Which process is responsible for the adaptations seen in a population of birds over time?

Natural selection is the process that drives the adaptations observed in a population of birds over time. This mechanism occurs when individuals within a species exhibit variations in traits that affect their survival and reproduction. For example, birds with beak shapes better suited to their food source will have a higher chance of survival and producing offspring with similar advantageous traits. Over generations, these traits become more common in the population, leading to adaptations that enhance the birds' fit within their environment.

Migration, while important for the movement of species in search of resources or breeding grounds, does not directly cause adaptations. Evolution is a broader term that encompasses the changes in species over time, including natural selection as one of its driving forces. Competition refers to the struggle between individuals for limited resources, which can influence survival, but it is natural selection that specifically accounts for the differential survival and reproduction based on advantageous traits. Thus, natural selection is the key process responsible for the adaptations seen in a population of birds over time.