Populations of seabirds at a colony reached a stable point after growing quickly. The number of birds coming and going changed over time, with about 1400 new birds arriving each year. The survival of local birds and the chance of birds leaving were most affected by how many birds were already there. The number of new birds joining the colony was lower when the population was growing, but sometimes unexpectedly high when the population was large. The amount of babies born changed with how much food was available, not just how many birds were around. Overall, the number of birds in the colony affected about a third of the changes in bird populations each year, showing that both random events and the number of birds play a role in how populations change.