The "island syndrome" refers to similarity in the biology of island organisms, but its generality is questionable, as the scope of species and traits examined are often limited. Here, I show that birds breeding exclusively on islands (breeding island endemics) evolved smaller clutches, using a dataset of 4,530 bird species. Using an inclusive definition of a breeding island endemic, which also encompasses migratory species and seabirds, I examine the evolution of clutch sizes in island breeding species using phylogenetic generalized linear models. Across disparate phylogenetic hypotheses, and after accounting for biological and geographical co-variables, I show that breeding island endemic landbirds (470 species) evolved smaller clutch sizes than continental breeding species (3,818 species). I show that the evolution of clutch size follows the expectations of the island syndrome, as among breeding island endemic landbirds there is a positive relationship between clutch size and breeding range area. Finally, I reinforce the view that the island syndrome is a general pattern in birds, spanning diverse phylogenetic and ecological groups, by showing that in a seabird-only dataset (242 species), breeding island endemic seabirds show evolution of smaller clutch sizes. In a model of the full dataset of both landbirds and seabirds (4,530 species) there was no evidence of an interaction of being a seabird with breeding island endemicity, showing that seabirds and landbirds respond in the same way. This study, using more than 40% of all bird species, provides the first evidence of a general evolutionary response in a life history trait, clearly showing the island syndrome as a general evolutionary tendency associated with island environments. center dot The island syndrome is a phenomenon of similarity in morphology, behavior, and life history between island organisms.center dot Even among birds, its extent is not fully studied; it is not clear which traits (particularly behavioral and life history traits) are involved, and how generally it applies across the whole diversity of birds (more than 10,000 species).center dot Using a dataset of 4,530 species with established molecular phylogenies, I examined whether birds that nest exclusively on islands consistently differ from continental-breeding species in terms of a key life history trait: clutch size.center dot I found that breeding island endemic landbirds have smaller clutches. Among island breeding endemic landbirds, breeding range area was positively correlated with clutch size, as expected in the island syndrome. Seabirds show the same evolutionary response in clutch size as landbirds.center dot This study shows that a life history trait is part of the island syndrome across a representative sample of the avian phylogeny, and shows that seabirds also exhibit the island syndrome. El "sindrome de la isla" se refiere a la similitud en la biologia de los organismos de las islas, pero su generalidad es cuestionable, ya que la amplitud de especies y rasgos examinados a menudo es limitada. Aqui, muestro que las aves que se reproducen exclusivamente en islas (reproductoras endemicas de islas) evolucionaron nidadas mas pequenas, utilizando un set de datos de 4.530 especies de aves. Utilizando una definicion inclusiva de reproductoras endemicas de islas, que tambien abarca especies migratorias y aves marinas, examino la evolucion de los tamanos de nidada en especies reproductoras de islas mediante modelos lineales generalizados filogeneticos. A traves de diversas hipotesis filogeneticas, y despues de tener en cuenta las covariables biologicas y geograficas, demuestro que las aves terrestres reproductoras endemicas de islas (470 especies) evolucionaron tamanos de nidada mas pequenos que las especies continentales (3.818 especies). Demuestro que la evolucion del tamano de la nidada sigue las expectativas del sindrome de la isla, ya que entre las aves terrestres reproductoras endemicas de islas hay una relacion positiva entre el tamano de la nidada y el area de reproduccion. Finalmente, refuerzo la idea de que el sindrome de la isla es un patron general en las aves, abarcando diversos grupos filogeneticos y ecologicos, al mostrar que en un set de datos exclusivo de aves marinas (242 especies), las aves marinas reproductoras endemicas de islas muestran la evolucion de tamanos de nidada mas pequenos. En un modelo del set completo de datos de aves terrestres y marinas (4.530 especies), no hubo evidencia de una interaccion entre ser un ave marina y ser un ave reproductora endemica de islas, mostrando que las aves marinas y las aves terrestres responden de la misma manera. Este estudio, que utiliza mas del 40% de todas las especies de aves, proporciona la primera evidencia de una respuesta evolutiva general en un rasgo de la historia de vida, mostrando claramente el sindrome de la isla como una tendencia evolutiva general asociada con ambientes islenos.
