Contribution of woody habitat islands to the conservation of birds and their potential ecosystem services in an extensive Colombian rangeland

•Gallery forests conserve forest birds and large-seeded tree species.•Tree islands and hedges are habitat of shrubland birds and small-seeded tree species.•Savanna acts as ecological filter between gallery forests, tree islands and hedges.•Tree islands and hedges facilitate spillover of avian ecological functions to the rangeland, including tree seed dispersal.