Sound is used as a medium for communication by taxa as varied as insects, fish, amphibians, birds and mammals. In some birds like the suboscines, song is genetically encoded, whereas in parrots, hummingbirds and oscines, it is learnt. The diversity and plasticity of birdsong continues to generate interest amongst ornithologists, and many questions remain unresolved. For instance, why do some species sing hundreds of different songs while others use simple, stereotyped ones for the same purposes? Why do some birds learn not only their own species’ song, but also the songs of heterospecifics? There are several anecdotal reports of such vocal mimicry in wild birds, where a species imitates the song or call of heterospecifics in its natural habitat, but much has yet to be learnt about this intriguing phenomenon. There has been a recent surge of interest and research into avian vocal mimicry. Despite having several species of birds that are known to produce mimicry, there is a dearth of research on this field in India. The Greater Racket-tailed drongo’s loud song and ability to mimic other species of birds with great accuracy has drawn the attention of many birdwatchers, but other than a few phonetic descriptions, no study has focussed on their song. Therefore, this thesis focuses on the structure, contexts and functions of vocal mimicry in this species. In order to understand the functions of vocal mimicry in any species, we require certain fundamental data, which are often overlooked in many studies of bird song. Since this is the first study focusing on the racket-tailed drongo in India, I began with collecting natural history data on the ecology and breeding biology of the species. Then, I attempted to arrive at an objective and quantitative definition and classification of the racket-tailed drongo’s vocal repertoire, especially its mimicry. It is also essential to have information on the contexts in which this mimicry is used. Using a combination of focal animal sampling and sound recordings, I documented the contexts in which the racket-tailed drongo imitates other species in the wild. I also examined the diversity of the species that were mimicked across these contexts. Building on the data from these observations, I used playback experiments to test hypotheses for the functions of mimicry in multiple contexts. Results from these show that greater racket-tailed drongos use mimicry in a flexible manner according to the intended audience. Drongos use two different sets of mimicked calls with distinct syntax directed at conspecifics and heterospecifics respectively, the former in territorial song and the latter to attract members of mixed-species flocks. These results also imply that mimicry may be driven by both sexual and natural selection within the same species, and have implications for the definition of avian vocal mimicry, which remains highly debated.

La migration cellulaire joue un rôle essentiel dans le développement des organismes multicellulaires et dans certaines pathologies comme le cancer, où elle permet la formation de métastases. Le trafic vésiculaire est un régulateur clé de la migration cellulaire, notamment en contrôlant la localisation de protéines impliquées dans la migration telles que les intégrines, les cadhérines et les récepteurs transmembranaires. En particulier, notre laboratoire a montré que l'endocytose contrôle l'orientation et la communication cellulaire durant la migration cellulaire collective. Notre hypothèse est que d'autres événements du trafic vésiculaire pourraient aussi être impliqués dans ce type de migration. Ainsi, le but de cette thèse a été de déterminer la fonction des petites GTPases Arf, importantes pour la formation de vésicules et le tri de cargo dans ces vésicules et de leurs régulateurs dans la migration cellulaire collective. Un modèle pour étudier la migration cellulaire collective est les chambres d’œufs de Drosophila melanogaster. En effet, lors de l’ovogénèse, des cellules folliculaires appelées cellules de bordure migrent à travers les cellules nourricières pour atteindre l’ovocyte. Conformément à notre hypothèse, un fort défaut de migration est observé lorsque les Arfs sont déplétées spécifiquement dans les cellules de bordure. De plus, un constat similaire est observé après la déplétion de certains régulateurs des Arfs (ArfGAPs et ArfGEFs). Notamment, nous avons démontré que l’ArfGAP Drongo et sa fonction d'activation de l’activité GTPase sont essentielles pour le détachement initial des cellules de bordure du tissu folliculaire. Drongo promeut le détachement en contrôlant la localisation de la myosine phosphatase afin de réguler l’activité de la myosine II à l’arrière des cellules. De plus, nous avons montré que Drongo agit sur l’Arf de classe III (Arf51F) de manière antagoniste à l’ArfGEF Steppke pour déplacer la myosine phosphatase de l’arrière du groupe de cellules. D’un autre côté, nous avons aussi démontré qu’une autre GAP, ArfGAP1, contrôle la directionnalité de migration. Cette ArfGAP agit potentiellement en régulant la localisation de certains déterminants de la migration tels que l’E-cadhérine et les récepteurs tyrosine kinase. Ainsi, nos recherches ont démontré un rôle essentiel des Arfs ainsi que des rôles spécifiques de deux ArfGAPs dans la migration cellulaire collective. / Cell migration is implicated in various important biological processes, notably it is central for the dissemination of cancer cells. Vesicular trafficking is a key regulator of cell migration, notably by controlling the localisation of proteins involved in migration such as integrins, cadherins and transmembrane receptors. In particular, our laboratory has shown that endocytosis controls orientation and cellular communication during collective cell migration. Our hypothesis is that other events of vesicular trafficking might be implicated in collective cell migration. Thus, the purpose of this thesis was to assess the function of small GTPases Arf, important for vesicle formation and cargo sorting into those vesicles, and their regulators in collective cell migration. A powerful model to study collective cell migration is the migration of follicular cells named border cells during oogenesis in Drosophila melanogaster. Border cells (BCs) detach from the follicle epithelium surrounding the egg chambers and form a small cluster of six to ten cells that migrates invasively between the giant nurse cells that compose the center of the egg chamber, toward the oocyte. Accordingly to our hypothesis, a strong migration defect is observed when the Arfs are depleted specifically in the border cells. Moreover, a similar finding is observed after depletion of some Arfs regulators (ArfGAPs and ArfGEFs). In particular, the ArfGAP Drongo and its GTPase-activating function are essential for the initial detachment of the border cell cluster from the basal lamina. We demonstrated through protein localization and genetic interactions that Drongo controls the localisation of the myosin phosphatase in order to regulate myosin II activity at the back of the cluster and promote border cells detachment. Moreover, we showed that Drongo acts on the class III Arf (Arf51F) antagonistically to the guanine exchange factor Steppke to displace myosin phosphatase from the back of the cluster. On the other hand, we have also demonstrated that the GAP ArfGAP1 controls the directionality of migration. This ArfGAP potentially acts by regulating the localization of certain determinants of migration such as E-cadherin and receptors tyrosine kinase. Thus, our research has demonstrated an essential role for Arfs in collective cell migration and specific contributions of two ArfGAPs in this migration process.