This thesis includes several research works on population dynamics and conservation of seabirds in two small islands of the western Mediterranean: the Columbretes Islands (Castellón) and the island of Benidorm (Alicante). Most of the works included in the thesis report have been already published in scientific journals. This thesis includes both works discussing the role of certain biological factors (Section I) on seabird population dynamics and assessing the effects of human activities (Section II) on population numbers. Each section is preceded by an introductory chapter dealing, in a more informal manner, with the topic approached by that section. Section I approaches specifically the influence of immigration and environmental stochasticity on the population dynamics of shags Phalacrocorax aristotelis (Chapter 1), the role of philopatry and conspecific atraction in breeding site selection in Audouin's gull Larus audouinii (Chapter 2), the determinants of colony-site dynamics in Audouin's gull (Chapter 3), the movements of yellow-legged gulls Larus michahellis from the Mediterranean to the Atlantic (Chapter 4), the presence of immigrant Cory's shearwaters Calonectris diomedea from Atlantic colonies in Mediterranean colonies (Chapter 5). Section II includes works discussing the role of landfills in disease dispersal among yellow-legged gulls (Chapter 6), the role of discards of the trawling fishing fleet as a food source during the summer for gulls (Chapter 7), the role of fishing discards on the dynamics of a colony of Audouin's gull facing local extinction (Chapter 8) and the impact of tourism on a small colony of the Eleonora's falcon Falco eleonorae (Chapter 9). A number of conclusions are listed. Basically we found that adult survival is the demographic parameter with the largest impact on the growth rate of colonies, although transfer processes play a very important role as well since seabirds are structured in subpopulations (patches) separated in space. Human activities can affect all demographic parameters (fecundity/mortality and immigration/emigration) and must be taken into account to understand seabird dynamics, specially fishing activities which may even produce the local extinction of colonies. Past is also a factor to be taken into account since present-day dynamics result from the interplay between local, regional and historical factors. Most of the seabird colonies in the Mediterranean are small sized and hence prone to genetic erosion and stochasticity besides being composed by species and subspecies with a high degree of endemism. Colonies not occupied at present must also be included in conservation plans since seabirds have high dispersal capabilities. Finally, the tremendous importance of the social factor in seabird colonies can be used as an effective conservation tool.