El uso de señales múltiples de calidad es común en numerosas especies, pero este hecho no es fácil de explicar. Las presiones fluctuantes del ambiente podrían tener un rol importante en el origen y mantenimiento de las señales múltiples, y las implicaciones evolutivas de esta hipótesis enfatizan la importancia de realizar nuevos estudios sobre señales múltiples. En esta tesis se realizaron diferentes estudios sobre la señalización múltiple del plumaje de dos especies de aves: El carbonero común (Parus major) y el lúgano (Carduelis spinus). En el primer capítulo se examinó la banda alar amarilla del lúgano, un ornamento sexual basado en carotenoides. Se vio que machos con bandas alares mayores eran más rápidos resolviendo un problema de forrajeo. Esto sugiere la existencia de una selección sexual de habilidades cognitivas en el lúgano. En el segundo capítulo, estudiamos la función del babero negro del lúgano como señal de la personalidad del individuo. Descubrimos que lúganos con baberos de mayor tamaño mostraban tendencias exploratorias más rápidas. Este hallazgo apoya la idea de que los ornamentos del plumaje pueden funcionar como señales de la personalidad del individuo. En el tercer capítulo se examinó cómo las señales múltiples pueden definir el comportamiento sexual extra pareja. Se pudo constatar que la respuesta agonística y la paternidad de machos de carbonero dependía de la interacción entre la corbata negra y la coloración amarilla del pecho del macho, sugiriendo la existencia de diferentes estrategias reproductivas de acuerdo a la calidad individual. Por último, examinamos las posibles interacciones entre ornamentos sexuales y la familiaridad, mediante experimentos de selección de pareja en lúganos. Nuestros resultados sugieren que la familiaridad puede ser tan importante como los ornamentos sexuales a la hora de determinar los criterios de selección de la hembra. En conclusión, esta tesis profundiza en el hecho de que las diferentes cualidades de un individuo, reflejadas en distintas señales múltiples, tienen el potencial de ser parte de diferentes estrategias válidas en selección sexual y social, permitiendo una selección más plástica. Esta plasticidad de selección podría ser uno de los mecanismos tras el origen y mantenimiento de las señales múltiples. / In sexual or social selection processes, animals employ traits that signal qualities. The use of multiple signals is known to be well spread in numerous species, but the ultimate reasons for using more than one single signal are not easy to explain. Why would animals invest in several different signals, instead of focusing on only one? Traditionally, the adaptive hypotheses formulated to explain the existence of multiple signalling have been framed into static scenarios, under constant selection pressures. However, there is increasing awareness of the need to widen this static framework, by taking into account the selection plasticity produced by spatial and temporal environmental heterogeneity. Fluctuating ecological and social environments could have an important role in the origin and maintenance of multiple signals. In an environment where changes happen faster every day, multiple signalling would allow a more flexible, dynamic selection, increasing the odds of survival and fitness. The evolutionary implications of this hypothesis highlight the importance of novel studies in multiple signalling. In this thesis, we performed several such experiments on the Eurasian siskin (Carduelis spinus) and the Great tit (Parus major). They are two passerine species that show ornaments based on melanins and carotenoids, which makes them a particularly suitable models for the study of multiple signalling. In the first chapter, the signalling function of the yellow wing stripe of the Eurasian siskin, a carotenoid based sexual ornament, was examined. A problem solving experiment showed that male siskins with longer yellow wing stripes were better foragers. These resuls suggest the existence of sexual selection for cognitive abilities. In the second chapter, we studied the function of the black bib of the Eurasian siskin as a potential source of information about the personality of the individual. We examined the exploratory behaviour of male siskins, finding that those with larger black bibs had a shorter approaching latency to a novel object. These results support the idea that plumage structures could give information about the personality of its bearer. The results of these two chapters show that colour signals based on different pigments have separate functions in the siskin, allowing multiple criteria in sexual (and social) selection. The third chapter examined the effects of different multiple colour signals on the extra pair sexual behaviour of the great tit. By measuring the response of different males to conspecific male intrusions, and the within pair paternity of males depending on their colour traits, we could observe that different combinations of traits where related to the intensity of nest defence, and implied differences in paternity loss. These findings suggest that males might be using different strategies to maximize their qualities. In the last chapter, we tested the potential interactions between colour sexual ornaments and the previous experience, by sexual selection experiments in siskins. Our results suggest that familiarity adds to attractiveness when female siskins choose a mate. At least in some species, females are using multiple criteria in mate choice, not necessarily related to external signals of quality. In conclusion, this thesis stress that multiple qualities, signalled by different colour traits, have the potential to be part of different successful social and sexual mate choice strategies, aiding a more plastic selection. Our results support the hypothesis that selection plasticity could be one of the mechanisms behind the origin and maintenance of multiple signals