Female coloration indicates female reproductive capacity in blue tits.

It is poorly understood whether female morphological and behavioural traits can be used as 'signals'. In particular, experimental tests of the hypothesis that female ornaments reflect quality are scarce. Here, we experimentally examine whether female plumage coloration might signal maternal quality in the blue tit, Cyanistes caeruleus by forcing half of the females breeding in our population to produce a replacement clutch. Using statistical models that controlled for the effects of male coloration, and the effects of age and condition of both parents, we found that carotenoid-based female coloration was positively linked to key proxies of bird lifetime reproductive success: clutch size, fledgling success and recruitment. Importantly, the relationships between maternal yellow carotenoid coloration and both clutch size and recruitment were stronger in the experimental group than in the control group, indicating that breeding females with higher values of yellow coloration were better able to handle the cost of producing a second clutch. Finally, UV-blue female coloration was positively linked to female survival and marginally linked to laying date. Taken together, these results show for the first time in a natural population that female coloration can indicate individual and maternal quality under natural and adverse reproductive conditions. They highlight the potential for the evolution of female ornamental traits through sexual selection.

Non-photoperiodic factors and timing of breeding in blue tits: impact of environmental and social influences in semi-natural conditions.

The seasonal development of life-history traits is influenced by many environmental factors. The impact of photoperiodic and non-photoperiodic factors on nest building and egg laying has been rarely investigated in non-domesticated avian species for which long term field data sets are available. Former investigations showed that blue tits originating from geographically close populations in the Mediterranean region do not respond in the same way to photoperiodic factors in semi-natural outdoor conditions. Here we show experimentally that nest building and onset of egg laying in captive blue tits is also proximately influenced by non-photoperiodic factors, including aspects related to aviary characteristics and social interactions between birds of the two sexes originating from different local Mediterranean study populations. In two successive experiments, we show that (1) increasing the volume of the aviary advanced the egg laying period of one specific population by almost 1 month, and (2) crossing pairs of birds from different origins strongly reduced the nest building and egg laying behaviours. These results indicate that obtaining biologically relevant breeding results in captivity with wild birds requires the control and experimental manipulation of a wide array of complex environmental cues.

Simultaneous pituitary-gonadal recrudescence in two Corsican populations of male blue tits with asynchronous breeding dates.

Animal populations living in geographically variable environments respond to different selection pressures. The adaptive character of the responses to environmental information determines the degree of synchrony of the breeding period with local optimal conditions. An example is provided by two populations of Mediterranean blue tits (Parus caeruleus) in Corsica, breeding in different habitats, with a 1-month difference in the onset of egg laying. This difference in the onset of lay is supposed to be adaptive because, although chicks from both populations are raised mostly on caterpillars, the timing of the appearance of caterpillars is earlier for populations of tits associated with deciduous oak trees than those associated with evergreen oak trees. Here, we show that, despite the difference in the timing of egg laying, males from these two populations start seasonal hypothalamo-hypophysial-testicular development at approximately the same time, in late winter. Specifically, the vernal recrudescence of brain GnRH-I perikarya and fibers, testes volume and song activity began around the same dates and proceeded at the same pace in late winter in both populations. Plasma testosterone and LH levels displayed seasonal variations that were shifted by less than 2 weeks compared to the 1-month difference in egg laying periods. We hypothesize that the strong selection pressures on these two populations to adapt the timing of their breeding seasons to their local environment may have acted mostly on the female egg laying dates, and not so much on the initiation and rate of seasonal recrudescence of the hypothalamo-hypophysial-testicular activity in males.

Olfactory conditioning experiments in a food-searching passerine bird in semi-natural conditions.

Because passerine birds have a very small relative olfactory bulb size, they have been considered to have weak olfactory capacities for decades. Recent investigations however suggest that breeding female blue tits (Parus caeruleus) are sensitive to lavender odour in the reproductive context of building and maintaining the nest. Here, we present results of an olfactory conditioning experiment in blue tits held in semi-natural conditions during the breeding season. We show that captive male blue tits, trained to associate lavender odour with a food reward, are more attracted to an empty feeder box emitting lavender odour than an odourless empty feeder box. Females did not distinguish significantly between empty feeders with and without lavender odour during the test phase, although they responded positively at the end of the training phase. These results suggest that male blue tits can use olfaction in a context not related to nest building. Additional experiments will be required to better understand the observed sex differences in response to the experimental set up, and in what context free-ranging individuals use olfaction.

Testing for microevolution in body size in three blue tit populations.

Quantifying the genetic variation and selection acting on phenotypes is a prerequisite for understanding microevolutionary processes. Surprisingly, long-term comparisons across conspecific populations exposed to different environments are still lacking, hampering evolutionary studies of population differentiation in natural conditions. Here, we present analyses of additive genetic variation and selection using two body-size traits in three blue tit (Parus caeruleus) populations from distinct habitats. Chick tarsus length and body mass at fledging showed substantial levels of genetic variation in the three populations. Estimated heritabilities of body mass increased with habitat quality. The poorer habitats showed weak positive selection on tarsus length, and strong positive selection on body mass, but there was no significant selection on either trait in the good habitat. However, there was no evidence of any microevolutionary response to selection in any population during the study periods. Potential explanations for this absence of a response to selection are discussed, including the effects of spatial heterogeneity associated with gene flow between habitats.

Energetic and fitness costs of mismatching resource supply and demand in seasonally breeding birds.

By advancing spring leaf flush and ensuing food availability, climatic warming results in a mismatch between the timing of peak food supply and nestling demand, shifting the optimal time for reproduction in birds. Two populations of blue tits (Parus caeruleus) that breed at different dates in similar, but spatially distinct, habitat types in Corsica and southern France provide a unique opportunity to quantify the energetic and fitness consequences when breeding is mismatched with local productivity. As food supply and demand become progressively mismatched, the increased cost of rearing young pushes the metabolic effort of adults beyond their apparent sustainable limit, drastically reducing the persistence of adults in the breeding population. We provide evidence that the economics of parental foraging and limits to sustainable metabolic effort are key selective forces underlying synchronized seasonal breeding and long-term shifts in breeding date in response to climatic change.

A long photoperiod overrides non-photoperiodic factors in blue tits' timing of reproduction.

Endocrinological studies have contributed considerably to the development of theory concerning the proximate aspects of the timing of reproduction. In non-domesticated, avian species, the relative importance of the photoperiodic and non-photoperiodic factors influencing later stages of the breeding cycle, such as the onset of egg laying, remains unclear because egg laying is difficult to obtain with captive populations and laboratory experiments of breeding are rarely carried out in the framework of long-term field studies. We set up a special experimental design such that captive Mediterranean blue tits (Parus caeruleus) can breed with success in large outdoor aviaries at similar latitudes and altitudes to their wild counterparts. Here we demonstrate experimentally that the non-photoperiodic factors responsible for large and consistent differences in the expression of natural breeding responses between three captive outdoor blue tit populations are ignored during long-day treatment. Based on these findings, an evolutionary explanation is provided for why the relative importance of the non-photoperiodic factors decreases with the progress of the season. The hypothesis can explain observed maladapted breeding dates in free-living populations and could possibly be used to increase the success of breeding programmes with some endangered, captive, non-domesticated, photoperiodic species.

Risk-taking restraints in a bird with reduced egg-hatching success.

Risk taking, as is any other phenotypic and/or behavioural trait, is determined by proximate constraints related to time or resource availability and by evolutionary adaptive restraints related to the differences in the costs of risk taking and its benefits in terms of fitness. Because risk taking is influenced by many confounding variables related to experimental design, environment, parents and offspring, few field studies have been reported which unambiguously separate the effects of restraints from those of constraints. We compared parental risk taking in blue tits (Parus caeruleus) during brood defence towards a nest predator in broods with experimentally reduced and natural egg-hatching success leaving the original number of eggs in the nest. The experimentally reduced broods had more time or resources available and lower risk-taking benefits compared to the control broods. 'Constraint' would predict more risk taking in broods having experimentally reduced egg-hatching success, whereas 'restraint' would predict the opposite effect with more risk taking in broods with natural egg-hatching success. We report, to our knowledge, the first field study experimentally demonstrating a brood defence restraint in response to reduced egg-hatching success. This demonstration was only possible after controlling for more than 20 potential confounding variables showing once more how complicated it is to separate proximate from evolutionary levels of analyses in natural populations.

A single response mechanism is responsible for evolutionary adaptive variation in a bird's laying date.

Many proximate factors determine a bird's laying date, including environmental and social stimuli as well as individual responses to internal and external factors. However, the relative importance of these factors has not been experimentally demonstrated. Here we show that (i) large differences in the onset of first clutches between different populations result from variation in different responses to photoperiod and not from variation in responses to any other proximate factors and (ii) the same response mechanism causes maladaptive laying dates in habitats modified by humans. We present, to our knowledge, the first experimental demonstration that a single response mechanism is responsible for evolutionary adaptive intraspecific variation in a vertebrate life history trait.