The best example of animals competing for resources is option A. The cattle egret benefits from the presence of grazing animals, as they stir up insects that the egret can then eat. This is an example of commensalism, where one species benefits from the presence of another species without harming or benefiting it. However, it still reflects competition for resources, as multiple species are utilizing the same space and resources (in this case, insects). Option B describes a form of mimicry used for reproduction, while option C describes the negative impact of human activity on a specific species. Option D describes the successful adaptation of a species to a new environment, but it does not involve competition for resources.