Bird colonies. Suppose (this is too simple to be realistic) that the number y of new birds that join a colony this year has the same straight-line relationship with the percent x1 of returning birds in colonies of two different bird species. An indicator variable shows which species we observe: x2 0 for one and x2= 1 for the other. Write a population regression model that describes this setting. Explain in words what each ß in your model means.