binding region (PBR). A maximum of three alleles per individual was detected by sequencing and the RFLP pattern consisted of 4-7 fragments, indicating a minimum number of 2-4 loci per individual. A phylogenetic analysis, demonstrated that the blue tit sequences are divergent compared to sequences from other passerines resembling a different MHC lineage than those possessed by most passerines studied to date.

What do we really know about the signalling role of plumage colour in blue tits? A case study of impediments to progress in evolutionary biology.

Evolutionary biologists seek to explain the origin and maintenance of phenotypes, and a substantial portion of this research is accomplished by thorough study of individual species. For instance, many researchers study individual species to understand evolution of ornamental traits which appear to be products of sexual selection. I explored our understanding of sexual ornaments in a well-studied vertebrate species that may serve as a case study for research programs in evolutionary biology. I attempted to located all published papers examining plumage colour and variables related to sexual selection hypotheses in a common European songbird, the blue tit (Cyanistes caeruleus). Researchers have estimated over 1200 statistical relationships with plumage colour of blue tits in 52 studies. However, of the approximately 1000 main-effect relationships from the 48 studies that are candidates for inclusion in this meta-analysis, more than 400 were reported without details of strength and direction. Circumstantial evidence suggests that an unknown number of other estimated effects remain unpublished. Missing information is a substantial barrier to interpretation of these papers and to meta-analytic synthesis. Examination and analysis of funnel plots indicated that unpublished effects may be a biased sample of all effects, especially for comparisons of plumage colour to age and individual quality, and possibly also to measures of mate choice. Further, type I error was likely elevated by the large number of statistical comparisons evaluated, the frequent use of iterative model-building procedures, and a willingness to interpret a wide variety of results as support for a hypothesis. Type I errors were made more problematic because blue tit plumage researchers only rarely have attempted to replicate important findings in their own work or that of others. Replication is essential to drawing robust scientific conclusions, especially in probabilistic systems with moderate to weak effects or a likelihood of bias. Last, researchers studying blue tit plumage have often developed ad hoc explanations for deviations of results from their predictions. Revising hypotheses in light of data is appropriate, but these revised hypotheses were rarely tested with new data. The only highly robust conclusion supported by meta-analysis is that male blue tits have plumage that reflects more light in the ultraviolet and yellow wavelengths than the plumage of females. Various other effects, including condition-dependence of plumage colour expression and a tendency for females to adjust the sex ratio of their offspring in response to male colour, remain uncertain. These obstacles to progress in the blue tit plumage literature are likely common in evolutionary biology, and so I recommend changes to incentive structures which may improve progress towards scientific understanding in this discipline.

NATURAL SELECTION ON AVIAN BREEDING TIME: CAUSALITY, FECUNDITY-DEPENDENT, AND FECUNDITY-INDEPENDENT SELECTION.

Natural selection on breeding time through postfledgling survival was investigated in a population of blue tits, Parus caeruleus, between 1993 and 1996. In particular, selection acting through large brood sizes among early breeders (fecundity-dependent selection) was separated from selection acting through fitness benefits unrelated to the numerical benefits of large brood sizes (fecundity-independent selection). There was evidence that natural selection favored early breeding in 1993 and 1995 (significantly so in 1993), whereas in 1994 there was a tendency for selection to be stabilizing. Analyses of selection gradients further showed that there was direct selection for both early breeding and large brood sizes in 1993. Experimentally delayed pairs produced a lower proportion of surviving young among their fledglings compared to early pairs, indicating that the seasonal fitness trend is a causal one and not caused by differences between early and late breeders in territory or individual quality. There was no evidence for significant heritability for four reproductive traits (clutch sizes, laying dates, hatching dates, and brood size at fledging) although repeatability values ranged from 0.26 to 0.53. The importance of year-specific selective regimes in the evolution of avian breedings times are discussed and a role for population density in this context is proposed.