A study examined the relationship between the percent of adult birds in a 
colony who return from the previous year and the number of new adults who
join the colony.  The data were taken from a graph in 
   Saether, B-E., Engen, S., and Mattysen, E. 2002.  Demographic
   characteristcs and population dynamical patterns of solitary birds.
   Science, 295, pp. 2070-2073
and are used in Moore's Basic Practice of Statistics.

For each of 13 colonies of sparrowhawks, the variables are:
   percent return
   new adults