Decisiones reproductivas del herrerillo común en un ambiente mediterráneo

Resumen

Universidad de Castilla-La Mancha García-Navas Corrales, Vicente Correo electrónico: vicente.garcianavas@gmail.com Director: Sanz Cid, J. J. Reproductive decisions of Blue Tits in a Mediterranean environment Decisiones reproductivas del Herrerillo Común en un ambiente Mediterráneo Abstract: Two of the key issues that any organism has to face at least once in its life are where and with whom do breed. These two decisions (when? whom?) are especially crucial for short-living species. This thesis analyzes the causes and consequences associated with these two aspects in a small forest insectivorous bird, the blue tit Cyanistes caeruleus, in populations located at Montes de Toledo. These Mediterranean forests are characterized by a longer duration of the breeding period (a wider reproductive window) and the existence of a high diversity of Lepidoptera species, which may have an important role in shaping the evolution of birds¿ vital traits in these populations. First, I addressed the phenotypic plasticity of blue tits in relation to the phenology of their main prey (Lepidoptera larvae) and its consequences in terms of prey choice and breeding success. Second, I focused on mate choice and sexual selection in this species considering i) the benefits derived from pairing with individuals showing high levels of genetic diversity (¿good genes¿), ii) the mechanisms used by males to reveal information about their individual quality to their partner, and iii) the processes involved in the maintenance or breakup of the pair-bond. Concerning the first issue, focused on timing of breeding, we found that the degree of synchronization between blue tits and Lepidoptera larvae (caterpillars) may largely determine the time at which these start the incubation period, suggesting that birds may change to some extent their breeding sequence in order to achieve a better synchronization with the time of maximal food abundance (caterpillar peak). The degree of synchrony in relation to the phenology of caterpillars influenced prey choice decisions of blue tits and had an impact on individuals in terms of fitness. Those birds that were able to adjust correctly their timing tended to exploit caterpillars requiring longer searching efforts (noctuids) and their nestlings fledged in better condition. Whilst, those birds that bred too early or too late were forced to exploit tortricid larvae, a type of caterpillar of small size and easy to get. The results obtained indicate that the fact that birds feed their young with one or another prey type is not trivial. The type of caterpillar brought to the nest could have important consequences for the parents as the costs associated with obtaining different types of prey influenced the rate at which they provisioned food to their offspring. Food quality could also have a significant impact on nestling condition. On the other hand, the fact that in these Mediterranean woodlands tits do not solely depend on one species (multiple caterpillar peak) allow them to adjust their provisioning strategy as plastic response to environmental unpredictability. I manipulated experimentally nestling demand (by means of daily manipulations of brood size) in a short term (i.e. from one day to the next) and I observed that parents adopted their foraging tactic (in terms of both parental effort and food selection) to cope with the new scenario (reduced or enlarged brood size). Total provisioning rates were lowest when broods were reduced in size and greatest when broods were enlarged. Prey choice was also affected by the brood size changes. Increase of nestling demand compelled parents to work harder and be less selective in prey choice; I found that birds with a high level of feeding responsibility opted to concentrate on more readily available food items (tortricids, pupae). Whilst, their immediate reaction when faced with a low level of feeding responsibility was to decrease these prey types in the diet, so that the percentage of other preys (noctuids, spiders) in the diet increased. These results emphasize the importance of considering the role of alternative prey and thus, the flexibility in the foraging behaviour of birds, when exploring the ecological interactions between the different levels of the oak-caterpillar-tit food chain and the consequences of disrupted synchrony, particularly in those regions such as the Mediterranean Basin where Lepidoptera larval assemblages show a high diversity in comparison with northern latitudes. Regarding the second question (mate choice), I found that individual heterozygosity of blue tits was positively correlated between social mates, suggesting that in these populations blue tits base their mating preferences on partner¿s heterozygosity. The results obtained indicate that the observed heterozygosity-based assortative mating could be maintained by both direct and indirect benefits since heterozygosity reflected individual quality in both sexes: egg production and quality increased with female heterozygosity while more heterozygous males showed higher feeding rates during the brood-rearing period. In the case of males, their genetic quality was correlated with expression of a secondary sexual trait (plumage crown coloration) so males with a superior genotype may reveal such information through this ornament. Although, I also found that in these populations males can use non-body ornaments (¿extended phenotype¿) to indicate their parental quality, experience or genetic quality and thereby, induce females to increase their reproductive investment. In these populations, male blue tits carry feathers to the nest and place them outside the nest cup which enhances the conspicuousness of this ornament. I found that female blue tits increased their clutch size in response to the addition of feathers (either experimentally or naturally) suggesting that this puzzling behaviour may function as an honest indicator of male quality and thus as a post-mating signal of good parents in this species. Lastly, the high divorce rate observed in these populations (63%) could be driven by the intense competition for good nesting sites. The abandonment of the current partner to find a male settled in a better territory seems to be the main reason that prompts females to break the pair-bond. Thus, divorce could be viewed a side effect of females seeking to improve their breeding performance by acquiring higher quality sites. ---Curso académico: 2011-2012